Nix Package Manager Guide

Version 1.11.9

Eelco Dolstra


I. Introduction
1. About Nix
2. Quick Start
II. Installation
3. Supported Platforms
4. Installing a Binary Distribution
5. Installing Nix from Source
5.1. Prerequisites
5.2. Obtaining a Source Distribution
5.3. Building Nix from Source
6. Security
6.1. Single-User Mode
6.2. Multi-User Mode
7. Environment Variables
III. Package Management
8. Basic Package Management
9. Profiles
10. Garbage Collection
10.1. Garbage Collector Roots
11. Channels
12. One-Click Installation
13. Sharing Packages Between Machines
13.1. Serving a Nix store via HTTP
13.2. Copying Closures Via SSH
13.3. Serving a Nix store via SSH
IV. Writing Nix Expressions
14. A Simple Nix Expression
14.1. Expression Syntax
14.2. Build Script
14.3. Arguments and Variables
14.4. Building and Testing
14.4.1. Debugging Build Failures
14.5. Generic Builder Syntax
15. Nix Expression Language
15.1. Values
15.2. Language Constructs
15.3. Operators
15.4. Derivations
15.4.1. Advanced Attributes
15.5. Built-in Functions
V. Advanced Topics
16. Distributed Builds
VI. Command Reference
17. Common Options
18. Common Environment Variables
19. Main Commands
nix-env — manipulate or query Nix user environments
nix-build — build a Nix expression
nix-shell — start an interactive shell based on a Nix expression
nix-store — manipulate or query the Nix store
20. Utilities
nix-channel — manage Nix channels
nix-collect-garbage — delete unreachable store paths
nix-copy-closure — copy a closure to or from a remote machine via SSH
nix-daemon — Nix multi-user support daemon
nix-hash — compute the cryptographic hash of a path
nix-install-package — install a Nix Package file
nix-instantiate — instantiate store derivations from Nix expressions
nix-prefetch-url — copy a file from a URL into the store and print its hash
nix-pull — register availability of pre-built binaries (deprecated)
nix-push — generate a binary cache
21. Files
nix.conf — Nix configuration file
A. Troubleshooting
A.1. Collisions in nix-env
A.2. Too many links Error in the Nix store
B. Glossary
C. Hacking
D. Nix Release Notes
D.1. Release 1.11 (2016-01-19)
D.2. Release 1.10 (2015-09-03)
D.3. Release 1.9 (2015-06-12)
D.4. Release 1.8 (2014-12-14)
D.5. Release 1.7 (2014-04-11)
D.6. Release 1.6.1 (2013-10-28)
D.7. Release 1.6 (2013-09-10)
D.8. Release 1.5.2 (2013-05-13)
D.9. Release 1.5 (2013-02-27)
D.10. Release 1.4 (2013-02-26)
D.11. Release 1.3 (2013-01-04)
D.12. Release 1.2 (2012-12-06)
D.13. Release 1.1 (2012-07-18)
D.14. Release 1.0 (2012-05-11)
D.15. Release 0.16 (2010-08-17)
D.16. Release 0.15 (2010-03-17)
D.17. Release 0.14 (2010-02-04)
D.18. Release 0.13 (2009-11-05)
D.19. Release 0.12 (2008-11-20)
D.20. Release 0.11 (2007-12-31)
D.21. Release 0.10.1 (2006-10-11)
D.22. Release 0.10 (2006-10-06)
D.23. Release 0.9.2 (2005-09-21)
D.24. Release 0.9.1 (2005-09-20)
D.25. Release 0.9 (2005-09-16)
D.26. Release 0.8.1 (2005-04-13)
D.27. Release 0.8 (2005-04-11)
D.28. Release 0.7 (2005-01-12)
D.29. Release 0.6 (2004-11-14)
D.30. Release 0.5 and earlier

Part I. Introduction

Chapter 1. About Nix

Nix is a purely functional package manager. This means that it treats packages like values in purely functional programming languages such as Haskell — they are built by functions that don’t have side-effects, and they never change after they have been built. Nix stores packages in the Nix store, usually the directory /nix/store, where each package has its own unique subdirectory such as


where b6gvzjyb2pg0… is a unique identifier for the package that captures all its dependencies (it’s a cryptographic hash of the package’s build dependency graph). This enables many powerful features.

Multiple versions

You can have multiple versions or variants of a package installed at the same time. This is especially important when different applications have dependencies on different versions of the same package — it prevents the “DLL hell”. Because of the hashing scheme, different versions of a package end up in different paths in the Nix store, so they don’t interfere with each other.

An important consequence is that operations like upgrading or uninstalling an application cannot break other applications, since these operations never “destructively” update or delete files that are used by other packages.

Complete dependencies

Nix helps you make sure that package dependency specifications are complete. In general, when you’re making a package for a package management system like RPM, you have to specify for each package what its dependencies are, but there are no guarantees that this specification is complete. If you forget a dependency, then the package will build and work correctly on your machine if you have the dependency installed, but not on the end user's machine if it's not there.

Since Nix on the other hand doesn’t install packages in “global” locations like /usr/bin but in package-specific directories, the risk of incomplete dependencies is greatly reduced. This is because tools such as compilers don’t search in per-packages directories such as /nix/store/5lbfaxb722zp…-openssl-0.9.8d/include, so if a package builds correctly on your system, this is because you specified the dependency explicitly.

Runtime dependencies are found by scanning binaries for the hash parts of Nix store paths (such as r8vvq9kq…). This sounds risky, but it works extremely well.

Multi-user support

Nix has multi-user support. This means that non-privileged users can securely install software. Each user can have a different profile, a set of packages in the Nix store that appear in the user’s PATH. If a user installs a package that another user has already installed previously, the package won’t be built or downloaded a second time. At the same time, it is not possible for one user to inject a Trojan horse into a package that might be used by another user.

Atomic upgrades and rollbacks

Since package management operations never overwrite packages in the Nix store but just add new versions in different paths, they are atomic. So during a package upgrade, there is no time window in which the package has some files from the old version and some files from the new version — which would be bad because a program might well crash if it’s started during that period.

And since package aren’t overwritten, the old versions are still there after an upgrade. This means that you can roll back to the old version:

$ nix-env --upgrade some-packages
$ nix-env --rollback

Garbage collection

When you uninstall a package like this…

$ nix-env --uninstall firefox

the package isn’t deleted from the system right away (after all, you might want to do a rollback, or it might be in the profiles of other users). Instead, unused packages can be deleted safely by running the garbage collector:

$ nix-collect-garbage

This deletes all packages that aren’t in use by any user profile or by a currently running program.

Functional package language

Packages are built from Nix expressions, which is a simple functional language. A Nix expression describes everything that goes into a package build action (a “derivation”): other packages, sources, the build script, environment variables for the build script, etc. Nix tries very hard to ensure that Nix expressions are deterministic: building a Nix expression twice should yield the same result.

Because it’s a functional language, it’s easy to support building variants of a package: turn the Nix expression into a function and call it any number of times with the appropriate arguments. Due to the hashing scheme, variants don’t conflict with each other in the Nix store.

Transparent source/binary deployment

Nix expressions generally describe how to build a package from source, so an installation action like

$ nix-env --install firefox

could cause quite a bit of build activity, as not only Firefox but also all its dependencies (all the way up to the C library and the compiler) would have to built, at least if they are not already in the Nix store. This is a source deployment model. For most users, building from source is not very pleasant as it takes far too long. However, Nix can automatically skip building from source and instead use a binary cache, a web server that provides pre-built binaries. For instance, when asked to build /nix/store/b6gvzjyb2pg0…-firefox-33.1 from source, Nix would first check if the file….narinfo exists, and if so, fetch the pre-built binary referenced from there; otherwise, it would fall back to building from source.

Nix Packages collection

We provide a large set of Nix expressions containing hundreds of existing Unix packages, the Nix Packages collection (Nixpkgs).

Managing build environments

Nix is extremely useful for developers as it makes it easy to automatically set up the build environment for a package. Given a Nix expression that describes the dependencies of your package, the command nix-shell will build or download those dependencies if they’re not already in your Nix store, and then start a Bash shell in which all necessary environment variables (such as compiler search paths) are set.

For example, the following command gets all dependencies of the Pan newsreader, as described by its Nix expression:

$ nix-shell '<nixpkgs>' -A pan

You’re then dropped into a shell where you can edit, build and test the package:

[nix-shell]$ tar xf $src
[nix-shell]$ cd pan-*
[nix-shell]$ ./configure
[nix-shell]$ make
[nix-shell]$ ./pan/gui/pan


Nix runs on Linux and Mac OS X.


NixOS is a Linux distribution based on Nix. It uses Nix not just for package management but also to manage the system configuration (e.g., to build configuration files in /etc). This means, among other things, that it is easy to roll back the entire configuration of the system to an earlier state. Also, users can install software without root privileges. For more information and downloads, see the NixOS homepage.


Nix is released under the terms of the GNU LGPLv2.1 or (at your option) any later version.

Chapter 2. Quick Start

This chapter is for impatient people who don't like reading documentation. For more in-depth information you are kindly referred to subsequent chapters.

  1. Install single-user Nix by running the following:

    $ curl | sh

    This will install Nix in /nix. The install script will create /nix using sudo, so make sure you have sufficient rights. (For other installation methods, see Part II, “Installation”.)

  2. See what installable packages are currently available in the channel:

    $ nix-env -qa

  3. Install some packages from the channel:

    $ nix-env -i hello ... 

    This should download pre-built packages; it should not build them locally (if it does, something went wrong).

  4. Test that they work:

    $ which hello
    $ hello
    Hello, world!

  5. Uninstall a package:

    $ nix-env -e hello

  6. You can also test a package without installing it:

    $ nix-shell -p hello

    This builds or downloads GNU Hello and its dependencies, then drops you into a Bash shell where the hello command is present, all without affecting your normal environment:

    [nix-shell:~]$ hello
    Hello, world!
    [nix-shell:~]$ exit
    $ hello
    hello: command not found

  7. To keep up-to-date with the channel, do:

    $ nix-channel --update nixpkgs
    $ nix-env -u '*'

    The latter command will upgrade each installed package for which there is a “newer” version (as determined by comparing the version numbers).

  8. If you're unhappy with the result of a nix-env action (e.g., an upgraded package turned out not to work properly), you can go back:

    $ nix-env --rollback

  9. You should periodically run the Nix garbage collector to get rid of unused packages, since uninstalls or upgrades don't actually delete them:

    $ nix-collect-garbage -d

Part II. Installation

This section describes how to install and configure Nix for first-time use.

Chapter 3. Supported Platforms

Nix is currently supported on the following platforms:

  • Linux (i686, x86_64).

  • Mac OS X (x86_64).

Nix is fairly portable, so it should work on most platforms that support POSIX threads and have a C++11 compiler.

Chapter 4. Installing a Binary Distribution

If you are using Linux or Mac OS X, the easiest way to install Nix is to run the following command:

$ bash <(curl

This will perform a single-user installation of Nix, meaning that /nix is owned by the invoking user. You should run this under your usual user account, not as root. The script will invoke sudo to create /nix if it doesn’t already exist. If you don’t have sudo, you should manually create /nix first as root, e.g.:

$ mkdir /nix
$ chown alice /nix

You can uninstall Nix simply by running:

$ rm -rf /nix

Chapter 5. Installing Nix from Source

If no binary package is available, you can download and compile a source distribution.

5.1. Prerequisites

  • GNU Make.

  • A version of GCC or Clang that supports C++11.

  • Perl 5.8 or higher.

  • pkg-config to locate dependencies. If your distribution does not provide it, you can get it from

  • The bzip2 compressor program and the libbz2 library. Thus you must have bzip2 installed, including development headers and libraries. If your distribution does not provide these, you can obtain bzip2 from

  • The SQLite embedded database library, version 3.6.19 or higher. If your distribution does not provide it, please install it from

  • The Perl DBI, DBD::SQLite, and WWW::Curl libraries, which are available from CPAN if your distribution does not provide them.

  • The Boehm garbage collector to reduce the evaluator’s memory consumption (optional). To enable it, install pkgconfig and the Boehm garbage collector, and pass the flag --enable-gc to configure.

  • The xmllint and xsltproc programs to build this manual and the man-pages. These are part of the libxml2 and libxslt packages, respectively. You also need the DocBook XSL stylesheets and optionally the DocBook 5.0 RELAX NG schemas. Note that these are only required if you modify the manual sources or when you are building from the Git repository.

  • Recent versions of Bison and Flex to build the parser. (This is because Nix needs GLR support in Bison and reentrancy support in Flex.) For Bison, you need version 2.6, which can be obtained from the GNU FTP server. For Flex, you need version 2.5.35, which is available on SourceForge. Slightly older versions may also work, but ancient versions like the ubiquitous 2.5.4a won't. Note that these are only required if you modify the parser or when you are building from the Git repository.

5.2. Obtaining a Source Distribution

The source tarball of the most recent stable release can be downloaded from the Nix homepage. You can also grab the most recent development release.

Alternatively, the most recent sources of Nix can be obtained from its Git repository. For example, the following command will check out the latest revision into a directory called nix:

$ git clone

Likewise, specific releases can be obtained from the tags of the repository.

5.3. Building Nix from Source

After unpacking or checking out the Nix sources, issue the following commands:

$ ./configure options...
$ make
$ make install

Nix requires GNU Make so you may need to invoke gmake instead.

When building from the Git repository, these should be preceded by the command:

$ ./

The installation path can be specified by passing the --prefix=prefix to configure. The default installation directory is /usr/local. You can change this to any location you like. You must have write permission to the prefix path.

Nix keeps its store (the place where packages are stored) in /nix/store by default. This can be changed using --with-store-dir=path.


It is best not to change the Nix store from its default, since doing so makes it impossible to use pre-built binaries from the standard Nixpkgs channels — that is, all packages will need to be built from source.

Nix keeps state (such as its database and log files) in /nix/var by default. This can be changed using --localstatedir=path.

Chapter 6. Security

Nix has two basic security models. First, it can be used in “single-user mode”, which is similar to what most other package management tools do: there is a single user (typically root) who performs all package management operations. All other users can then use the installed packages, but they cannot perform package management operations themselves.

Alternatively, you can configure Nix in “multi-user mode”. In this model, all users can perform package management operations — for instance, every user can install software without requiring root privileges. Nix ensures that this is secure. For instance, it’s not possible for one user to overwrite a package used by another user with a Trojan horse.

6.1. Single-User Mode

In single-user mode, all Nix operations that access the database in prefix/var/nix/db or modify the Nix store in prefix/store must be performed under the user ID that owns those directories. This is typically root. (If you install from RPM packages, that’s in fact the default ownership.) However, on single-user machines, it is often convenient to chown those directories to your normal user account so that you don’t have to su to root all the time.

6.2. Multi-User Mode

To allow a Nix store to be shared safely among multiple users, it is important that users are not able to run builders that modify the Nix store or database in arbitrary ways, or that interfere with builds started by other users. If they could do so, they could install a Trojan horse in some package and compromise the accounts of other users.

To prevent this, the Nix store and database are owned by some privileged user (usually root) and builders are executed under special user accounts (usually named nixbld1, nixbld2, etc.). When a unprivileged user runs a Nix command, actions that operate on the Nix store (such as builds) are forwarded to a Nix daemon running under the owner of the Nix store/database that performs the operation.


Multi-user mode has one important limitation: only root and a set of trusted users specified in nix.conf can specify arbitrary binary caches. So while unprivileged users may install packages from arbitrary Nix expressions, they may not get pre-built binaries.

Setting up the build users

The build users are the special UIDs under which builds are performed. They should all be members of the build users group nixbld. This group should have no other members. The build users should not be members of any other group. On Linux, you can create the group and users as follows:

$ groupadd -r nixbld
$ for n in $(seq 1 10); do useradd -c "Nix build user $n" \
    -d /var/empty -g nixbld -G nixbld -M -N -r -s "$(which nologin)" \
    nixbld$n; done

This creates 10 build users. There can never be more concurrent builds than the number of build users, so you may want to increase this if you expect to do many builds at the same time.

On Mac OS X, you can create the required group and users by running the following script:

#! /bin/bash -e

dseditgroup -o create nixbld -q

gid=$(dscl . -read /Groups/nixbld | awk '($1 == "PrimaryGroupID:") {print $2 }')

echo "created nixbld group with gid $gid"

for i in $(seq 1 10); do
    uid="$((30000 + $i))"
    dscl . create $user
    dscl . create $user RealName "Nix build user $i"
    dscl . create $user PrimaryGroupID "$gid"
    dscl . create $user UserShell /usr/bin/false
    dscl . create $user NFSHomeDirectory /var/empty
    dscl . create $user UniqueID "$uid"
    dseditgroup -o edit -a nixbld$i -t user nixbld
    echo "created nixbld$i user with uid $uid"

Running the daemon

The Nix daemon should be started as follows (as root):

$ nix-daemon

You’ll want to put that line somewhere in your system’s boot scripts.

To let unprivileged users use the daemon, they should set the NIX_REMOTE environment variable to daemon. So you should put a line like

export NIX_REMOTE=daemon

into the users’ login scripts.

Restricting access

To limit which users can perform Nix operations, you can use the permissions on the directory /nix/var/nix/daemon-socket. For instance, if you want to restrict the use of Nix to the members of a group called nix-users, do

$ chgrp nix-users /nix/var/nix/daemon-socket
$ chmod ug=rwx,o= /nix/var/nix/daemon-socket

This way, users who are not in the nix-users group cannot connect to the Unix domain socket /nix/var/nix/daemon-socket/socket, so they cannot perform Nix operations.

Chapter 7. Environment Variables

To use Nix, some environment variables should be set. In particular, PATH should contain the directories prefix/bin and ~/.nix-profile/bin. The first directory contains the Nix tools themselves, while ~/.nix-profile is a symbolic link to the current user environment (an automatically generated package consisting of symlinks to installed packages). The simplest way to set the required environment variables is to include the file prefix/etc/profile.d/ in your ~/.profile (or similar), like this:

source prefix/etc/profile.d/

Part III. Package Management

This chapter discusses how to do package management with Nix, i.e., how to obtain, install, upgrade, and erase packages. This is the “user’s” perspective of the Nix system — people who want to create packages should consult Part IV, “Writing Nix Expressions”.

Chapter 8. Basic Package Management

The main command for package management is nix-env. You can use it to install, upgrade, and erase packages, and to query what packages are installed or are available for installation.

In Nix, different users can have different “views” on the set of installed applications. That is, there might be lots of applications present on the system (possibly in many different versions), but users can have a specific selection of those active — where “active” just means that it appears in a directory in the user’s PATH. Such a view on the set of installed applications is called a user environment, which is just a directory tree consisting of symlinks to the files of the active applications.

Components are installed from a set of Nix expressions that tell Nix how to build those packages, including, if necessary, their dependencies. There is a collection of Nix expressions called the Nix Package collection that contains packages ranging from basic development stuff such as GCC and Glibc, to end-user applications like Mozilla Firefox. (Nix is however not tied to the Nix Package collection; you could write your own Nix expressions based on it, or completely new ones.)

You can manually download the latest version of Nixpkgs from However, it’s much more convenient to use the Nixpkgs channel, since it makes it easy to stay up to date with new versions of Nixpkgs. (Channels are described in more detail in Chapter 11, Channels.) Nixpkgs is automatically added to your list of “subscribed” channels when you install Nix. If this is not the case for some reason, you can add it as follows:

$ nix-channel --add
$ nix-channel --update


On NixOS, you’re automatically subscribed to a NixOS channel corresponding to your NixOS major release (e.g. A NixOS channel is identical to the Nixpkgs channel, except that it contains only Linux binaries and is updated only if a set of regression tests succeed.

You can view the set of available packages in Nixpkgs:

$ nix-env -qa

The flag -q specifies a query operation, and -a means that you want to show the “available” (i.e., installable) packages, as opposed to the installed packages. If you downloaded Nixpkgs yourself, or if you checked it out from GitHub, then you need to pass the path to your Nixpkgs tree using the -f flag:

$ nix-env -qaf /path/to/nixpkgs

where /path/to/nixpkgs is where you’ve unpacked or checked out Nixpkgs.

You can select specific packages by name:

$ nix-env -qa firefox

and using regular expressions:

$ nix-env -qa 'firefox.*'

It is also possible to see the status of available packages, i.e., whether they are installed into the user environment and/or present in the system:

$ nix-env -qas
-PS bash-3.0
--S binutils-2.15
IPS bison-1.875d

The first character (I) indicates whether the package is installed in your current user environment. The second (P) indicates whether it is present on your system (in which case installing it into your user environment would be a very quick operation). The last one (S) indicates whether there is a so-called substitute for the package, which is Nix’s mechanism for doing binary deployment. It just means that Nix knows that it can fetch a pre-built package from somewhere (typically a network server) instead of building it locally.

You can install a package using nix-env -i. For instance,

$ nix-env -i subversion

will install the package called subversion (which is, of course, the Subversion version management system).


When you ask Nix to install a package, it will first try to get it in pre-compiled form from a binary cache. By default, Nix will use the binary cache; it contains binaries for most packages in Nixpkgs. Only if no binary is available in the binary cache, Nix will build the package from source. So if nix-env -i subversion results in Nix building stuff from source, then either the package is not built for your platform by the Nixpkgs build servers, or your version of Nixpkgs is too old or too new. For instance, if you have a very recent checkout of Nixpkgs, then the Nixpkgs build servers may not have had a chance to build everything and upload the resulting binaries to The Nixpkgs channel is only updated after all binaries have been uploaded to the cache, so if you stick to the Nixpkgs channel (rather than using a Git checkout of the Nixpkgs tree), you will get binaries for most packages.

Naturally, packages can also be uninstalled:

$ nix-env -e subversion

Upgrading to a new version is just as easy. If you have a new release of Nix Packages, you can do:

$ nix-env -u subversion

This will only upgrade Subversion if there is a “newer” version in the new set of Nix expressions, as defined by some pretty arbitrary rules regarding ordering of version numbers (which generally do what you’d expect of them). To just unconditionally replace Subversion with whatever version is in the Nix expressions, use -i instead of -u; -i will remove whatever version is already installed.

You can also upgrade all packages for which there are newer versions:

$ nix-env -u

Sometimes it’s useful to be able to ask what nix-env would do, without actually doing it. For instance, to find out what packages would be upgraded by nix-env -u, you can do

$ nix-env -u --dry-run
(dry run; not doing anything)
upgrading `libxslt-1.1.0' to `libxslt-1.1.10'
upgrading `graphviz-1.10' to `graphviz-1.12'
upgrading `coreutils-5.0' to `coreutils-5.2.1'

Chapter 9. Profiles

Profiles and user environments are Nix’s mechanism for implementing the ability to allow different users to have different configurations, and to do atomic upgrades and rollbacks. To understand how they work, it’s useful to know a bit about how Nix works. In Nix, packages are stored in unique locations in the Nix store (typically, /nix/store). For instance, a particular version of the Subversion package might be stored in a directory /nix/store/dpmvp969yhdqs7lm2r1a3gng7pyq6vy4-subversion-1.1.3/, while another version might be stored in /nix/store/5mq2jcn36ldlmh93yj1n8s9c95pj7c5s-subversion-1.1.2. The long strings prefixed to the directory names are cryptographic hashes[1] of all inputs involved in building the package — sources, dependencies, compiler flags, and so on. So if two packages differ in any way, they end up in different locations in the file system, so they don’t interfere with each other. Figure 9.1, “User environments” shows a part of a typical Nix store.

Figure 9.1. User environments

User environments

Of course, you wouldn’t want to type

$ /nix/store/dpmvp969yhdq...-subversion-1.1.3/bin/svn

every time you want to run Subversion. Of course we could set up the PATH environment variable to include the bin directory of every package we want to use, but this is not very convenient since changing PATH doesn’t take effect for already existing processes. The solution Nix uses is to create directory trees of symlinks to activated packages. These are called user environments and they are packages themselves (though automatically generated by nix-env), so they too reside in the Nix store. For instance, in Figure 9.1, “User environments” the user environment /nix/store/0c1p5z4kda11...-user-env contains a symlink to just Subversion 1.1.2 (arrows in the figure indicate symlinks). This would be what we would obtain if we had done

$ nix-env -i subversion

on a set of Nix expressions that contained Subversion 1.1.2.

This doesn’t in itself solve the problem, of course; you wouldn’t want to type /nix/store/0c1p5z4kda11...-user-env/bin/svn either. That’s why there are symlinks outside of the store that point to the user environments in the store; for instance, the symlinks default-42-link and default-43-link in the example. These are called generations since every time you perform a nix-env operation, a new user environment is generated based on the current one. For instance, generation 43 was created from generation 42 when we did

$ nix-env -i subversion firefox

on a set of Nix expressions that contained Firefox and a new version of Subversion.

Generations are grouped together into profiles so that different users don’t interfere with each other if they don’t want to. For example:

$ ls -l /nix/var/nix/profiles/
lrwxrwxrwx  1 eelco ... default-42-link -> /nix/store/0c1p5z4kda11...-user-env
lrwxrwxrwx  1 eelco ... default-43-link -> /nix/store/3aw2pdyx2jfc...-user-env
lrwxrwxrwx  1 eelco ... default -> default-43-link

This shows a profile called default. The file default itself is actually a symlink that points to the current generation. When we do a nix-env operation, a new user environment and generation link are created based on the current one, and finally the default symlink is made to point at the new generation. This last step is atomic on Unix, which explains how we can do atomic upgrades. (Note that the building/installing of new packages doesn’t interfere in any way with old packages, since they are stored in different locations in the Nix store.)

If you find that you want to undo a nix-env operation, you can just do

$ nix-env --rollback

which will just make the current generation link point at the previous link. E.g., default would be made to point at default-42-link. You can also switch to a specific generation:

$ nix-env --switch-generation 43

which in this example would roll forward to generation 43 again. You can also see all available generations:

$ nix-env --list-generations

You generally wouldn’t have /nix/var/nix/profiles/some-profile/bin in your PATH. Rather, there is a symlink ~/.nix-profile that points to your current profile. This means that you should put ~/.nix-profile/bin in your PATH (and indeed, that’s what the initialisation script /nix/etc/profile.d/ does). This makes it easier to switch to a different profile. You can do that using the command nix-env --switch-profile:

$ nix-env --switch-profile /nix/var/nix/profiles/my-profile

$ nix-env --switch-profile /nix/var/nix/profiles/default

These commands switch to the my-profile and default profile, respectively. If the profile doesn’t exist, it will be created automatically. You should be careful about storing a profile in another location than the profiles directory, since otherwise it might not be used as a root of the garbage collector (see Chapter 10, Garbage Collection).

All nix-env operations work on the profile pointed to by ~/.nix-profile, but you can override this using the --profile option (abbreviation -p):

$ nix-env -p /nix/var/nix/profiles/other-profile -i subversion

This will not change the ~/.nix-profile symlink.

[1] 160-bit truncations of SHA-256 hashes encoded in a base-32 notation, to be precise.

Chapter 10. Garbage Collection

nix-env operations such as upgrades (-u) and uninstall (-e) never actually delete packages from the system. All they do (as shown above) is to create a new user environment that no longer contains symlinks to the “deleted” packages.

Of course, since disk space is not infinite, unused packages should be removed at some point. You can do this by running the Nix garbage collector. It will remove from the Nix store any package not used (directly or indirectly) by any generation of any profile.

Note however that as long as old generations reference a package, it will not be deleted. After all, we wouldn’t be able to do a rollback otherwise. So in order for garbage collection to be effective, you should also delete (some) old generations. Of course, this should only be done if you are certain that you will not need to roll back.

To delete all old (non-current) generations of your current profile:

$ nix-env --delete-generations old

Instead of old you can also specify a list of generations, e.g.,

$ nix-env --delete-generations 10 11 14

To delete all generations older than a specified number of days (except the current generation), use the d suffix. For example,

$ nix-env --delete-generations 14d

deletes all generations older than two weeks.

After removing appropriate old generations you can run the garbage collector as follows:

$ nix-store --gc

If you are feeling uncertain, you can also first view what files would be deleted:

$ nix-store --gc --print-dead

Likewise, the option --print-live will show the paths that won’t be deleted.

There is also a convenient little utility nix-collect-garbage, which when invoked with the -d (--delete-old) switch deletes all old generations of all profiles in /nix/var/nix/profiles. So

$ nix-collect-garbage -d

is a quick and easy way to clean up your system.

10.1. Garbage Collector Roots

The roots of the garbage collector are all store paths to which there are symlinks in the directory prefix/nix/var/nix/gcroots. For instance, the following command makes the path /nix/store/d718ef...-foo a root of the collector:

$ ln -s /nix/store/d718ef...-foo /nix/var/nix/gcroots/bar

That is, after this command, the garbage collector will not remove /nix/store/d718ef...-foo or any of its dependencies.

Subdirectories of prefix/nix/var/nix/gcroots are also searched for symlinks. Symlinks to non-store paths are followed and searched for roots, but symlinks to non-store paths inside the paths reached in that way are not followed to prevent infinite recursion.

Chapter 11. Channels

If you want to stay up to date with a set of packages, it’s not very convenient to manually download the latest set of Nix expressions for those packages and upgrade using nix-env. Fortunately, there’s a better way: Nix channels.

A Nix channel is just a URL that points to a place that contains a set of Nix expressions and a manifest. Using the command nix-channel you can automatically stay up to date with whatever is available at that URL.

You can “subscribe” to a channel using nix-channel --add, e.g.,

$ nix-channel --add

subscribes you to a channel that always contains that latest version of the Nix Packages collection. (Subscribing really just means that the URL is added to the file ~/.nix-channels, where it is read by subsequent calls to nix-channel --update.) You can “unsubscribe” using nix-channel --remove:

$ nix-channel --remove nixpkgs

To obtain the latest Nix expressions available in a channel, do

$ nix-channel --update

This downloads and unpacks the Nix expressions in every channel (downloaded from url/nixexprs.tar.bz2). It also makes the union of each channel’s Nix expressions available by default to nix-env operations (via the symlink ~/.nix-defexpr/channels). Consequently, you can then say

$ nix-env -u

to upgrade all packages in your profile to the latest versions available in the subscribed channels.

Chapter 12. One-Click Installation

Often, when you want to install a specific package (e.g., from the Nix Packages collection), subscribing to a channel is a bit cumbersome. And channels don’t help you at all if you want to install an older version of a package than the one provided by the current contents of the channel, or a package that has been removed from the channel. That’s when one-click installs come in handy: you can just go to the web page that contains the package, click on it, and it will be installed with all the necessary dependencies.

For instance, you can go to and click on any link for the individual packages for your platform. The first time you do this, your browser will ask what to do with application/nix-package files. You should open them with /nix/bin/nix-install-package. This will open a window that asks you to confirm that you want to install the package. When you answer Y, the package and all its dependencies will be installed. This is a binary deployment mechanism — you get packages pre-compiled for the selected platform type.

You can also install application/nix-package files from the command line directly. See nix-install-package(1) for details.

Chapter 13. Sharing Packages Between Machines

Sometimes you want to copy a package from one machine to another. Or, you want to install some packages and you know that another machine already has some or all of those packages or their dependencies. In that case there are mechanisms to quickly copy packages between machines.

13.1. Serving a Nix store via HTTP

You can easily share the Nix store of a machine via HTTP. This allows other machines to fetch store paths from that machine to speed up installations. It uses the same binary cache mechanism that Nix usually uses to fetch pre-built binaries from

The daemon that handles binary cache requests via HTTP, nix-serve, is not part of the Nix distribution, but you can install it from Nixpkgs:

$ nix-env -i nix-serve

You can then start the server, listening for HTTP connections on whatever port you like:

$ nix-serve -p 8080

To check whether it works, try the following on the client:

$ curl http://avalon:8080/nix-cache-info

which should print something like:

StoreDir: /nix/store
WantMassQuery: 1
Priority: 30

On the client side, you can tell Nix to use your binary cache using --option extra-binary-caches, e.g.:

$ nix-env -i firefox --option extra-binary-caches http://avalon:8080/

The option extra-binary-caches tells Nix to use this binary cache in addition to your default caches, such as Thus, for any path in the closure of Firefox, Nix will first check if the path is available on the server avalon or another binary caches. If not, it will fall back to building from source.

You can also tell Nix to always use your binary cache by adding a line to the nix.conf configuration file like this:

binary-caches = http://avalon:8080/

13.2. Copying Closures Via SSH

The command nix-copy-closure copies a Nix store path along with all its dependencies to or from another machine via the SSH protocol. It doesn’t copy store paths that are already present on the target machine. For example, the following command copies Firefox with all its dependencies:

$ nix-copy-closure --to $(type -p firefox)

See nix-copy-closure(1) for details.

With nix-store --export and nix-store --import you can write the closure of a store path (that is, the path and all its dependencies) to a file, and then unpack that file into another Nix store. For example,

$ nix-store --export $(nix-store -qR $(type -p firefox)) > firefox.closure

writes the closure of Firefox to a file. You can then copy this file to another machine and install the closure:

$ nix-store --import < firefox.closure

Any store paths in the closure that are already present in the target store are ignored. It is also possible to pipe the export into another command, e.g. to copy and install a closure directly to/on another machine:

$ nix-store --export $(nix-store -qR $(type -p firefox)) | bzip2 | \
    ssh "bunzip2 | nix-store --import"

However, nix-copy-closure is generally more efficient because it only copies paths that are not already present in the target Nix store.

13.3. Serving a Nix store via SSH

You can tell Nix to automatically fetch needed binaries from a remote Nix store via SSH. For example, the following installs Firefox, automatically fetching any store paths in Firefox’s closure if they are available on the server avalon:

$ nix-env -i firefox --option ssh-substituter-hosts alice@avalon

This works similar to the binary cache substituter that Nix usually uses, only using SSH instead of HTTP: if a store path P is needed, Nix will first check if it’s available in the Nix store on avalon. If not, it will fall back to using the binary cache substituter, and then to building from source.


The SSH substituter currently does not allow you to enter an SSH passphrase interactively. Therefore, you should use ssh-add to load the decrypted private key into ssh-agent.

You can also copy the closure of some store path, without installing it into your profile, e.g.

$ nix-store -r /nix/store/m85bxg…-firefox-34.0.5 --option ssh-substituter-hosts alice@avalon

This is essentially equivalent to doing

$ nix-copy-closure --from alice@avalon /nix/store/m85bxg…-firefox-34.0.5

You can use SSH’s forced command feature to set up a restricted user account for SSH substituter access, allowing read-only access to the local Nix store, but nothing more. For example, add the following lines to sshd_config to restrict the user nix-ssh:

Match User nix-ssh
  AllowAgentForwarding no
  AllowTcpForwarding no
  PermitTTY no
  PermitTunnel no
  X11Forwarding no
  ForceCommand nix-store --serve
Match All

On NixOS, you can accomplish the same by adding the following to your configuration.nix:

nix.sshServe.enable = true;
nix.sshServe.keys = [ "ssh-dss AAAAB3NzaC1k..." ];

where the latter line lists the public keys of users that are allowed to connect.

Part IV. Writing Nix Expressions

This chapter shows you how to write Nix expressions, which instruct Nix how to build packages. It starts with a simple example (a Nix expression for GNU Hello), and then moves on to a more in-depth look at the Nix expression language.


This chapter is mostly about the Nix expression language. For more extensive information on adding packages to the Nix Packages collection (such as functions in the standard environment and coding conventions), please consult its manual.

Chapter 14. A Simple Nix Expression

This section shows how to add and test the GNU Hello package to the Nix Packages collection. Hello is a program that prints out the text Hello, world!.

To add a package to the Nix Packages collection, you generally need to do three things:

  1. Write a Nix expression for the package. This is a file that describes all the inputs involved in building the package, such as dependencies, sources, and so on.

  2. Write a builder. This is a shell script[2] that actually builds the package from the inputs.

  3. Add the package to the file pkgs/top-level/all-packages.nix. The Nix expression written in the first step is a function; it requires other packages in order to build it. In this step you put it all together, i.e., you call the function with the right arguments to build the actual package.

14.1. Expression Syntax

Example 14.1. Nix expression for GNU Hello (default.nix)

{ stdenv, fetchurl, perl }: 1

stdenv.mkDerivation { 2
  name = "hello-2.1.1"; 3
  builder = ./; 4
  src = fetchurl { 5
    url =;
    md5 = "70c9ccf9fac07f762c24f2df2290784d";
  inherit perl; 6

Example 14.1, “Nix expression for GNU Hello (default.nix)” shows a Nix expression for GNU Hello. It's actually already in the Nix Packages collection in pkgs/applications/misc/hello/ex-1/default.nix. It is customary to place each package in a separate directory and call the single Nix expression in that directory default.nix. The file has the following elements (referenced from the figure by number):


This states that the expression is a function that expects to be called with three arguments: stdenv, fetchurl, and perl. They are needed to build Hello, but we don't know how to build them here; that's why they are function arguments. stdenv is a package that is used by almost all Nix Packages packages; it provides a standard environment consisting of the things you would expect in a basic Unix environment: a C/C++ compiler (GCC, to be precise), the Bash shell, fundamental Unix tools such as cp, grep, tar, etc. fetchurl is a function that downloads files. perl is the Perl interpreter.

Nix functions generally have the form { x, y, ..., z }: e where x, y, etc. are the names of the expected arguments, and where e is the body of the function. So here, the entire remainder of the file is the body of the function; when given the required arguments, the body should describe how to build an instance of the Hello package.


So we have to build a package. Building something from other stuff is called a derivation in Nix (as opposed to sources, which are built by humans instead of computers). We perform a derivation by calling stdenv.mkDerivation. mkDerivation is a function provided by stdenv that builds a package from a set of attributes. A set is just a list of key/value pairs where each key is a string and each value is an arbitrary Nix expression. They take the general form { name1 = expr1; ... nameN = exprN; }.


The attribute name specifies the symbolic name and version of the package. Nix doesn't really care about these things, but they are used by for instance nix-env -q to show a human-readable name for packages. This attribute is required by mkDerivation.


The attribute builder specifies the builder. This attribute can sometimes be omitted, in which case mkDerivation will fill in a default builder (which does a configure; make; make install, in essence). Hello is sufficiently simple that the default builder would suffice, but in this case, we will show an actual builder for educational purposes. The value ./ refers to the shell script shown in Example 14.2, “Build script for GNU Hello (”, discussed below.


The builder has to know what the sources of the package are. Here, the attribute src is bound to the result of a call to the fetchurl function. Given a URL and an MD5 hash of the expected contents of the file at that URL, this function builds a derivation that downloads the file and checks its hash. So the sources are a dependency that like all other dependencies is built before Hello itself is built.

Instead of src any other name could have been used, and in fact there can be any number of sources (bound to different attributes). However, src is customary, and it's also expected by the default builder (which we don't use in this example).


Since the derivation requires Perl, we have to pass the value of the perl function argument to the builder. All attributes in the set are actually passed as environment variables to the builder, so declaring an attribute

perl = perl;

will do the trick: it binds an attribute perl to the function argument which also happens to be called perl. However, it looks a bit silly, so there is a shorter syntax. The inherit keyword causes the specified attributes to be bound to whatever variables with the same name happen to be in scope.

14.2. Build Script

Example 14.2. Build script for GNU Hello (

source $stdenv/setup 1

PATH=$perl/bin:$PATH 2

tar xvfz $src 3
cd hello-*
./configure --prefix=$out 4
make 5
make install

Example 14.2, “Build script for GNU Hello (” shows the builder referenced from Hello's Nix expression (stored in pkgs/applications/misc/hello/ex-1/ The builder can actually be made a lot shorter by using the generic builder functions provided by stdenv, but here we write out the build steps to elucidate what a builder does. It performs the following steps:


When Nix runs a builder, it initially completely clears the environment (except for the attributes declared in the derivation). For instance, the PATH variable is empty[3]. This is done to prevent undeclared inputs from being used in the build process. If for example the PATH contained /usr/bin, then you might accidentally use /usr/bin/gcc.

So the first step is to set up the environment. This is done by calling the setup script of the standard environment. The environment variable stdenv points to the location of the standard environment being used. (It wasn't specified explicitly as an attribute in Example 14.1, “Nix expression for GNU Hello (default.nix)”, but mkDerivation adds it automatically.)


Since Hello needs Perl, we have to make sure that Perl is in the PATH. The perl environment variable points to the location of the Perl package (since it was passed in as an attribute to the derivation), so $perl/bin is the directory containing the Perl interpreter.


Now we have to unpack the sources. The src attribute was bound to the result of fetching the Hello source tarball from the network, so the src environment variable points to the location in the Nix store to which the tarball was downloaded. After unpacking, we cd to the resulting source directory.

The whole build is performed in a temporary directory created in /tmp, by the way. This directory is removed after the builder finishes, so there is no need to clean up the sources afterwards. Also, the temporary directory is always newly created, so you don't have to worry about files from previous builds interfering with the current build.


GNU Hello is a typical Autoconf-based package, so we first have to run its configure script. In Nix every package is stored in a separate location in the Nix store, for instance /nix/store/9a54ba97fb71b65fda531012d0443ce2-hello-2.1.1. Nix computes this path by cryptographically hashing all attributes of the derivation. The path is passed to the builder through the out environment variable. So here we give configure the parameter --prefix=$out to cause Hello to be installed in the expected location.


Finally we build Hello (make) and install it into the location specified by out (make install).

If you are wondering about the absence of error checking on the result of various commands called in the builder: this is because the shell script is evaluated with Bash's -e option, which causes the script to be aborted if any command fails without an error check.

14.3. Arguments and Variables

Example 14.3. Composing GNU Hello (all-packages.nix)


rec { 1

  hello = import ../applications/misc/hello/ex-1 2 { 3
    inherit fetchurl stdenv perl;

  perl = import ../development/interpreters/perl { 4
    inherit fetchurl stdenv;

  fetchurl = import ../build-support/fetchurl {
    inherit stdenv; ...

  stdenv = ...;


The Nix expression in Example 14.1, “Nix expression for GNU Hello (default.nix)” is a function; it is missing some arguments that have to be filled in somewhere. In the Nix Packages collection this is done in the file pkgs/top-level/all-packages.nix, where all Nix expressions for packages are imported and called with the appropriate arguments. Example 14.3, “Composing GNU Hello (all-packages.nix)” shows some fragments of all-packages.nix.


This file defines a set of attributes, all of which are concrete derivations (i.e., not functions). In fact, we define a mutually recursive set of attributes. That is, the attributes can refer to each other. This is precisely what we want since we want to plug the various packages into each other.


Here we import the Nix expression for GNU Hello. The import operation just loads and returns the specified Nix expression. In fact, we could just have put the contents of Example 14.1, “Nix expression for GNU Hello (default.nix)” in all-packages.nix at this point. That would be completely equivalent, but it would make the file rather bulky.

Note that we refer to ../applications/misc/hello/ex-1, not ../applications/misc/hello/ex-1/default.nix. When you try to import a directory, Nix automatically appends /default.nix to the file name.


This is where the actual composition takes place. Here we call the function imported from ../applications/misc/hello/ex-1 with a set containing the things that the function expects, namely fetchurl, stdenv, and perl. We use inherit again to use the attributes defined in the surrounding scope (we could also have written fetchurl = fetchurl;, etc.).

The result of this function call is an actual derivation that can be built by Nix (since when we fill in the arguments of the function, what we get is its body, which is the call to stdenv.mkDerivation in Example 14.1, “Nix expression for GNU Hello (default.nix)”).


Nixpkgs has a convenience function callPackage that imports and calls a function, filling in any missing arguments by passing the corresponding attribute from the Nixpkgs set, like this:

hello = callPackage ../applications/misc/hello/ex-1 { };

If necessary, you can set or override arguments:

hello = callPackage ../applications/misc/hello/ex-1 { stdenv = myStdenv; };


Likewise, we have to instantiate Perl, fetchurl, and the standard environment.

14.4. Building and Testing

You can now try to build Hello. Of course, you could do nix-env -i hello, but you may not want to install a possibly broken package just yet. The best way to test the package is by using the command nix-build, which builds a Nix expression and creates a symlink named result in the current directory:

$ nix-build -A hello
building path `/nix/store/632d2b22514d...-hello-2.1.1'

$ ls -l result
lrwxrwxrwx ... 2006-09-29 10:43 result -> /nix/store/632d2b22514d...-hello-2.1.1

$ ./result/bin/hello
Hello, world!

The -A option selects the hello attribute. This is faster than using the symbolic package name specified by the name attribute (which also happens to be hello) and is unambiguous (there can be multiple packages with the symbolic name hello, but there can be only one attribute in a set named hello).

nix-build registers the ./result symlink as a garbage collection root, so unless and until you delete the ./result symlink, the output of the build will be safely kept on your system. You can use nix-build’s -o switch to give the symlink another name.

Nix has a transactional semantics. Once a build finishes successfully, Nix makes a note of this in its database: it registers that the path denoted by out is now valid. If you try to build the derivation again, Nix will see that the path is already valid and finish immediately. If a build fails, either because it returns a non-zero exit code, because Nix or the builder are killed, or because the machine crashes, then the output paths will not be registered as valid. If you try to build the derivation again, Nix will remove the output paths if they exist (e.g., because the builder died half-way through make install) and try again. Note that there is no negative caching: Nix doesn't remember that a build failed, and so a failed build can always be repeated. This is because Nix cannot distinguish between permanent failures (e.g., a compiler error due to a syntax error in the source) and transient failures (e.g., a disk full condition).

Nix also performs locking. If you run multiple Nix builds simultaneously, and they try to build the same derivation, the first Nix instance that gets there will perform the build, while the others block (or perform other derivations if available) until the build finishes:

$ nix-build -A hello
waiting for lock on `/nix/store/0h5b7hp8d4hqfrw8igvx97x1xawrjnac-hello-2.1.1x'

So it is always safe to run multiple instances of Nix in parallel (which isn’t the case with, say, make).

If you have a system with multiple CPUs, you may want to have Nix build different derivations in parallel (insofar as possible). Just pass the option -j N, where N is the maximum number of jobs to be run in parallel, or set. Typically this should be the number of CPUs.

14.4.1. Debugging Build Failures

At the beginning of each phase of the build (such as unpacking, building or installing), the set of all shell variables is written to the file env-vars at the top-level build directory. This is useful for debugging: it allows you to recreate the environment in which a build was performed. For instance, if a build fails, then assuming you used the -K flag, you can go to the output directory and switch to the environment of the builder:

$ nix-build -K ./foo.nix
... fails, keeping build directory `/tmp/nix-1234-0'

$ cd /tmp/nix-1234-0

$ source env-vars

(edit some files...)

$ make

(execution continues with the same GCC, make, etc.)

14.5. Generic Builder Syntax

Recall from Example 14.2, “Build script for GNU Hello (” that the builder looked something like this:

tar xvfz $src
cd hello-*
./configure --prefix=$out
make install

The builders for almost all Unix packages look like this — set up some environment variables, unpack the sources, configure, build, and install. For this reason the standard environment provides some Bash functions that automate the build process. A builder using the generic build facilities in shown in Example 14.4, “Build script using the generic build functions”.

Example 14.4. Build script using the generic build functions

buildInputs="$perl" 1

source $stdenv/setup 2

genericBuild 3


The buildInputs variable tells setup to use the indicated packages as inputs. This means that if a package provides a bin subdirectory, it's added to PATH; if it has a include subdirectory, it's added to GCC's header search path; and so on.[4]


The function genericBuild is defined in the file $stdenv/setup.


The final step calls the shell function genericBuild, which performs the steps that were done explicitly in Example 14.2, “Build script for GNU Hello (”. The generic builder is smart enough to figure out whether to unpack the sources using gzip, bzip2, etc. It can be customised in many ways; see the Nixpkgs manual for details.

Discerning readers will note that the buildInputs could just as well have been set in the Nix expression, like this:

  buildInputs = [ perl ];

The perl attribute can then be removed, and the builder becomes even shorter:

source $stdenv/setup

In fact, mkDerivation provides a default builder that looks exactly like that, so it is actually possible to omit the builder for Hello entirely.

[2] In fact, it can be written in any language, but typically it's a bash shell script.

[3] Actually, it's initialised to /path-not-set to prevent Bash from setting it to a default value.

[4] How does it work? setup tries to source the file pkg/nix-support/setup-hook of all dependencies. These “setup hooks” can then set up whatever environment variables they want; for instance, the setup hook for Perl sets the PERL5LIB environment variable to contain the lib/site_perl directories of all inputs.

Chapter 15. Nix Expression Language

The Nix expression language is a pure, lazy, functional language. Purity means that operations in the language don't have side-effects (for instance, there is no variable assignment). Laziness means that arguments to functions are evaluated only when they are needed. Functional means that functions are normal values that can be passed around and manipulated in interesting ways. The language is not a full-featured, general purpose language. Its main job is to describe packages, compositions of packages, and the variability within packages.

This section presents the various features of the language.

15.1. Values

Simple Values

Nix has the following basic data types:

  • Strings can be written in three ways.

    The most common way is to enclose the string between double quotes, e.g., "foo bar". Strings can span multiple lines. The special characters " and \ and the character sequence ${ must be escaped by prefixing them with a backslash (\). Newlines, carriage returns and tabs can be written as \n, \r and \t, respectively.

    You can include the result of an expression into a string by enclosing it in ${...}, a feature known as antiquotation. The enclosed expression must evaluate to something that can be coerced into a string (meaning that it must be a string, a path, or a derivation). For instance, rather than writing

    "--with-freetype2-library=" + freetype + "/lib"

    (where freetype is a derivation), you can instead write the more natural


    The latter is automatically translated to the former. A more complicated example (from the Nix expression for Qt):

    configureFlags = "
      -system-zlib -system-libpng -system-libjpeg
      ${if openglSupport then "-dlopen-opengl
        -L${mesa}/lib -I${mesa}/include
        -L${libXmu}/lib -I${libXmu}/include" else ""}
      ${if threadSupport then "-thread" else "-no-thread"}

    Note that Nix expressions and strings can be arbitrarily nested; in this case the outer string contains various antiquotations that themselves contain strings (e.g., "-thread"), some of which in turn contain expressions (e.g., ${mesa}).

    The second way to write string literals is as an indented string, which is enclosed between pairs of double single-quotes, like so:

      This is the first line.
      This is the second line.
        This is the third line.

    This kind of string literal intelligently strips indentation from the start of each line. To be precise, it strips from each line a number of spaces equal to the minimal indentation of the string as a whole (disregarding the indentation of empty lines). For instance, the first and second line are indented two space, while the third line is indented four spaces. Thus, two spaces are stripped from each line, so the resulting string is

    "This is the first line.\nThis is the second line.\n  This is the third line.\n"

    Note that the whitespace and newline following the opening '' is ignored if there is no non-whitespace text on the initial line.

    Antiquotation (${expr}) is supported in indented strings.

    Since ${ and '' have special meaning in indented strings, you need a way to quote them. ${ can be escaped by prefixing it with '' (that is, two single quotes), i.e., ''${. '' can be escaped by prefixing it with ', i.e., '''. Finally, linefeed, carriage-return and tab characters can be written as ''\n, ''\r, ''\t.

    Indented strings are primarily useful in that they allow multi-line string literals to follow the indentation of the enclosing Nix expression, and that less escaping is typically necessary for strings representing languages such as shell scripts and configuration files because '' is much less common than ". Example:

    stdenv.mkDerivation {
      postInstall =
          mkdir $out/bin $out/etc
          cp foo $out/bin
          echo "Hello World" > $out/etc/foo.conf
          ${if enableBar then "cp bar $out/bin" else ""}

    Finally, as a convenience, URIs as defined in appendix B of RFC 2396 can be written as is, without quotes. For instance, the string "" can also be written as

  • Integers, e.g., 123.

  • Paths, e.g., /bin/sh or ./ A path must contain at least one slash to be recognised as such; for instance, is not a path[5]. If the file name is relative, i.e., if it does not begin with a slash, it is made absolute at parse time relative to the directory of the Nix expression that contained it. For instance, if a Nix expression in /foo/bar/bla.nix refers to ../xyzzy/fnord.nix, the absolute path is /foo/xyzzy/fnord.nix.

    If the first component of a path is a ~, it is interpreted as if the rest of the path were relative to the user's home directory. e.g. ~/foo would be equivalent to /home/edolstra/foo for a user whose home directory is /home/edolstra.

  • Booleans with values true and false.

  • The null value, denoted as null.


Lists are formed by enclosing a whitespace-separated list of values between square brackets. For example,

[ 123 ./foo.nix "abc" (f { x = y; }) ]

defines a list of four elements, the last being the result of a call to the function f. Note that function calls have to be enclosed in parentheses. If they had been omitted, e.g.,

[ 123 ./foo.nix "abc" f { x = y; } ]

the result would be a list of five elements, the fourth one being a function and the fifth being a set.

Note that lists are only lazy in values, and they are strict in length.


Sets are really the core of the language, since ultimately the Nix language is all about creating derivations, which are really just sets of attributes to be passed to build scripts.

Sets are just a list of name/value pairs (called attributes) enclosed in curly brackets, where each value is an arbitrary expression terminated by a semicolon. For example:

{ x = 123;
  text = "Hello";
  y = f { bla = 456; };

This defines a set with attributes named x, text, y. The order of the attributes is irrelevant. An attribute name may only occur once.

Attributes can be selected from a set using the . operator. For instance,

{ a = "Foo"; b = "Bar"; }.a

evaluates to "Foo". It is possible to provide a default value in an attribute selection using the or keyword. For example,

{ a = "Foo"; b = "Bar"; }.c or "Xyzzy"

will evaluate to "Xyzzy" because there is no c attribute in the set.

You can use arbitrary double-quoted strings as attribute names:

{ "foo ${bar}" = 123; "nix-1.0" = 456; }."foo ${bar}"

This will evaluate to 123 (Assuming bar is antiquotable). In the case where an attribute name is just a single antiquotation, the quotes can be dropped:

{ foo = 123; }.${bar} or 456 

This will evaluate to 123 if bar evaluates to "foo" when coerced to a string and 456 otherwise (again assuming bar is antiquotable).

In the special case where an attribute name inside of a set declaration evaluates to null (which is normally an error, as null is not antiquotable), that attribute is simply not added to the set:

{ ${if foo then "bar" else null} = true; }

This will evaluate to {} if foo evaluates to false.

15.2. Language Constructs

Recursive sets

Recursive sets are just normal sets, but the attributes can refer to each other. For example,

rec {
  x = y;
  y = 123;

evaluates to 123. Note that without rec the binding x = y; would refer to the variable y in the surrounding scope, if one exists, and would be invalid if no such variable exists. That is, in a normal (non-recursive) set, attributes are not added to the lexical scope; in a recursive set, they are.

Recursive sets of course introduce the danger of infinite recursion. For example,

rec {
  x = y;
  y = x;

does not terminate[6].


A let-expression allows you define local variables for an expression. For instance,

  x = "foo";
  y = "bar";
in x + y

evaluates to "foobar".

Inheriting attributes

When defining a set it is often convenient to copy variables from the surrounding lexical scope (e.g., when you want to propagate attributes). This can be shortened using the inherit keyword. For instance,

let x = 123; in
{ inherit x;
  y = 456;

evaluates to { x = 123; y = 456; }. (Note that this works because x is added to the lexical scope by the let construct.) It is also possible to inherit attributes from another set. For instance, in this fragment from all-packages.nix,

  graphviz = (import ../tools/graphics/graphviz) {
    inherit fetchurl stdenv libpng libjpeg expat x11 yacc;
    inherit (xlibs) libXaw;

  xlibs = {
    libX11 = ...;
    libXaw = ...;

  libpng = ...;
  libjpg = ...;

the set used in the function call to the function defined in ../tools/graphics/graphviz inherits a number of variables from the surrounding scope (fetchurl ... yacc), but also inherits libXaw (the X Athena Widgets) from the xlibs (X11 client-side libraries) set.


Functions have the following form:

pattern: body

The pattern specifies what the argument of the function must look like, and binds variables in the body to (parts of) the argument. There are three kinds of patterns:

  • If a pattern is a single identifier, then the function matches any argument. Example:

    let negate = x: !x;
        concat = x: y: x + y;
    in if negate true then concat "foo" "bar" else ""

    Note that concat is a function that takes one argument and returns a function that takes another argument. This allows partial parameterisation (i.e., only filling some of the arguments of a function); e.g.,

    map (concat "foo") [ "bar" "bla" "abc" ]

    evaluates to [ "foobar" "foobla" "fooabc" ].

  • A set pattern of the form { name1, name2, …, nameN } matches a set containing the listed attributes, and binds the values of those attributes to variables in the function body. For example, the function

    { x, y, z }: z + y + x

    can only be called with a set containing exactly the attributes x, y and z. No other attributes are allowed. If you want to allow additional arguments, you can use an ellipsis (...):

    { x, y, z, ... }: z + y + x

    This works on any set that contains at least the three named attributes.

    It is possible to provide default values for attributes, in which case they are allowed to be missing. A default value is specified by writing name ? e, where e is an arbitrary expression. For example,

    { x, y ? "foo", z ? "bar" }: z + y + x

    specifies a function that only requires an attribute named x, but optionally accepts y and z.

  • An @-pattern provides a means of referring to the whole value being matched:

    args@{ x, y, z, ... }: z + y + x + args.a

    Here args is bound to the entire argument, which is further matched against the pattern { x, y, z, ... }.

Note that functions do not have names. If you want to give them a name, you can bind them to an attribute, e.g.,

let concat = { x, y }: x + y;
in concat { x = "foo"; y = "bar"; }

A set that has a __functor attribute whose value is callable (i.e. is itself a function or a set with a __functor attribute whose value is callable) can be applied as if it were a function, with the set itself passed in first , e.g.,

let add = { __functor = self: x: x + self.x; };
    inc = add // { x = 1; };
in inc 1

evaluates to 2. This can be used to attach metadata to a function without the caller needing to treat it specially, or to implement a form of object-oriented programming, for example.


Conditionals look like this:

if e1 then e2 else e3

where e1 is an expression that should evaluate to a Boolean value (true or false).


Assertions are generally used to check that certain requirements on or between features and dependencies hold. They look like this:

assert e1; e2

where e1 is an expression that should evaluate to a Boolean value. If it evaluates to true, e2 is returned; otherwise expression evaluation is aborted and a backtrace is printed.

Example 15.1. Nix expression for Subversion

{ localServer ? false
, httpServer ? false
, sslSupport ? false
, pythonBindings ? false
, javaSwigBindings ? false
, javahlBindings ? false
, stdenv, fetchurl
, openssl ? null, httpd ? null, db4 ? null, expat, swig ? null, j2sdk ? null

assert localServer -> db4 != null; 1
assert httpServer -> httpd != null && httpd.expat == expat; 2
assert sslSupport -> openssl != null && (httpServer -> httpd.openssl == openssl); 3
assert pythonBindings -> swig != null && swig.pythonSupport;
assert javaSwigBindings -> swig != null && swig.javaSupport;
assert javahlBindings -> j2sdk != null;

stdenv.mkDerivation {
  name = "subversion-1.1.1";
  openssl = if sslSupport then openssl else null; 4

Example 15.1, “Nix expression for Subversion” show how assertions are used in the Nix expression for Subversion.


This assertion states that if Subversion is to have support for local repositories, then Berkeley DB is needed. So if the Subversion function is called with the localServer argument set to true but the db4 argument set to null, then the evaluation fails.


This is a more subtle condition: if Subversion is built with Apache (httpServer) support, then the Expat library (an XML library) used by Subversion should be same as the one used by Apache. This is because in this configuration Subversion code ends up being linked with Apache code, and if the Expat libraries do not match, a build- or runtime link error or incompatibility might occur.


This assertion says that in order for Subversion to have SSL support (so that it can access https URLs), an OpenSSL library must be passed. Additionally, it says that if Apache support is enabled, then Apache's OpenSSL should match Subversion's. (Note that if Apache support is not enabled, we don't care about Apache's OpenSSL.)


The conditional here is not really related to assertions, but is worth pointing out: it ensures that if SSL support is disabled, then the Subversion derivation is not dependent on OpenSSL, even if a non-null value was passed. This prevents an unnecessary rebuild of Subversion if OpenSSL changes.


A with-expression,

with e1; e2

introduces the set e1 into the lexical scope of the expression e2. For instance,

let as = { x = "foo"; y = "bar"; };
in with as; x + y

evaluates to "foobar" since the with adds the x and y attributes of as to the lexical scope in the expression x + y. The most common use of with is in conjunction with the import function. E.g.,

with (import ./definitions.nix); ...

makes all attributes defined in the file definitions.nix available as if they were defined locally in a rec-expression.


Comments can be single-line, started with a # character, or inline/multi-line, enclosed within /* ... */.

15.3. Operators

Table 15.1, “Operators” lists the operators in the Nix expression language, in order of precedence (from strongest to weakest binding).

Table 15.1. Operators

e . attrpath [ or def ] noneSelect attribute denoted by the attribute path attrpath from set e. (An attribute path is a dot-separated list of attribute names.) If the attribute doesn’t exist, return def if provided, otherwise abort evaluation.
e1 e2leftCall function e1 with argument e2.
e ? attrpathnoneTest whether set e contains the attribute denoted by attrpath; return true or false.
e1 ++ e2rightList concatenation.
e1 + e2leftString or path concatenation.
! eleftBoolean negation.
e1 // e2rightReturn a set consisting of the attributes in e1 and e2 (with the latter taking precedence over the former in case of equally named attributes).
e1 == e2noneEquality.
e1 != e2noneInequality.
e1 && e2leftLogical AND.
e1 || e2leftLogical OR.
e1 -> e2noneLogical implication (equivalent to !e1 || e2).

15.4. Derivations

The most important built-in function is derivation, which is used to describe a single derivation (a build action). It takes as input a set, the attributes of which specify the inputs of the build.

  • There must be an attribute named system whose value must be a string specifying a Nix platform identifier, such as "i686-linux" or "powerpc-darwin"[7] The build can only be performed on a machine and operating system matching the platform identifier. (Nix can automatically forward builds for other platforms by forwarding them to other machines; see Chapter 16, Distributed Builds.)

  • There must be an attribute named name whose value must be a string. This is used as a symbolic name for the package by nix-env, and it is appended to the output paths of the derivation.

  • There must be an attribute named builder that identifies the program that is executed to perform the build. It can be either a derivation or a source (a local file reference, e.g., ./

  • Every attribute is passed as an environment variable to the builder. Attribute values are translated to environment variables as follows:

    • Strings and integers are just passed verbatim.

    • A path (e.g., ../foo/sources.tar) causes the referenced file to be copied to the store; its location in the store is put in the environment variable. The idea is that all sources should reside in the Nix store, since all inputs to a derivation should reside in the Nix store.

    • A derivation causes that derivation to be built prior to the present derivation; its default output path is put in the environment variable.

    • Lists of the previous types are also allowed. They are simply concatenated, separated by spaces.

    • true is passed as the string 1, false and null are passed as an empty string.

  • The optional attribute args specifies command-line arguments to be passed to the builder. It should be a list.

  • The optional attribute outputs specifies a list of symbolic outputs of the derivation. By default, a derivation produces a single output path, denoted as out. However, derivations can produce multiple output paths. This is useful because it allows outputs to be downloaded or garbage-collected separately. For instance, imagine a library package that provides a dynamic library, header files, and documentation. A program that links against the library doesn’t need the header files and documentation at runtime, and it doesn’t need the documentation at build time. Thus, the library package could specify:

    outputs = [ "lib" "headers" "doc" ];

    This will cause Nix to pass environment variables lib, headers and doc to the builder containing the intended store paths of each output. The builder would typically do something like

    ./configure --libdir=$lib/lib --includedir=$headers/include --docdir=$doc/share/doc

    for an Autoconf-style package. You can refer to each output of a derivation by selecting it as an attribute, e.g.

    buildInputs = [ pkg.lib pkg.headers ];

    The first element of output determines the default output. Thus, you could also write

    buildInputs = [ pkg pkg.headers ];

    since pkg is equivalent to pkg.lib.

The function mkDerivation in the Nixpkgs standard environment is a wrapper around derivation that adds a default value for system and always uses Bash as the builder, to which the supplied builder is passed as a command-line argument. See the Nixpkgs manual for details.

The builder is executed as follows:

  • A temporary directory is created under the directory specified by TMPDIR (default /tmp) where the build will take place. The current directory is changed to this directory.

  • The environment is cleared and set to the derivation attributes, as specified above.

  • In addition, the following variables are set:

    • NIX_BUILD_TOP contains the path of the temporary directory for this build.

    • Also, TMPDIR, TEMPDIR, TMP, TEMP are set to point to the temporary directory. This is to prevent the builder from accidentally writing temporary files anywhere else. Doing so might cause interference by other processes.

    • PATH is set to /path-not-set to prevent shells from initialising it to their built-in default value.

    • HOME is set to /homeless-shelter to prevent programs from using /etc/passwd or the like to find the user's home directory, which could cause impurity. Usually, when HOME is set, it is used as the location of the home directory, even if it points to a non-existent path.

    • NIX_STORE is set to the path of the top-level Nix store directory (typically, /nix/store).

    • For each output declared in outputs, the corresponding environment variable is set to point to the intended path in the Nix store for that output. Each output path is a concatenation of the cryptographic hash of all build inputs, the name attribute and the output name. (The output name is omitted if it’s out.)

  • If an output path already exists, it is removed. Also, locks are acquired to prevent multiple Nix instances from performing the same build at the same time.

  • A log of the combined standard output and error is written to /nix/var/log/nix.

  • The builder is executed with the arguments specified by the attribute args. If it exits with exit code 0, it is considered to have succeeded.

  • The temporary directory is removed (unless the -K option was specified).

  • If the build was successful, Nix scans each output path for references to input paths by looking for the hash parts of the input paths. Since these are potential runtime dependencies, Nix registers them as dependencies of the output paths.

  • After the build, Nix sets the last-modified timestamp on all files in the build result to 1 (00:00:01 1/1/1970 UTC), sets the group to the default group, and sets the mode of the file to 0444 or 0555 (i.e., read-only, with execute permission enabled if the file was originally executable). Note that possible setuid and setgid bits are cleared. Setuid and setgid programs are not currently supported by Nix. This is because the Nix archives used in deployment have no concept of ownership information, and because it makes the build result dependent on the user performing the build.

15.4.1. Advanced Attributes

Derivations can declare some infrequently used optional attributes.


The optional attribute allowedReferences specifies a list of legal references (dependencies) of the output of the builder. For example,

allowedReferences = [];

enforces that the output of a derivation cannot have any runtime dependencies on its inputs. To allow an output to have a runtime dependency on itself, use "out" as a list item. This is used in NixOS to check that generated files such as initial ramdisks for booting Linux don’t have accidental dependencies on other paths in the Nix store.


This attribute is similar to allowedReferences, but it specifies the legal requisites of the whole closure, so all the dependencies recursively. For example,

allowedRequisites = [ foobar ];

enforces that the output of a derivation cannot have any other runtime dependency than foobar, and in addition it enforces that foobar itself doesn't introduce any other dependency itself.


This attribute allows builders access to the references graph of their inputs. The attribute is a list of inputs in the Nix store whose references graph the builder needs to know. The value of this attribute should be a list of pairs [ name1 path1 name2 path2 ... ]. The references graph of each pathN will be stored in a text file nameN in the temporary build directory. The text files have the format used by nix-store --register-validity (with the deriver fields left empty). For example, when the following derivation is built:

derivation {
  exportReferencesGraph = [ "libfoo-graph" libfoo ];

the references graph of libfoo is placed in the file libfoo-graph in the temporary build directory.

exportReferencesGraph is useful for builders that want to do something with the closure of a store path. Examples include the builders in NixOS that generate the initial ramdisk for booting Linux (a cpio archive containing the closure of the boot script) and the ISO-9660 image for the installation CD (which is populated with a Nix store containing the closure of a bootable NixOS configuration).


This attribute allows you to specify a list of environment variables that should be passed from the environment of the calling user to the builder. Usually, the environment is cleared completely when the builder is executed, but with this attribute you can allow specific environment variables to be passed unmodified. For example, fetchurl in Nixpkgs has the line

impureEnvVars = [ "http_proxy" "https_proxy" ... ];

to make it use the proxy server configuration specified by the user in the environment variables http_proxy and friends.

This attribute is only allowed in fixed-output derivations, where impurities such as these are okay since (the hash of) the output is known in advance. It is ignored for all other derivations.

outputHash, outputHashAlgo, outputHashMode

These attributes declare that the derivation is a so-called fixed-output derivation, which means that a cryptographic hash of the output is already known in advance. When the build of a fixed-output derivation finishes, Nix computes the cryptographic hash of the output and compares it to the hash declared with these attributes. If there is a mismatch, the build fails.

The rationale for fixed-output derivations is derivations such as those produced by the fetchurl function. This function downloads a file from a given URL. To ensure that the downloaded file has not been modified, the caller must also specify a cryptographic hash of the file. For example,

fetchurl {
  url =;
  md5 = "70c9ccf9fac07f762c24f2df2290784d";

It sometimes happens that the URL of the file changes, e.g., because servers are reorganised or no longer available. We then must update the call to fetchurl, e.g.,

fetchurl {
  url =;
  md5 = "70c9ccf9fac07f762c24f2df2290784d";

If a fetchurl derivation was treated like a normal derivation, the output paths of the derivation and all derivations depending on it would change. For instance, if we were to change the URL of the Glibc source distribution in Nixpkgs (a package on which almost all other packages depend) massive rebuilds would be needed. This is unfortunate for a change which we know cannot have a real effect as it propagates upwards through the dependency graph.

For fixed-output derivations, on the other hand, the name of the output path only depends on the outputHash* and name attributes, while all other attributes are ignored for the purpose of computing the output path. (The name attribute is included because it is part of the path.)

As an example, here is the (simplified) Nix expression for fetchurl:

{ stdenv, curl }: # The curl program is used for downloading.

{ url, md5 }:

stdenv.mkDerivation {
  name = baseNameOf (toString url);
  builder = ./;
  buildInputs = [ curl ];

  # This is a fixed-output derivation; the output must be a regular
  # file with MD5 hash md5.
  outputHashMode = "flat";
  outputHashAlgo = "md5";
  outputHash = md5;

  inherit url;

The outputHashAlgo attribute specifies the hash algorithm used to compute the hash. It can currently be "md5", "sha1" or "sha256".

The outputHashMode attribute determines how the hash is computed. It must be one of the following two values:


The output must be a non-executable regular file. If it isn’t, the build fails. The hash is simply computed over the contents of that file (so it’s equal to what Unix commands like md5sum or sha1sum produce).

This is the default.


The hash is computed over the NAR archive dump of the output (i.e., the result of nix-store --dump). In this case, the output can be anything, including a directory tree.

The outputHash attribute, finally, must be a string containing the hash in either hexadecimal or base-32 notation. (See the nix-hash command for information about converting to and from base-32 notation.)


A list of names of attributes that should be passed via files rather than environment variables. For example, if you have

passAsFile = ["big"];
big = "a very long string";

then when the builder runs, the environment variable bigPath will contain the absolute path to a temporary file containing a very long string. That is, for any attribute x listed in passAsFile, Nix will pass an environment variable xPath holding the path of the file containing the value of attribute x. This is useful when you need to pass large strings to a builder, since most operating systems impose a limit on the size of the environment (typically, a few hundred kilobyte).


If this attribute is set to true, it has two effects. First, the derivation will always be built, not substituted, even if a substitute is available. Second, if distributed building is enabled, then, if possible, the derivaton will be built locally instead of forwarded to a remote machine. This is appropriate for trivial builders where the cost of doing a download or remote build would exceed the cost of building locally.

15.5. Built-in Functions

This section lists the functions and constants built into the Nix expression evaluator. (The built-in function derivation is discussed above.) Some built-ins, such as derivation, are always in scope of every Nix expression; you can just access them right away. But to prevent polluting the namespace too much, most built-ins are not in scope. Instead, you can access them through the builtins built-in value, which is a set that contains all built-in functions and values. For instance, derivation is also available as builtins.derivation.

abort s

Abort Nix expression evaluation, print error message s.

builtins.add e1 e2

Return the sum of the integers e1 and e2.

builtins.all pred list

Return true if the function pred returns true for all elements of list, and false otherwise.

builtins.any pred list

Return true if the function pred returns true for at least one element of list, and false otherwise.

builtins.attrNames set

Return the names of the attributes in the set set in a sorted list. For instance, builtins.attrNames { y = 1; x = "foo"; } evaluates to [ "x" "y" ].

builtins.attrValues set

Return the values of the attributes in the set set in the order corresponding to the sorted attribute names.

baseNameOf s

Return the base name of the string s, that is, everything following the final slash in the string. This is similar to the GNU basename command.


The set builtins contains all the built-in functions and values. You can use builtins to test for the availability of features in the Nix installation, e.g.,

if builtins ? getEnv then builtins.getEnv "PATH" else ""

This allows a Nix expression to fall back gracefully on older Nix installations that don’t have the desired built-in function.

builtins.compareVersions s1 s2

Compare two strings representing versions and return -1 if version s1 is older than version s2, 0 if they are the same, and 1 if s1 is newer than s2. The version comparison algorithm is the same as the one used by nix-env -u.

builtins.concatLists lists

Concatenate a list of lists into a single list.


The built-in value currentSystem evaluates to the Nix platform identifier for the Nix installation on which the expression is being evaluated, such as "i686-linux" or "powerpc-darwin".

builtins.deepSeq e1 e2

This is like seq e1 e2, except that e1 is evaluated deeply: if it’s a list or set, its elements or attributes are also evaluated recursively.

derivation attrs

derivation is described in Section 15.4, “Derivations”.

dirOf s

Return the directory part of the string s, that is, everything before the final slash in the string. This is similar to the GNU dirname command.

builtins.div e1 e2

Return the quotient of the integers e1 and e2.

builtins.elem x xs

Return true if a value equal to x occurs in the list xs, and false otherwise.

builtins.elemAt xs n

Return element n from the list xs. Elements are counted starting from 0. A fatal error occurs in the index is out of bounds.

builtins.fetchurl url

Download the specified URL and return the path of the downloaded file. This function is not available if restricted evaluation mode is enabled.

fetchTarball url

Download the specified URL, unpack it and return the path of the unpacked tree. The file must be a tape archive (.tar) compressed with gzip, bzip2 or xz. The top-level path component of the files in the tarball is removed, so it is best if the tarball contains a single directory at top level. The typical use of the function is to obtain external Nix expression dependencies, such as a particular version of Nixpkgs, e.g.

with import (fetchTarball {};

stdenv.mkDerivation { … }

This function is not available if restricted evaluation mode is enabled.

builtins.filter f xs

Return a list consisting of the elements of xs for which the function f returns true.

builtins.filterSource e1 e2

This function allows you to copy sources into the Nix store while filtering certain files. For instance, suppose that you want to use the directory source-dir as an input to a Nix expression, e.g.

stdenv.mkDerivation {
  src = ./source-dir;

However, if source-dir is a Subversion working copy, then all those annoying .svn subdirectories will also be copied to the store. Worse, the contents of those directories may change a lot, causing lots of spurious rebuilds. With filterSource you can filter out the .svn directories:

  src = builtins.filterSource
    (path: type: type != "directory" || baseNameOf path != ".svn")

Thus, the first argument e1 must be a predicate function that is called for each regular file, directory or symlink in the source tree e2. If the function returns true, the file is copied to the Nix store, otherwise it is omitted. The function is called with two arguments. The first is the full path of the file. The second is a string that identifies the type of the file, which is either "regular", "directory", "symlink" or "unknown" (for other kinds of files such as device nodes or fifos — but note that those cannot be copied to the Nix store, so if the predicate returns true for them, the copy will fail).

builtins.foldl’ op nul list

Reduce a list by applying a binary operator, from left to right, e.g. foldl’ op nul [x0 x1 x2 ...] = op (op (op nul x0) x1) x2) .... The operator is applied strictly, i.e., its arguments are evaluated first. For example, foldl’ (x: y: x + y) 0 [1 2 3] evaluates to 6.

builtins.functionArgs f

Return a set containing the names of the formal arguments expected by the function f. The value of each attribute is a Boolean denoting whether the corresponding argument has a default value. For instance, functionArgs ({ x, y ? 123}: ...) = { x = false; y = true; }.

"Formal argument" here refers to the attributes pattern-matched by the function. Plain lambdas are not included, e.g. functionArgs (x: ...) = { }.

builtins.fromJSON e

Convert a JSON string to a Nix value. For example,

builtins.fromJSON ''{"x": [1, 2, 3], "y": null}''

returns the value { x = [ 1 2 3 ]; y = null; }. Floating point numbers are not supported.

builtins.genList generator length

Generate list of size length, with each element i> equal to the value returned by generator i. For example,

builtins.genList (x: x * x) 5

returns the list [ 0 1 4 9 16 ].

builtins.getAttr s set

getAttr returns the attribute named s from set. Evaluation aborts if the attribute doesn’t exist. This is a dynamic version of the . operator, since s is an expression rather than an identifier.

builtins.getEnv s

getEnv returns the value of the environment variable s, or an empty string if the variable doesn’t exist. This function should be used with care, as it can introduce all sorts of nasty environment dependencies in your Nix expression.

getEnv is used in Nix Packages to locate the file ~/.nixpkgs/config.nix, which contains user-local settings for Nix Packages. (That is, it does a getEnv "HOME" to locate the user’s home directory.)

builtins.hasAttr s set

hasAttr returns true if set has an attribute named s, and false otherwise. This is a dynamic version of the ? operator, since s is an expression rather than an identifier.

builtins.hashString type s

Return a base-16 representation of the cryptographic hash of string s. The hash algorithm specified by type must be one of "md5", "sha1" or "sha256".

builtins.head list

Return the first element of a list; abort evaluation if the argument isn’t a list or is an empty list. You can test whether a list is empty by comparing it with [].

import path

Load, parse and return the Nix expression in the file path. If path is a directory, the file default.nix in that directory is loaded. Evaluation aborts if the file doesn’t exist or contains an incorrect Nix expression. import implements Nix’s module system: you can put any Nix expression (such as a set or a function) in a separate file, and use it from Nix expressions in other files.

A Nix expression loaded by import must not contain any free variables (identifiers that are not defined in the Nix expression itself and are not built-in). Therefore, it cannot refer to variables that are in scope at the call site. For instance, if you have a calling expression

rec {
  x = 123;
  y = import ./foo.nix;

then the following foo.nix will give an error:

x + 456

since x is not in scope in foo.nix. If you want x to be available in foo.nix, you should pass it as a function argument:

rec {
  x = 123;
  y = import ./foo.nix x;


x: x + 456

(The function argument doesn’t have to be called x in foo.nix; any name would work.)

builtins.intersectAttrs e1 e2

Return a set consisting of the attributes in the set e2 that also exist in the set e1.

builtins.isAttrs e

Return true if e evaluates to a set, and false otherwise.

builtins.isList e

Return true if e evaluates to a list, and false otherwise.

builtins.isFunction e

Return true if e evaluates to a function, and false otherwise.

builtins.isString e

Return true if e evaluates to a string, and false otherwise.

builtins.isInt e

Return true if e evaluates to an int, and false otherwise.

builtins.isBool e

Return true if e evaluates to a bool, and false otherwise.

isNull e

Return true if e evaluates to null, and false otherwise.


This function is deprecated; just write e == null instead.

builtins.length e

Return the length of the list e.

builtins.lessThan e1 e2

Return true if the integer e1 is less than the integer e2, and false otherwise. Evaluation aborts if either e1 or e2 does not evaluate to an integer.

builtins.listToAttrs e

Construct a set from a list specifying the names and values of each attribute. Each element of the list should be a set consisting of a string-valued attribute name specifying the name of the attribute, and an attribute value specifying its value. Example:

  [ { name = "foo"; value = 123; }
    { name = "bar"; value = 456; }

evaluates to

{ foo = 123; bar = 456; }

map f list

Apply the function f to each element in the list list. For example,

map (x: "foo" + x) [ "bar" "bla" "abc" ]

evaluates to [ "foobar" "foobla" "fooabc" ].

builtins.mul e1 e2

Return the product of the integers e1 and e2.

builtins.parseDrvName s

Split the string s into a package name and version. The package name is everything up to but not including the first dash followed by a digit, and the version is everything following that dash. The result is returned in a set { name, version }. Thus, builtins.parseDrvName "nix-0.12pre12876" returns { name = "nix"; version = "0.12pre12876"; }.

builtins.pathExists path

Return true if the path path exists, and false otherwise. One application of this function is to conditionally include a Nix expression containing user configuration:

  fileName = builtins.getEnv "CONFIG_FILE";
  config =
    if fileName != "" && builtins.pathExists (builtins.toPath fileName)
    then import (builtins.toPath fileName)
    else { someSetting = false; }; # default configuration
in config.someSetting

(Note that CONFIG_FILE must be an absolute path for this to work.)

builtins.readDir path

Return the contents of the directory path as a set mapping directory entries to the corresponding file type. For instance, if directory A contains a regular file B and another directory C, then builtins.readDir ./A will return the set

{ B = "regular"; C = "directory"; }

The possible values for the file type are "regular", "directory", "symlink" and "unknown".

builtins.readFile path

Return the contents of the file path as a string.

removeAttrs set list

Remove the attributes listed in list from set. The attributes don’t have to exist in set. For instance,

removeAttrs { x = 1; y = 2; z = 3; } [ "a" "x" "z" ]

evaluates to { y = 2; }.

builtins.replaceStrings from to s

Given string s, replace every occurrence of the strings in from with the corresponding string in to. For example,

builtins.replaceStrings ["oo" "a"] ["a" "i"] "foobar"

evaluates to "fabir".

builtins.seq e1 e2

Evaluate e1, then evaluate and return e2. This ensures that a computation is strict in the value of e1.

builtins.sort comparator list

Return list in sorted order. It repeatedly calls the function comparator with two elements. The comparator should return true if the first element is less than the second, and false otherwise. For example,

builtins.sort builtins.lessThan [ 483 249 526 147 42 77 ]

produces the list [ 42 77 147 249 483 526 ].

This is a stable sort: it preserves the relative order of elements deemed equal by the comparator.

builtins.stringLength e

Return the length of the string e. If e is not a string, evaluation is aborted.

builtins.sub e1 e2

Return the difference between the integers e1 and e2.

builtins.substring start len s

Return the substring of s from character position start (zero-based) up to but not including start + len. If start is greater than the length of the string, an empty string is returned, and if start + len lies beyond the end of the string, only the substring up to the end of the string is returned. start must be non-negative.

builtins.tail list

Return the second to last elements of a list; abort evaluation if the argument isn’t a list or is an empty list.

throw s

Throw an error message s. This usually aborts Nix expression evaluation, but in nix-env -qa and other commands that try to evaluate a set of derivations to get information about those derivations, a derivation that throws an error is silently skipped (which is not the case for abort).

builtins.toFile name s

Store the string s in a file in the Nix store and return its path. The file has suffix name. This file can be used as an input to derivations. One application is to write builders “inline”. For instance, the following Nix expression combines Example 14.1, “Nix expression for GNU Hello (default.nix)” and Example 14.2, “Build script for GNU Hello (” into one file:

{ stdenv, fetchurl, perl }:

stdenv.mkDerivation {
  name = "hello-2.1.1";

  builder = builtins.toFile "" "
    source $stdenv/setup


    tar xvfz $src
    cd hello-*
    ./configure --prefix=$out
    make install

  src = fetchurl {
    url =;
    md5 = "70c9ccf9fac07f762c24f2df2290784d";
  inherit perl;

It is even possible for one file to refer to another, e.g.,

  builder = let
    configFile = builtins.toFile "foo.conf" "
      # This is some dummy configuration file.
  in builtins.toFile "" "
    source $stdenv/setup
    cp ${configFile} $out/etc/foo.conf

Note that ${configFile} is an antiquotation (see Section 15.1, “Values”), so the result of the expression configFile (i.e., a path like /nix/store/m7p7jfny445k...-foo.conf) will be spliced into the resulting string.

It is however not allowed to have files mutually referring to each other, like so:

  foo = builtins.toFile "foo" "...${bar}...";
  bar = builtins.toFile "bar" "...${foo}...";
in foo

This is not allowed because it would cause a cyclic dependency in the computation of the cryptographic hashes for foo and bar.

builtins.toJSON e

Return a string containing a JSON representation of e. Strings, integers, booleans, nulls and lists are mapped to their JSON equivalents. Sets (except derivations) are represented as objects. Derivations are translated to a JSON string containing the derivation’s output path. Paths are copied to the store and represented as a JSON string of the resulting store path.

builtins.toPath s

Convert the string value s into a path value. The string s must represent an absolute path (i.e., must start with /). The path need not exist. The resulting path is canonicalised, e.g., builtins.toPath "//foo/xyzzy/../bar/" returns /foo/bar.

toString e

Convert the expression e to a string. e can be a string (in which case toString is a no-op) or a path (e.g., toString /foo/bar yields "/foo/bar".

builtins.toXML e

Return a string containing an XML representation of e. The main application for toXML is to communicate information with the builder in a more structured format than plain environment variables.

Example 15.2, “Passing information to a builder using toXML shows an example where this is the case. The builder is supposed to generate the configuration file for a Jetty servlet container. A servlet container contains a number of servlets (*.war files) each exported under a specific URI prefix. So the servlet configuration is a list of sets containing the path and war of the servlet (3). This kind of information is difficult to communicate with the normal method of passing information through an environment variable, which just concatenates everything together into a string (which might just work in this case, but wouldn’t work if fields are optional or contain lists themselves). Instead the Nix expression is converted to an XML representation with toXML, which is unambiguous and can easily be processed with the appropriate tools. For instance, in the example an XSLT stylesheet (2) is applied to it (1) to generate the XML configuration file for the Jetty server. The XML representation produced from 3 by toXML is shown in Example 15.3, “XML representation produced by toXML.

Note that Example 15.2, “Passing information to a builder using toXML uses the toFile built-in to write the builder and the stylesheet “inline” in the Nix expression. The path of the stylesheet is spliced into the builder at xsltproc ${stylesheet} ....

Example 15.2. Passing information to a builder using toXML

{ stdenv, fetchurl, libxslt, jira, uberwiki }:

stdenv.mkDerivation (rec {
  name = "web-server";

  buildInputs = [ libxslt ];

  builder = builtins.toFile "" "
    source $stdenv/setup
    mkdir $out
    echo $servlets | xsltproc ${stylesheet} - > $out/server-conf.xml 1 

  stylesheet = builtins.toFile "stylesheet.xsl" 2 
   "<?xml version='1.0' encoding='UTF-8'?>
    <xsl:stylesheet xmlns:xsl='' version='1.0'>
      <xsl:template match='/'>
          <xsl:for-each select='/expr/list/attrs'>
            <Call name='addWebApplication'>
              <Arg><xsl:value-of select=\"attr[@name = 'path']/string/@value\" /></Arg>
              <Arg><xsl:value-of select=\"attr[@name = 'war']/path/@value\" /></Arg>

  servlets = builtins.toXML [ 3 
    { path = "/bugtracker"; war = jira + "/lib/atlassian-jira.war"; }
    { path = "/wiki"; war = uberwiki + "/uberwiki.war"; }

Example 15.3. XML representation produced by toXML

<?xml version='1.0' encoding='utf-8'?>
      <attr name="path">
        <string value="/bugtracker" />
      <attr name="war">
        <path value="/nix/store/d1jh9pasa7k2...-jira/lib/atlassian-jira.war" />
      <attr name="path">
        <string value="/wiki" />
      <attr name="war">
        <path value="/nix/store/y6423b1yi4sx...-uberwiki/uberwiki.war" />

builtins.trace e1 e2

Evaluate e1 and print its abstract syntax representation on standard error. Then return e2. This function is useful for debugging.

builtins.typeOf e

Return a string representing the type of the value e, namely "int", "bool", "string", "path", "null", "set", "list" or "lambda".

[5] It's parsed as an expression that selects the attribute sh from the variable builder.

[6] Actually, Nix detects infinite recursion in this case and aborts (infinite recursion encountered).

[7] To figure out your platform identifier, look at the line Checking for the canonical Nix system name in the output of Nix's configure script.

Part V. Advanced Topics

Chapter 16. Distributed Builds

Nix supports distributed builds, where a local Nix installation can forward Nix builds to other machines over the network. This allows multiple builds to be performed in parallel (thus improving performance) and allows Nix to perform multi-platform builds in a semi-transparent way. For instance, if you perform a build for a powerpc-darwin on an i686-linux machine, Nix can automatically forward the build to a powerpc-darwin machine, if available.

You can enable distributed builds by setting the environment variable NIX_BUILD_HOOK to point to a program that Nix will call whenever it wants to build a derivation. The build hook (typically a shell or Perl script) can decline the build, in which Nix will perform it in the usual way if possible, or it can accept it, in which case it is responsible for somehow getting the inputs of the build to another machine, doing the build there, and getting the results back. The details of the build hook protocol are described in the documentation of the NIX_BUILD_HOOK variable.

Example 16.1. Remote machine configuration: remote-systems.conf  powerpc-darwin  /home/nix/.ssh/id_quarterpounder_auto  2  i686-linux      /home/nix/.ssh/id_scratchy_auto        8 1 kvm     i686-linux      /home/nix/.ssh/id_scratchy_auto        8 2   i686-linux      /home/nix/.ssh/id_scratchy_auto        8 2 kvm perf

Nix ships with a build hook that should be suitable for most purposes. It uses ssh and nix-copy-closure to copy the build inputs and outputs and perform the remote build. To use it, you should set NIX_BUILD_HOOK to prefix/libexec/nix/ You should also define a list of available build machines and point the environment variable NIX_REMOTE_SYSTEMS to it. An example configuration is shown in Example 16.1, “Remote machine configuration: remote-systems.conf. Each line in the file specifies a machine, with the following bits of information:

  1. The name of the remote machine, with optionally the user under which the remote build should be performed. This is actually passed as an argument to ssh, so it can be an alias defined in your ~/.ssh/config.

  2. A comma-separated list of Nix platform type identifiers, such as powerpc-darwin. It is possible for a machine to support multiple platform types, e.g., i686-linux,x86_64-linux.

  3. The SSH private key to be used to log in to the remote machine. Since builds should be non-interactive, this key should not have a passphrase!

  4. The maximum number of builds that will execute in parallel on the machine. Typically this should be equal to the number of CPU cores. For instance, the machine itchy in the example will execute up to 8 builds in parallel.

  5. The “speed factor”, indicating the relative speed of the machine. If there are multiple machines of the right type, Nix will prefer the fastest, taking load into account.

  6. A comma-separated list of supported features. If a derivation has the requiredSystemFeatures attribute, then will only perform the derivation on a machine that has the specified features. For instance, the attribute

    requiredSystemFeatures = [ "kvm" ];

    will cause the build to be performed on a machine that has the kvm feature (i.e., scratchy in the example above).

  7. A comma-separated list of mandatory features. A machine will only be used to build a derivation if all of the machine’s mandatory features appear in the derivation’s requiredSystemFeatures attribute. Thus, in the example, the machine poochie will only do derivations that have requiredSystemFeatures set to ["kvm" "perf"] or ["perf"].

You should also set up the environment variable NIX_CURRENT_LOAD to point at a directory (e.g., /var/run/nix/current-load) that uses to remember how many builds it is currently executing remotely. It doesn't look at the actual load on the remote machine, so if you have multiple instances of Nix running, they should use the same NIX_CURRENT_LOAD file. Maybe in the future will look at the actual remote load.

Part VI. Command Reference

This section lists commands and options that you can use when you work with Nix.

Chapter 17. Common Options

Most Nix commands accept the following command-line options:


Prints out a summary of the command syntax and exits.


Prints out the Nix version number on standard output and exits.

--verbose, -v

Increases the level of verbosity of diagnostic messages printed on standard error. For each Nix operation, the information printed on standard output is well-defined; any diagnostic information is printed on standard error, never on standard output.

This option may be specified repeatedly. Currently, the following verbosity levels exist:


“Errors only”: only print messages explaining why the Nix invocation failed.


“Informational”: print useful messages about what Nix is doing. This is the default.


“Talkative”: print more informational messages.


“Chatty”: print even more informational messages.


“Debug”: print debug information.


“Vomit”: print vast amounts of debug information.

--no-build-output, -Q

By default, output written by builders to standard output and standard error is echoed to the Nix command's standard error. This option suppresses this behaviour. Note that the builder's standard output and error are always written to a log file in prefix/nix/var/log/nix.

--max-jobs, -j

Sets the maximum number of build jobs that Nix will perform in parallel to the specified number. The default is specified by the build-max-jobs configuration setting, which itself defaults to 1. A higher value is useful on SMP systems or to exploit I/O latency.


Sets the value of the NIX_BUILD_CORES environment variable in the invocation of builders. Builders can use this variable at their discretion to control the maximum amount of parallelism. For instance, in Nixpkgs, if the derivation attribute enableParallelBuilding is set to true, the builder passes the -jN flag to GNU Make. It defaults to the value of the build-cores configuration setting, if set, or 1 otherwise. The value 0 means that the builder should use all available CPU cores in the system.


Sets the maximum number of seconds that a builder can go without producing any data on standard output or standard error. The default is specified by the build-max-silent-time configuration setting. 0 means no time-out.


Sets the maximum number of seconds that a builder can run. The default is specified by the build-timeout configuration setting. 0 means no timeout.

--keep-going, -k

Keep going in case of failed builds, to the greatest extent possible. That is, if building an input of some derivation fails, Nix will still build the other inputs, but not the derivation itself. Without this option, Nix stops if any build fails (except for builds of substitutes), possibly killing builds in progress (in case of parallel or distributed builds).

--keep-failed, -K

Specifies that in case of a build failure, the temporary directory (usually in /tmp) in which the build takes place should not be deleted. The path of the build directory is printed as an informational message.


Whenever Nix attempts to build a derivation for which substitutes are known for each output path, but realising the output paths through the substitutes fails, fall back on building the derivation.

The most common scenario in which this is useful is when we have registered substitutes in order to perform binary distribution from, say, a network repository. If the repository is down, the realisation of the derivation will fail. When this option is specified, Nix will build the derivation instead. Thus, installation from binaries falls back on installation from source. This option is not the default since it is generally not desirable for a transient failure in obtaining the substitutes to lead to a full build from source (with the related consumption of resources).


When this option is used, no attempt is made to open the Nix database. Most Nix operations do need database access, so those operations will fail.

--log-type type

This option determines how the output written to standard error is formatted. Nix’s diagnostic messages are typically nested. For instance, when tracing Nix expression evaluation (nix-env -vvvvv, messages from subexpressions are nested inside their parent expressions. Nix builder output is also often nested. For instance, the Nix Packages generic builder nests the various build tasks (unpack, configure, compile, etc.), and the GNU Make in stdenv-linux has been patched to provide nesting for recursive Make invocations.

type can be one of the following:


Pretty-print the output, indicating different nesting levels using spaces. This is the default.


Indicate nesting using escape codes that can be interpreted by the nix-log2xml tool in the Nix source distribution. The resulting XML file can be fed into the log2html.xsl stylesheet to create an HTML file that can be browsed interactively, using JavaScript to expand and collapse parts of the output.


Remove all nesting.

--arg name value

This option is accepted by nix-env, nix-instantiate and nix-build. When evaluating Nix expressions, the expression evaluator will automatically try to call functions that it encounters. It can automatically call functions for which every argument has a default value (e.g., { argName ? defaultValue }: ...). With --arg, you can also call functions that have arguments without a default value (or override a default value). That is, if the evaluator encounters a function with an argument named name, it will call it with value value.

For instance, the file pkgs/top-level/all-packages.nix in Nixpkgs is actually a function:

{ # The system (e.g., `i686-linux') for which to build the packages.
  system ? builtins.currentSystem
}: ...

So if you call this Nix expression (e.g., when you do nix-env -i pkgname), the function will be called automatically using the value builtins.currentSystem for the system argument. You can override this using --arg, e.g., nix-env -i pkgname --arg system \"i686-freebsd\". (Note that since the argument is a Nix string literal, you have to escape the quotes.)

--argstr name value

This option is like --arg, only the value is not a Nix expression but a string. So instead of --arg system \"i686-linux\" (the outer quotes are to keep the shell happy) you can say --argstr system i686-linux.

--attr / -A attrPath

Select an attribute from the top-level Nix expression being evaluated. (nix-env, nix-instantiate, nix-build and nix-shell only.) The attribute path attrPath is a sequence of attribute names separated by dots. For instance, given a top-level Nix expression e, the attribute path xorg.xorgserver would cause the expression e.xorg.xorgserver to be used. See nix-env --install for some concrete examples.

In addition to attribute names, you can also specify array indices. For instance, the attribute path selects the bar attribute of the fourth element of the array in the foo attribute of the top-level expression.

--expr / -E

Interpret the command line arguments as a list of Nix expressions to be parsed and evaluated, rather than as a list of file names of Nix expressions. (nix-instantiate, nix-build and nix-shell only.)


Causes Nix to print out a stack trace in case of Nix expression evaluation errors.

-I path

Add a path to the Nix expression search path. This option may be given multiple times. See the NIX_PATH environment variable for information on the semantics of the Nix search path. Paths added through -I take precedence over NIX_PATH.

--option name value

Set the Nix configuration option name to value. This overrides settings in the Nix configuration file (see nix.conf(5)).


Fix corrupted or missing store paths by redownloading or rebuilding them. Note that this is slow because it requires computing a cryptographic hash of the contents of every path in the closure of the build. Also note the warning under nix-store --repair-path.

Chapter 18. Common Environment Variables

Most Nix commands interpret the following environment variables:


A colon-separated list of directories used to look up Nix expressions enclosed in angle brackets (i.e., <path>). For instance, the value


will cause Nix to look for paths relative to /home/eelco/Dev and /etc/nixos, in that order. It is also possible to match paths against a prefix. For example, the value


will cause Nix to search for <nixpkgs/path> in /home/eelco/Dev/nixpkgs-branch/path and /etc/nixos/nixpkgs/path.

If a path in the Nix search path starts with http:// or https://, it is interpreted as the URL of a tarball that will be downloaded and unpacked to a temporary location. The tarball must consist of a single top-level directory. For example, setting NIX_PATH to


tells Nix to download the latest revision in the Nixpkgs/NixOS 14.12 channel.

The search path can be extended using the -I option, which takes precedence over NIX_PATH.


Normally, the Nix store directory (typically /nix/store) is not allowed to contain any symlink components. This is to prevent “impure” builds. Builders sometimes “canonicalise” paths by resolving all symlink components. Thus, builds on different machines (with /nix/store resolving to different locations) could yield different results. This is generally not a problem, except when builds are deployed to machines where /nix/store resolves differently. If you are sure that you’re not going to do that, you can set NIX_IGNORE_SYMLINK_STORE to 1.

Note that if you’re symlinking the Nix store so that you can put it on another file system than the root file system, on Linux you’re better off using bind mount points, e.g.,

$ mkdir /nix
$ mount -o bind /mnt/otherdisk/nix /nix

Consult the mount(8) manual page for details.


Overrides the location of the Nix store (default prefix/store).


Overrides the location of the Nix static data directory (default prefix/share).


Overrides the location of the Nix log directory (default prefix/log/nix).


Overrides the location of the Nix state directory (default prefix/var/nix).


Overrides the location of the Nix database (default $NIX_STATE_DIR/db, i.e., prefix/var/nix/db).


Overrides the location of the Nix configuration directory (default prefix/etc/nix).


Use the specified directory to store temporary files. In particular, this includes temporary build directories; these can take up substantial amounts of disk space. The default is /tmp.


Specifies the location of the build hook, which is a program (typically some script) that Nix will call whenever it wants to build a derivation. This is used to implement distributed builds (see Chapter 16, Distributed Builds).


This variable should be set to daemon if you want to use the Nix daemon to execute Nix operations. This is necessary in multi-user Nix installations. Otherwise, it should be left unset.


If set to 1, Nix will print some evaluation statistics, such as the number of values allocated.


If set to 1, Nix will print how often functions were called during Nix expression evaluation. This is useful for profiling your Nix expressions.


If Nix has been configured to use the Boehm garbage collector, this variable sets the initial size of the heap in bytes. It defaults to 384 MiB. Setting it to a low value reduces memory consumption, but will increase runtime due to the overhead of garbage collection.

Chapter 19. Main Commands

This section lists commands and options that you can use when you work with Nix.


nix-env — manipulate or query Nix user environments


nix-env [--help] [--version] [--verbose...] [-v...] [--no-build-output] [-Q] [ { --max-jobs | -j } number ] [ --cores number ] [ --max-silent-time number ] [ --timeout number ] [--keep-going] [-k] [--keep-failed] [-K] [--fallback] [--readonly-mode] [--log-type type] [--show-trace] [ -I path ] [ --option name value ]
[--arg name value] [--argstr name value] [ { --file | -f } path ] [ { --profile | -p } path ] [ --system-filter system ] [--dry-run] operation [options...] [arguments...]


The command nix-env is used to manipulate Nix user environments. User environments are sets of software packages available to a user at some point in time. In other words, they are a synthesised view of the programs available in the Nix store. There may be many user environments: different users can have different environments, and individual users can switch between different environments.

nix-env takes exactly one operation flag which indicates the subcommand to be performed. These are documented below.


Several commands, such as nix-env -q and nix-env -i, take a list of arguments that specify the packages on which to operate. These are extended regular expressions that must match the entire name of the package. (For details on regular expressions, see regex(7).) The match is case-sensitive. The regular expression can optionally be followed by a dash and a version number; if omitted, any version of the package will match. Here are some examples:


Matches the package name firefox and any version.


Matches the package name firefox and version 32.0.


Matches the package name gtk+. The + character must be escaped using a backslash to prevent it from being interpreted as a quantifier, and the backslash must be escaped in turn with another backslash to ensure that the shell passes it on.


Matches any package name. This is the default for most commands.


Matches any package name containing the string zip. Note the dots: '*zip*' does not work, because in a regular expression, the character * is interpreted as a quantifier.


Matches any package name containing the strings firefox or chromium.

Common options

This section lists the options that are common to all operations. These options are allowed for every subcommand, though they may not always have an effect. See also Chapter 17, Common Options.

--file, -f

Specifies the Nix expression (designated below as the active Nix expression) used by the --install, --upgrade, and --query --available operations to obtain derivations. The default is ~/.nix-defexpr.

If the argument starts with http:// or https://, it is interpreted as the URL of a tarball that will be downloaded and unpacked to a temporary location. The tarball must include a single top-level directory containing at least a file named default.nix.

--profile, -p

Specifies the profile to be used by those operations that operate on a profile (designated below as the active profile). A profile is a sequence of user environments called generations, one of which is the current generation.


For the --install, --upgrade, --uninstall, --switch-generation, --delete-generations and --rollback operations, this flag will cause nix-env to print what would be done if this flag had not been specified, without actually doing it.

--dry-run also prints out which paths will be substituted (i.e., downloaded) and which paths will be built from source (because no substitute is available).

--system-filter system

By default, operations such as --query --available show derivations matching any platform. This option allows you to use derivations for the specified platform system.



A directory that contains the default Nix expressions used by the --install, --upgrade, and --query --available operations to obtain derivations. The --file option may be used to override this default.

The Nix expressions in this directory are combined into a single set, with each file as an attribute that has the name of the file. Thus, if ~/.nix-defexpr contains two files, foo and bar, then the default Nix expression will essentially be

  foo = import ~/.nix-defexpr/foo;
  bar = import ~/.nix-defexpr/bar;

The command nix-channel places symlinks to the downloaded Nix expressions from each subscribed channel in this directory.


A symbolic link to the user's current profile. By default, this symlink points to prefix/var/nix/profiles/default. The PATH environment variable should include ~/.nix-profile/bin for the user environment to be visible to the user.

Operation --install


nix-env { --install | -i } [ { --prebuilt-only | -b } ] [ { --attr | -A } ] [--from-expression] [-E] [--from-profile path] [ --preserve-installed | -P ] [ --remove-all | -r ] args...


The install operation creates a new user environment, based on the current generation of the active profile, to which a set of store paths described by args is added. The arguments args map to store paths in a number of possible ways:

  • By default, args is a set of derivation names denoting derivations in the active Nix expression. These are realised, and the resulting output paths are installed. Currently installed derivations with a name equal to the name of a derivation being added are removed unless the option --preserve-installed is specified.

    If there are multiple derivations matching a name in args that have the same name (e.g., gcc-3.3.6 and gcc-4.1.1), then the derivation with the highest priority is used. A derivation can define a priority by declaring the meta.priority attribute. This attribute should be a number, with a higher value denoting a lower priority. The default priority is 0.

    If there are multiple matching derivations with the same priority, then the derivation with the highest version will be installed.

    You can force the installation of multiple derivations with the same name by being specific about the versions. For instance, nix-env -i gcc-3.3.6 gcc-4.1.1 will install both version of GCC (and will probably cause a user environment conflict!).

  • If --attr (-A) is specified, the arguments are attribute paths that select attributes from the top-level Nix expression. This is faster than using derivation names and unambiguous. To find out the attribute paths of available packages, use nix-env -qaP.

  • If --from-profile path is given, args is a set of names denoting installed store paths in the profile path. This is an easy way to copy user environment elements from one profile to another.

  • If --from-expression is given, args are Nix functions that are called with the active Nix expression as their single argument. The derivations returned by those function calls are installed. This allows derivations to be specified in an unambiguous way, which is necessary if there are multiple derivations with the same name.

  • If args are store derivations, then these are realised, and the resulting output paths are installed.

  • If args are store paths that are not store derivations, then these are realised and installed.

  • By default all outputs are installed for each derivation. That can be reduced by setting meta.outputsToInstall.


--prebuilt-only / -b

Use only derivations for which a substitute is registered, i.e., there is a pre-built binary available that can be downloaded in lieu of building the derivation. Thus, no packages will be built from source.

--preserve-installed, -P

Do not remove derivations with a name matching one of the derivations being installed. Usually, trying to have two versions of the same package installed in the same generation of a profile will lead to an error in building the generation, due to file name clashes between the two versions. However, this is not the case for all packages.

--remove-all, -r

Remove all previously installed packages first. This is equivalent to running nix-env -e '.*' first, except that everything happens in a single transaction.


To install a specific version of gcc from the active Nix expression:

$ nix-env --install gcc-3.3.2
installing `gcc-3.3.2'
uninstalling `gcc-3.1'

Note the previously installed version is removed, since --preserve-installed was not specified.

To install an arbitrary version:

$ nix-env --install gcc
installing `gcc-3.3.2'

To install using a specific attribute:

$ nix-env -i -A gcc40mips
$ nix-env -i -A xorg.xorgserver

To install all derivations in the Nix expression foo.nix:

$ nix-env -f ~/foo.nix -i '.*'

To copy the store path with symbolic name gcc from another profile:

$ nix-env -i --from-profile /nix/var/nix/profiles/foo -i gcc

To install a specific store derivation (typically created by nix-instantiate):

$ nix-env -i /nix/store/fibjb1bfbpm5mrsxc4mh2d8n37sxh91i-gcc-3.4.3.drv

To install a specific output path:

$ nix-env -i /nix/store/y3cgx0xj1p4iv9x0pnnmdhr8iyg741vk-gcc-3.4.3

To install from a Nix expression specified on the command-line:

$ nix-env -f ./foo.nix -i -E \
    'f: (f {system = "i686-linux";}).subversionWithJava'

I.e., this evaluates to (f: (f {system = "i686-linux";}).subversionWithJava) (import ./foo.nix), thus selecting the subversionWithJava attribute from the set returned by calling the function defined in ./foo.nix.

A dry-run tells you which paths will be downloaded or built from source:

$ nix-env -f pkgs/top-level/all-packages.nix -i f-spot --dry-run
(dry run; not doing anything)
installing `f-spot-0.0.10'
the following derivations will be built:
the following paths will be substituted:

To install Firefox from the latest revision in the Nixpkgs/NixOS 14.12 channel:

$ nix-env -f -iA firefox

(The GitHub repository nixpkgs-channels is updated automatically from the main nixpkgs repository after certain tests have succeeded and binaries have been built and uploaded to the binary cache at

Operation --upgrade


nix-env { --upgrade | -u } [ { --prebuilt-only | -b } ] [ { --attr | -A } ] [--from-expression] [-E] [--from-profile path] [ --lt | --leq | --eq | --always ] args...


The upgrade operation creates a new user environment, based on the current generation of the active profile, in which all store paths are replaced for which there are newer versions in the set of paths described by args. Paths for which there are no newer versions are left untouched; this is not an error. It is also not an error if an element of args matches no installed derivations.

For a description of how args is mapped to a set of store paths, see --install. If args describes multiple store paths with the same symbolic name, only the one with the highest version is installed.



Only upgrade a derivation to newer versions. This is the default.


In addition to upgrading to newer versions, also “upgrade” to derivations that have the same version. Version are not a unique identification of a derivation, so there may be many derivations that have the same version. This flag may be useful to force “synchronisation” between the installed and available derivations.


Only “upgrade” to derivations that have the same version. This may not seem very useful, but it actually is, e.g., when there is a new release of Nixpkgs and you want to replace installed applications with the same versions built against newer dependencies (to reduce the number of dependencies floating around on your system).


In addition to upgrading to newer versions, also “upgrade” to derivations that have the same or a lower version. I.e., derivations may actually be downgraded depending on what is available in the active Nix expression.

For the other flags, see --install.


$ nix-env --upgrade gcc
upgrading `gcc-3.3.1' to `gcc-3.4'

$ nix-env -u gcc-3.3.2 --always (switch to a specific version)
upgrading `gcc-3.4' to `gcc-3.3.2'

$ nix-env --upgrade pan
(no upgrades available, so nothing happens)

$ nix-env -u (try to upgrade everything)
upgrading `hello-2.1.2' to `hello-2.1.3'
upgrading `mozilla-1.2' to `mozilla-1.4'


The upgrade operation determines whether a derivation y is an upgrade of a derivation x by looking at their respective name attributes. The names (e.g., gcc-3.3.1 are split into two parts: the package name (gcc), and the version (3.3.1). The version part starts after the first dash not following by a letter. x is considered an upgrade of y if their package names match, and the version of y is higher that that of x.

The versions are compared by splitting them into contiguous components of numbers and letters. E.g., 3.3.1pre5 is split into [3, 3, 1, "pre", 5]. These lists are then compared lexicographically (from left to right). Corresponding components a and b are compared as follows. If they are both numbers, integer comparison is used. If a is an empty string and b is a number, a is considered less than b. The special string component pre (for pre-release) is considered to be less than other components. String components are considered less than number components. Otherwise, they are compared lexicographically (i.e., using case-sensitive string comparison).

This is illustrated by the following examples:

1.0 < 2.3
2.1 < 2.3
2.3 = 2.3
2.5 > 2.3
3.1 > 2.3
2.3.1 > 2.3
2.3.1 > 2.3a
2.3pre1 < 2.3
2.3pre3 < 2.3pre12
2.3a < 2.3c
2.3pre1 < 2.3c
2.3pre1 < 2.3q

Operation --uninstall


nix-env { --uninstall | -e } drvnames...


The uninstall operation creates a new user environment, based on the current generation of the active profile, from which the store paths designated by the symbolic names names are removed.


$ nix-env --uninstall gcc
$ nix-env -e '.*' (remove everything)

Operation --set


nix-env --set drvname


The --set operation modifies the current generation of a profile so that it contains exactly the specified derivation, and nothing else.


The following updates a profile such that its current generation will contain just Firefox:

$ nix-env -p /nix/var/nix/profiles/browser --set firefox

Operation --set-flag


nix-env --set-flag name value drvnames...


The --set-flag operation allows meta attributes of installed packages to be modified. There are several attributes that can be usefully modified, because they affect the behaviour of nix-env or the user environment build script:

  • priority can be changed to resolve filename clashes. The user environment build script uses the meta.priority attribute of derivations to resolve filename collisions between packages. Lower priority values denote a higher priority. For instance, the GCC wrapper package and the Binutils package in Nixpkgs both have a file bin/ld, so previously if you tried to install both you would get a collision. Now, on the other hand, the GCC wrapper declares a higher priority than Binutils, so the former’s bin/ld is symlinked in the user environment.

  • keep can be set to true to prevent the package from being upgraded or replaced. This is useful if you want to hang on to an older version of a package.

  • active can be set to false to “disable” the package. That is, no symlinks will be generated to the files of the package, but it remains part of the profile (so it won’t be garbage-collected). It can be set back to true to re-enable the package.


To prevent the currently installed Firefox from being upgraded:

$ nix-env --set-flag keep true firefox

After this, nix-env -u will ignore Firefox.

To disable the currently installed Firefox, then install a new Firefox while the old remains part of the profile:

$ nix-env -q
firefox- (the current one)

$ nix-env --preserve-installed -i firefox-
installing `firefox-'
building path(s) `/nix/store/myy0y59q3ig70dgq37jqwg1j0rsapzsl-user-environment'
collision between `/nix/store/...-firefox-'
  and `/nix/store/...-firefox-'.
(i.e., can’t have two active at the same time)

$ nix-env --set-flag active false firefox
setting flag on `firefox-'

$ nix-env --preserve-installed -i firefox-
installing `firefox-'

$ nix-env -q
firefox- (the enabled one)
firefox- (the disabled one)

To make files from binutils take precedence over files from gcc:

$ nix-env --set-flag priority 5 binutils
$ nix-env --set-flag priority 10 gcc

Operation --query


nix-env { --query | -q } [ --installed | --available | -a ]
[ { --status | -s } ] [ { --attr-path | -P } ] [--no-name] [ { --compare-versions | -c } ] [--system] [--drv-path] [--out-path] [--description] [--meta]
[--xml] [--json] [ { --prebuilt-only | -b } ] [ { --attr | -A } attribute-path ]


The query operation displays information about either the store paths that are installed in the current generation of the active profile (--installed), or the derivations that are available for installation in the active Nix expression (--available). It only prints information about derivations whose symbolic name matches one of names.

The derivations are sorted by their name attributes.

Source selection

The following flags specify the set of things on which the query operates.


The query operates on the store paths that are installed in the current generation of the active profile. This is the default.

--available, -a

The query operates on the derivations that are available in the active Nix expression.


The following flags specify what information to display about the selected derivations. Multiple flags may be specified, in which case the information is shown in the order given here. Note that the name of the derivation is shown unless --no-name is specified.


Print the result in an XML representation suitable for automatic processing by other tools. The root element is called items, which contains a item element for each available or installed derivation. The fields discussed below are all stored in attributes of the item elements.


Print the result in a JSON representation suitable for automatic processing by other tools.

--prebuilt-only / -b

Show only derivations for which a substitute is registered, i.e., there is a pre-built binary available that can be downloaded in lieu of building the derivation. Thus, this shows all packages that probably can be installed quickly.

--status, -s

Print the status of the derivation. The status consists of three characters. The first is I or -, indicating whether the derivation is currently installed in the current generation of the active profile. This is by definition the case for --installed, but not for --available. The second is P or -, indicating whether the derivation is present on the system. This indicates whether installation of an available derivation will require the derivation to be built. The third is S or -, indicating whether a substitute is available for the derivation.

--attr-path, -P

Print the attribute path of the derivation, which can be used to unambiguously select it using the --attr option available in commands that install derivations like nix-env --install.


Suppress printing of the name attribute of each derivation.

--compare-versions / -c

Compare installed versions to available versions, or vice versa (if --available is given). This is useful for quickly seeing whether upgrades for installed packages are available in a Nix expression. A column is added with the following meaning:

< version

A newer version of the package is available or installed.

= version

At most the same version of the package is available or installed.

> version

Only older versions of the package are available or installed.

- ?

No version of the package is available or installed.


Print the system attribute of the derivation.


Print the path of the store derivation.


Print the output path of the derivation.


Print a short (one-line) description of the derivation, if available. The description is taken from the meta.description attribute of the derivation.


Print all of the meta-attributes of the derivation. This option is only available with --xml.


To show installed packages:

$ nix-env -q

To show available packages:

$ nix-env -qa

To show the status of available packages:

$ nix-env -qas
-P- firefox-1.0.7   (not installed but present)
--S GConf-   (not present, but there is a substitute for fast installation)
--S MPlayer-1.0pre3 (i.e., this is not the installed MPlayer, even though the version is the same!)
IP- ORBit2-2.8.3    (installed and by definition present)

To show available packages in the Nix expression foo.nix:

$ nix-env -f ./foo.nix -qa

To compare installed versions to what’s available:

$ nix-env -qc
acrobat-reader-7.0 - ?      (package is not available at all)
autoconf-2.59      = 2.59   (same version)
firefox-1.0.4      < 1.0.7  (a more recent version is available)

To show all packages with “zip” in the name:

$ nix-env -qa '.*zip.*'

To show all packages with “firefox” or “chromium” in the name:

$ nix-env -qa '.*(firefox|chromium).*'

To show all packages in the latest revision of the Nixpkgs repository:

$ nix-env -f -qa

Operation --switch-profile


nix-env { --switch-profile | -S } {path}


This operation makes path the current profile for the user. That is, the symlink ~/.nix-profile is made to point to path.


$ nix-env -S ~/my-profile

Operation --list-generations


nix-env --list-generations


This operation print a list of all the currently existing generations for the active profile. These may be switched to using the --switch-generation operation. It also prints the creation date of the generation, and indicates the current generation.


$ nix-env --list-generations
  95   2004-02-06 11:48:24
  96   2004-02-06 11:49:01
  97   2004-02-06 16:22:45
  98   2004-02-06 16:24:33   (current)

Operation --delete-generations


nix-env --delete-generations generations...


This operation deletes the specified generations of the current profile. The generations can be a list of generation numbers, the special value old to delete all non-current generations, or a value such as 30d to delete all generations older than the specified number of days (except for the generation that was active at that point in time). Periodically deleting old generations is important to make garbage collection effective.


$ nix-env --delete-generations 3 4 8

$ nix-env --delete-generations 30d

$ nix-env -p other_profile --delete-generations old

Operation --switch-generation


nix-env { --switch-generation | -G } {generation}


This operation makes generation number generation the current generation of the active profile. That is, if the profile is the path to the active profile, then the symlink profile is made to point to profile-generation-link, which is in turn a symlink to the actual user environment in the Nix store.

Switching will fail if the specified generation does not exist.


$ nix-env -G 42
switching from generation 50 to 42

Operation --rollback


nix-env --rollback


This operation switches to the “previous” generation of the active profile, that is, the highest numbered generation lower than the current generation, if it exists. It is just a convenience wrapper around --list-generations and --switch-generation.


$ nix-env --rollback
switching from generation 92 to 91

$ nix-env --rollback
error: no generation older than the current (91) exists


nix-build — build a Nix expression


nix-build [--help] [--version] [--verbose...] [-v...] [--no-build-output] [-Q] [ { --max-jobs | -j } number ] [ --cores number ] [ --max-silent-time number ] [ --timeout number ] [--keep-going] [-k] [--keep-failed] [-K] [--fallback] [--readonly-mode] [--log-type type] [--show-trace] [ -I path ] [ --option name value ]
[--arg name value] [--argstr name value] [ { --attr | -A } attrPath ] [--drv-link drvlink] [--add-drv-link] [--no-out-link] [ { --out-link | -o } outlink ] paths...


The nix-build command builds the derivations described by the Nix expressions in paths. If the build succeeds, it places a symlink to the result in the current directory. The symlink is called result. If there are multiple Nix expressions, or the Nix expressions evaluate to multiple derivations, multiple sequentially numbered symlinks are created (result, result-2, and so on).

If no paths are specified, then nix-build will use default.nix in the current directory, if it exists.

If an element of paths starts with http:// or https://, it is interpreted as the URL of a tarball that will be downloaded and unpacked to a temporary location. The tarball must include a single top-level directory containing at least a file named default.nix.

nix-build is essentially a wrapper around nix-instantiate (to translate a high-level Nix expression to a low-level store derivation) and nix-store --realise (to build the store derivation).


The result of the build is automatically registered as a root of the Nix garbage collector. This root disappears automatically when the result symlink is deleted or renamed. So don’t rename the symlink.


All options not listed here are passed to nix-store --realise, except for --arg and --attr / -A which are passed to nix-instantiate. See also Chapter 17, Common Options.

--drv-link drvlink

Add a symlink named drvlink to the store derivation produced by nix-instantiate. The derivation is a root of the garbage collector until the symlink is deleted or renamed. If there are multiple derivations, numbers are suffixed to drvlink to distinguish between them.


Shorthand for --drv-link ./derivation.


Do not create a symlink to the output path. Note that as a result the output does not become a root of the garbage collector, and so might be deleted by nix-store --gc.

--out-link / -o outlink

Change the name of the symlink to the output path created from result to outlink.

The following common options are supported:


$ nix-build '<nixpkgs>' -A firefox
store derivation is /nix/store/qybprl8sz2lc...-firefox-

$ ls -l result
lrwxrwxrwx  ...  result -> /nix/store/d18hyl92g30l...-firefox-

$ ls ./result/bin/
firefox  firefox-config

If a derivation has multiple outputs, nix-build will build the default (first) output. You can also build all outputs:

$ nix-build '<nixpkgs>' -A openssl.all

This will create a symlink for each output named result-outputname. The suffix is omitted if the output name is out. So if openssl has outputs out, bin and man, nix-build will create symlinks result, result-bin and result-man. It’s also possible to build a specific output:

$ nix-build '<nixpkgs>' -A

This will create a symlink result-man.

Build a Nix expression given on the command line:

$ nix-build -E 'with import <nixpkgs> { }; runCommand "foo" { } "echo bar > $out"'
$ cat ./result

Build the GNU Hello package from the latest revision of the master branch of Nixpkgs:

$ nix-build -A hello


nix-shell — start an interactive shell based on a Nix expression


nix-shell [--arg name value] [--argstr name value] [ { --attr | -A } attrPath ] [--command cmd] [--run cmd] [--exclude regexp] [--pure] { [ --packages | -p ]packages | [path]}


The command nix-shell will build the dependencies of the specified derivation, but not the derivation itself. It will then start an interactive shell in which all environment variables defined by the derivation path have been set to their corresponding values, and the script $stdenv/setup has been sourced. This is useful for reproducing the environment of a derivation for development.

If path is not given, nix-shell defaults to shell.nix if it exists, and default.nix otherwise.

If path starts with http:// or https://, it is interpreted as the URL of a tarball that will be downloaded and unpacked to a temporary location. The tarball must include a single top-level directory containing at least a file named default.nix.

If the derivation defines the variable shellHook, it will be evaluated after $stdenv/setup has been sourced. Since this hook is not executed by regular Nix builds, it allows you to perform initialisation specific to nix-shell. For example, the derivation attribute

shellHook =
    echo "Hello shell"

will cause nix-shell to print Hello shell.


All options not listed here are passed to nix-store --realise, except for --arg and --attr / -A which are passed to nix-instantiate. See also Chapter 17, Common Options.

--command cmd

In the environment of the derivation, run the shell command cmd. This command is executed in an interactive shell. (Use --run to use a non-interactive shell instead.) However, a call to exit is implicitly added to the command, so the shell will exit after running the command. To prevent this, add return at the end; e.g. --command "echo Hello; return" will print Hello and then drop you into the interactive shell. This can be useful for doing any additional initialisation.

--run cmd

Like --command, but executes the command in a non-interactive shell. This means (among other things) that if you hit Ctrl-C while the command is running, the shell exits.

--exclude regexp

Do not build any dependencies whose store path matches the regular expression regexp. This option may be specified multiple times.


If this flag is specified, the environment is almost entirely cleared before the interactive shell is started, so you get an environment that more closely corresponds to the “real” Nix build. A few variables, in particular HOME, USER and DISPLAY, are retained. Note that ~/.bashrc and (depending on your Bash installation) /etc/bashrc are still sourced, so any variables set there will affect the interactive shell.

--packages / -p

Set up an environment in which the specified packages are present. The command line arguments are interpreted as attribute names inside the Nix Packages collection. Thus, nix-shell -p libjpeg openjdk will start a shell in which the packages denoted by the attribute names libjpeg and openjdk are present.

-i interpreter

The chained script interpreter to be invoked by nix-shell. Only applicable in #!-scripts (described below).

The following common options are supported:

Environment variables


Shell used to start the interactive environment. Defaults to the bash found in PATH.


To build the dependencies of the package Pan, and start an interactive shell in which to build it:

$ nix-shell '<nixpkgs>' -A pan
[nix-shell]$ unpackPhase
[nix-shell]$ cd pan-*
[nix-shell]$ configurePhase
[nix-shell]$ buildPhase
[nix-shell]$ ./pan/gui/pan

To clear the environment first, and do some additional automatic initialisation of the interactive shell:

$ nix-shell '<nixpkgs>' -A pan --pure \
    --command 'export NIX_DEBUG=1; export NIX_CORES=8; return'

Nix expressions can also be given on the command line. For instance, the following starts a shell containing the packages sqlite and libX11:

$ nix-shell -E 'with import <nixpkgs> { }; runCommand "dummy" { buildInputs = [ sqlite xorg.libX11 ]; } ""'

A shorter way to do the same is:

$ nix-shell -p sqlite xorg.libX11
[nix-shell]$ echo $NIX_LDFLAGS
… -L/nix/store/j1zg5v…-sqlite- -L/nix/store/0gmcz9…-libX11-1.6.1/lib …

The -p flag looks up Nixpkgs in the Nix search path. You can override it by passing -I or setting NIX_PATH. For example, the following gives you a shell containing the Pan package from a specific revision of Nixpkgs:

$ nix-shell -p pan -I nixpkgs=

[nix-shell:~]$ pan --version
Pan 0.139

Use as a #!-interpreter

You can use nix-shell as a script interpreter to allow scripts written in arbitrary languages to obtain their own dependencies via Nix. This is done by starting the script with the following lines:

#! /usr/bin/env nix-shell
#! nix-shell -i real-interpreter -p packages

where real-interpreter is the “real” script interpreter that will be invoked by nix-shell after it has obtained the dependencies and initialised the environment, and packages are the attribute names of the dependencies in Nixpkgs.

The lines starting with #! nix-shell specify nix-shell options (see above). Note that you cannot write #1 /usr/bin/env nix-shell -i ... because many operating systems only allow one argument in #! lines.

For example, here is a Python script that depends on Python and the prettytable package:

#! /usr/bin/env nix-shell
#! nix-shell -i python -p python pythonPackages.prettytable

import prettytable

# Print a simple table.
t = prettytable.PrettyTable(["N", "N^2"])
for n in range(1, 10): t.add_row([n, n * n])
print t

Similarly, the following is a Perl script that specifies that it requires Perl and the HTML::TokeParser::Simple and LWP packages:

#! /usr/bin/env nix-shell
#! nix-shell -i perl -p perl perlPackages.HTMLTokeParserSimple perlPackages.LWP

use HTML::TokeParser::Simple;

# Fetch and print all hrefs.
my $p = HTML::TokeParser::Simple->new(url => '');

while (my $token = $p->get_tag("a")) {
    my $href = $token->get_attr("href");
    print "$href\n" if $href;

Finally, the following Haskell script uses a specific branch of Nixpkgs/NixOS (the 14.12 stable branch):

#! /usr/bin/env nix-shell
#! nix-shell -i runghc -p haskellPackages.ghc haskellPackages.HTTP haskellPackages.tagsoup
#! nix-shell -I nixpkgs=

import Network.HTTP
import Text.HTML.TagSoup

-- Fetch and print all hrefs.
main = do
  resp <- Network.HTTP.simpleHTTP (getRequest "")
  body <- getResponseBody resp
  let tags = filter (isTagOpenName "a") $ parseTags body
  let tags' = map (fromAttrib "href") tags
  mapM_ putStrLn $ filter (/= "") tags'

If you want to be even more precise, you can specify a specific revision of Nixpkgs:

#! nix-shell -I nixpkgs=

The examples above all used -p to get dependencies from Nixpkgs. You can also use a Nix expression to build your own dependencies. For example, the Python example could have been written as:

#! /usr/bin/env nix-shell
#! nix-shell deps.nix -i python

where the file deps.nix in the same directory as the #!-script contains:

with import <nixpkgs> {};

runCommand "dummy" { buildInputs = [ python pythonPackages.prettytable ]; } ""


nix-store — manipulate or query the Nix store


nix-store [--help] [--version] [--verbose...] [-v...] [--no-build-output] [-Q] [ { --max-jobs | -j } number ] [ --cores number ] [ --max-silent-time number ] [ --timeout number ] [--keep-going] [-k] [--keep-failed] [-K] [--fallback] [--readonly-mode] [--log-type type] [--show-trace] [ -I path ] [ --option name value ]
[--add-root path] [--indirect] operation [options...] [arguments...]


The command nix-store performs primitive operations on the Nix store. You generally do not need to run this command manually.

nix-store takes exactly one operation flag which indicates the subcommand to be performed. These are documented below.

Common options

This section lists the options that are common to all operations. These options are allowed for every subcommand, though they may not always have an effect. See also Chapter 17, Common Options for a list of common options.

--add-root path

Causes the result of a realisation (--realise and --force-realise) to be registered as a root of the garbage collector (see Section 10.1, “Garbage Collector Roots”). The root is stored in path, which must be inside a directory that is scanned for roots by the garbage collector (i.e., typically in a subdirectory of /nix/var/nix/gcroots/) unless the --indirect flag is used.

If there are multiple results, then multiple symlinks will be created by sequentially numbering symlinks beyond the first one (e.g., foo, foo-2, foo-3, and so on).


In conjunction with --add-root, this option allows roots to be stored outside of the GC roots directory. This is useful for commands such as nix-build that place a symlink to the build result in the current directory; such a build result should not be garbage-collected unless the symlink is removed.

The --indirect flag causes a uniquely named symlink to path to be stored in /nix/var/nix/gcroots/auto/. For instance,

$ nix-store --add-root /home/eelco/bla/result --indirect -r ...

$ ls -l /nix/var/nix/gcroots/auto
lrwxrwxrwx    1 ... 2005-03-13 21:10 dn54lcypm8f8... -> /home/eelco/bla/result

$ ls -l /home/eelco/bla/result
lrwxrwxrwx    1 ... 2005-03-13 21:10 /home/eelco/bla/result -> /nix/store/1r11343n6qd4...-f-spot-0.0.10

Thus, when /home/eelco/bla/result is removed, the GC root in the auto directory becomes a dangling symlink and will be ignored by the collector.


Note that it is not possible to move or rename indirect GC roots, since the symlink in the auto directory will still point to the old location.

Operation --realise


nix-store { --realise | -r } paths... [--dry-run]


The operation --realise essentially “builds” the specified store paths. Realisation is a somewhat overloaded term:

  • If the store path is a derivation, realisation ensures that the output paths of the derivation are valid (i.e., the output path and its closure exist in the file system). This can be done in several ways. First, it is possible that the outputs are already valid, in which case we are done immediately. Otherwise, there may be substitutes that produce the outputs (e.g., by downloading them). Finally, the outputs can be produced by performing the build action described by the derivation.

  • If the store path is not a derivation, realisation ensures that the specified path is valid (i.e., it and its closure exist in the file system). If the path is already valid, we are done immediately. Otherwise, the path and any missing paths in its closure may be produced through substitutes. If there are no (successful) subsitutes, realisation fails.

The output path of each derivation is printed on standard output. (For non-derivations argument, the argument itself is printed.)

The following flags are available:


Print on standard error a description of what packages would be built or downloaded, without actually performing the operation.


If a non-derivation path does not have a substitute, then silently ignore it.


This option allows you to check whether a derivation is deterministic. It rebuilds the specified derivation and checks whether the result is bitwise-identical with the existing outputs, printing an error if that’s not the case. The outputs of the specified derivation must already exist. When used with -K, if an output path is not identical to the corresponding output from the previous build, the new output path is left in /nix/store/name-check.

See also the build-repeat configuration option, which repeats a derivation a number of times and prevents its outputs from being registered as “valid” in the Nix store unless they are identical.


This operation is typically used to build store derivations produced by nix-instantiate:

$ nix-store -r $(nix-instantiate ./test.nix)

This is essentially what nix-build does.

To test whether a previously-built derivation is deterministic:

$ nix-build -r '<nixpkgs>' -A hello --check -K

Operation --gc


nix-store --gc [ --print-roots | --print-live | --print-dead | --delete ] [--max-freed bytes]


Without additional flags, the operation --gc performs a garbage collection on the Nix store. That is, all paths in the Nix store not reachable via file system references from a set of “roots”, are deleted.

The following suboperations may be specified:


This operation prints on standard output the set of roots used by the garbage collector. What constitutes a root is described in Section 10.1, “Garbage Collector Roots”.


This operation prints on standard output the set of “live” store paths, which are all the store paths reachable from the roots. Live paths should never be deleted, since that would break consistency — it would become possible that applications are installed that reference things that are no longer present in the store.


This operation prints out on standard output the set of “dead” store paths, which is just the opposite of the set of live paths: any path in the store that is not live (with respect to the roots) is dead.


This operation performs an actual garbage collection. All dead paths are removed from the store. This is the default.

By default, all unreachable paths are deleted. The following options control what gets deleted and in what order:

--max-freed bytes

Keep deleting paths until at least bytes bytes have been deleted, then stop. The argument bytes can be followed by the multiplicative suffix K, M, G or T, denoting KiB, MiB, GiB or TiB units.

The behaviour of the collector is also influenced by the gc-keep-outputs and gc-keep-derivations variables in the Nix configuration file.

With --delete, the collector prints the total number of freed bytes when it finishes (or when it is interrupted). With --print-dead, it prints the number of bytes that would be freed.


To delete all unreachable paths, just do:

$ nix-store --gc
deleting `/nix/store/kq82idx6g0nyzsp2s14gfsc38npai7lf-cairo-1.0.4.tar.gz.drv'
8825586 bytes freed (8.42 MiB)

To delete at least 100 MiBs of unreachable paths:

$ nix-store --gc --max-freed $((100 * 1024 * 1024))

Operation --delete


nix-store --delete [--ignore-liveness] paths...


The operation --delete deletes the store paths paths from the Nix store, but only if it is safe to do so; that is, when the path is not reachable from a root of the garbage collector. This means that you can only delete paths that would also be deleted by nix-store --gc. Thus, --delete is a more targeted version of --gc.

With the option --ignore-liveness, reachability from the roots is ignored. However, the path still won’t be deleted if there are other paths in the store that refer to it (i.e., depend on it).


$ nix-store --delete /nix/store/zq0h41l75vlb4z45kzgjjmsjxvcv1qk7-mesa-6.4
0 bytes freed (0.00 MiB)
error: cannot delete path `/nix/store/zq0h41l75vlb4z45kzgjjmsjxvcv1qk7-mesa-6.4' since it is still alive

Operation --query


nix-store { --query | -q } { --outputs | --requisites | -R | --references | --referrers | --referrers-closure | --deriver | --deriver | --graph | --tree | --binding name | --hash | --size | --roots } [--use-output] [-u] [--force-realise] [-f] paths...


The operation --query displays various bits of information about the store paths . The queries are described below. At most one query can be specified. The default query is --outputs.

The paths paths may also be symlinks from outside of the Nix store, to the Nix store. In that case, the query is applied to the target of the symlink.

Common query options

--use-output, -u

For each argument to the query that is a store derivation, apply the query to the output path of the derivation instead.

--force-realise, -f

Realise each argument to the query first (see nix-store --realise).



Prints out the output paths of the store derivations paths. These are the paths that will be produced when the derivation is built.

--requisites, -R

Prints out the closure of the store path paths.

This query has one option:


Also include the output path of store derivations, and their closures.

This query can be used to implement various kinds of deployment. A source deployment is obtained by distributing the closure of a store derivation. A binary deployment is obtained by distributing the closure of an output path. A cache deployment (combined source/binary deployment, including binaries of build-time-only dependencies) is obtained by distributing the closure of a store derivation and specifying the option --include-outputs.


Prints the set of references of the store paths paths, that is, their immediate dependencies. (For all dependencies, use --requisites.)


Prints the set of referrers of the store paths paths, that is, the store paths currently existing in the Nix store that refer to one of paths. Note that contrary to the references, the set of referrers is not constant; it can change as store paths are added or removed.


Prints the closure of the set of store paths paths under the referrers relation; that is, all store paths that directly or indirectly refer to one of paths. These are all the path currently in the Nix store that are dependent on paths.


Prints the deriver of the store paths paths. If the path has no deriver (e.g., if it is a source file), or if the deriver is not known (e.g., in the case of a binary-only deployment), the string unknown-deriver is printed.


Prints the references graph of the store paths paths in the format of the dot tool of AT&T's Graphviz package. This can be used to visualise dependency graphs. To obtain a build-time dependency graph, apply this to a store derivation. To obtain a runtime dependency graph, apply it to an output path.


Prints the references graph of the store paths paths as a nested ASCII tree. References are ordered by descending closure size; this tends to flatten the tree, making it more readable. The query only recurses into a store path when it is first encountered; this prevents a blowup of the tree representation of the graph.

--binding name

Prints the value of the attribute name (i.e., environment variable) of the store derivations paths. It is an error for a derivation to not have the specified attribute.


Prints the SHA-256 hash of the contents of the store paths paths (that is, the hash of the output of nix-store --dump on the given paths). Since the hash is stored in the Nix database, this is a fast operation.


Prints the size in bytes of the contents of the store paths paths — to be precise, the size of the output of nix-store --dump on the given paths. Note that the actual disk space required by the store paths may be higher, especially on filesystems with large cluster sizes.


Prints the garbage collector roots that point, directly or indirectly, at the store paths paths.


Print the closure (runtime dependencies) of the svn program in the current user environment:

$ nix-store -qR $(which svn)

Print the build-time dependencies of svn:

$ nix-store -qR $(nix-store -qd $(which svn))
... lots of other paths ...

The difference with the previous example is that we ask the closure of the derivation (-qd), not the closure of the output path that contains svn.

Show the build-time dependencies as a tree:

$ nix-store -q --tree $(nix-store -qd $(which svn))
|   +---/nix/store/570hmhmx3v57605cqg9yfvvyh0nnb8k8-bash
|   +---/nix/store/

Show all paths that depend on the same OpenSSL library as svn:

$ nix-store -q --referrers $(nix-store -q --binding openssl $(nix-store -qd $(which svn)))

Show all paths that directly or indirectly depend on the Glibc (C library) used by svn:

$ nix-store -q --referrers-closure $(ldd $(which svn) | grep / | awk '{print $3}')

Note that ldd is a command that prints out the dynamic libraries used by an ELF executable.

Make a picture of the runtime dependency graph of the current user environment:

$ nix-store -q --graph ~/.nix-profile | dot -Tps >
$ gv

Show every garbage collector root that points to a store path that depends on svn:

$ nix-store -q --roots $(which svn)

Operation --add


nix-store --add paths...


The operation --add adds the specified paths to the Nix store. It prints the resulting paths in the Nix store on standard output.


$ nix-store --add ./foo.c

Operation --verify


nix-store --verify [--check-contents] [--repair]


The operation --verify verifies the internal consistency of the Nix database, and the consistency between the Nix database and the Nix store. Any inconsistencies encountered are automatically repaired. Inconsistencies are generally the result of the Nix store or database being modified by non-Nix tools, or of bugs in Nix itself.

This operation has the following options:


Checks that the contents of every valid store path has not been altered by computing a SHA-256 hash of the contents and comparing it with the hash stored in the Nix database at build time. Paths that have been modified are printed out. For large stores, --check-contents is obviously quite slow.


If any valid path is missing from the store, or (if --check-contents is given) the contents of a valid path has been modified, then try to repair the path by redownloading it. See nix-store --repair-path for details.

Operation --verify-path


nix-store --verify-path paths...


The operation --verify-path compares the contents of the given store paths to their cryptographic hashes stored in Nix’s database. For every changed path, it prints a warning message. The exit status is 0 if no path has changed, and 1 otherwise.


To verify the integrity of the svn command and all its dependencies:

$ nix-store --verify-path $(nix-store -qR $(which svn))

Operation --repair-path


nix-store --repair-path paths...


The operation --repair-path attempts to “repair” the specified paths by redownloading them using the available substituters. If no substitutes are available, then repair is not possible.


During repair, there is a very small time window during which the old path (if it exists) is moved out of the way and replaced with the new path. If repair is interrupted in between, then the system may be left in a broken state (e.g., if the path contains a critical system component like the GNU C Library).


$ nix-store --verify-path /nix/store/dj7a81wsm1ijwwpkks3725661h3263p5-glibc-2.13
path `/nix/store/dj7a81wsm1ijwwpkks3725661h3263p5-glibc-2.13' was modified!
  expected hash `2db57715ae90b7e31ff1f2ecb8c12ec1cc43da920efcbe3b22763f36a1861588',
  got `481c5aa5483ebc97c20457bb8bca24deea56550d3985cda0027f67fe54b808e4'

$ nix-store --repair-path /nix/store/dj7a81wsm1ijwwpkks3725661h3263p5-glibc-2.13
fetching path `/nix/store/d7a81wsm1ijwwpkks3725661h3263p5-glibc-2.13'...

Operation --dump


nix-store --dump path


The operation --dump produces a NAR (Nix ARchive) file containing the contents of the file system tree rooted at path. The archive is written to standard output.

A NAR archive is like a TAR or Zip archive, but it contains only the information that Nix considers important. For instance, timestamps are elided because all files in the Nix store have their timestamp set to 0 anyway. Likewise, all permissions are left out except for the execute bit, because all files in the Nix store have 644 or 755 permission.

Also, a NAR archive is canonical, meaning that “equal” paths always produce the same NAR archive. For instance, directory entries are always sorted so that the actual on-disk order doesn’t influence the result. This means that the cryptographic hash of a NAR dump of a path is usable as a fingerprint of the contents of the path. Indeed, the hashes of store paths stored in Nix’s database (see nix-store -q --hash) are SHA-256 hashes of the NAR dump of each store path.

NAR archives support filenames of unlimited length and 64-bit file sizes. They can contain regular files, directories, and symbolic links, but not other types of files (such as device nodes).

A Nix archive can be unpacked using nix-store --restore.

Operation --restore


nix-store --restore path


The operation --restore unpacks a NAR archive to path, which must not already exist. The archive is read from standard input.

Operation --export


nix-store --export paths...


The operation --export writes a serialisation of the specified store paths to standard output in a format that can be imported into another Nix store with nix-store --import. This is like nix-store --dump, except that the NAR archive produced by that command doesn’t contain the necessary meta-information to allow it to be imported into another Nix store (namely, the set of references of the path).

This command does not produce a closure of the specified paths, so if a store path references other store paths that are missing in the target Nix store, the import will fail. To copy a whole closure, do something like

$ nix-store --export $(nix-store -qR paths) > out

For an example of how --export and --import can be used, see the source of the nix-copy-closure command.

Operation --import


nix-store --import


The operation --import reads a serialisation of a set of store paths produced by nix-store --export from standard input and adds those store paths to the Nix store. Paths that already exist in the Nix store are ignored. If a path refers to another path that doesn’t exist in the Nix store, the import fails.

Operation --optimise


nix-store --optimise


The operation --optimise reduces Nix store disk space usage by finding identical files in the store and hard-linking them to each other. It typically reduces the size of the store by something like 25-35%. Only regular files and symlinks are hard-linked in this manner. Files are considered identical when they have the same NAR archive serialisation: that is, regular files must have the same contents and permission (executable or non-executable), and symlinks must have the same contents.

After completion, or when the command is interrupted, a report on the achieved savings is printed on standard error.

Use -vv or -vvv to get some progress indication.


$ nix-store --optimise
hashing files in `/nix/store/qhqx7l2f1kmwihc9bnxs7rc159hsxnf3-gcc-4.1.1'
541838819 bytes (516.74 MiB) freed by hard-linking 54143 files;
there are 114486 files with equal contents out of 215894 files in total

Operation --read-log


nix-store { --read-log | -l } paths...


The operation --read-log prints the build log of the specified store paths on standard output. The build log is whatever the builder of a derivation wrote to standard output and standard error. If a store path is not a derivation, the deriver of the store path is used.

Build logs are kept in /nix/var/log/nix/drvs. However, there is no guarantee that a build log is available for any particular store path. For instance, if the path was downloaded as a pre-built binary through a substitute, then the log is unavailable. If the log is not available locally, then nix-store will try to download the log from the servers specified in the Nix option log-servers. For example, if it’s set to, then Nix will check


$ nix-store -l $(which ktorrent)
building /nix/store/dhc73pvzpnzxhdgpimsd9sw39di66ph1-ktorrent-2.2.1
unpacking sources
unpacking source archive /nix/store/p8n1jpqs27mgkjw07pb5269717nzf5f8-ktorrent-2.2.1.tar.gz

Operation --dump-db


nix-store --dump-db


The operation --dump-db writes a dump of the Nix database to standard output. It can be loaded into an empty Nix store using --load-db. This is useful for making backups and when migrating to different database schemas.

Operation --load-db


nix-store --load-db


The operation --load-db reads a dump of the Nix database created by --dump-db from standard input and loads it into the Nix database.

Operation --print-env


nix-store --print-env drvpath


The operation --print-env prints out the environment of a derivation in a format that can be evaluated by a shell. The command line arguments of the builder are placed in the variable _args.


$ nix-store --print-env $(nix-instantiate '<nixpkgs>' -A firefox)

export src; src='/nix/store/plpj7qrwcz94z2psh6fchsi7s8yihc7k-firefox-12.0.source.tar.bz2'
export stdenv; stdenv='/nix/store/7c8asx3yfrg5dg1gzhzyq2236zfgibnn-stdenv'
export system; system='x86_64-linux'
export _args; _args='-e /nix/store/'

Operation --query-failed-paths


nix-store --query-failed-paths


If build failure caching is enabled through the build-cache-failure configuration option, the operation --query-failed-paths will print out all store paths that have failed to build.


$ nix-store --query-failed-paths

Operation --clear-failed-paths


nix-store --clear-failed-paths paths...


If build failure caching is enabled through the build-cache-failure configuration option, the operation --clear-failed-paths clears the “failed” state of the given store paths, allowing them to be built again. This is useful if the failure was actually transient (e.g. because the disk was full).

If a path denotes a derivation, its output paths are cleared. You can provide the argument * to clear all store paths.


$ nix-store --clear-failed-paths /nix/store/000zi5dcla86l92jn1g997jb06sidm7x-perl-PerlMagick-6.59
$ nix-store --clear-failed-paths *

Operation --generate-binary-cache-key


nix-store --generate-binary-cache-key key-name secret-key-file public-key-file


This command generates an Ed25519 key pair that can be used to create a signed binary cache. It takes three mandatory parameters:

  1. A key name, such as, that is used to look up keys on the client when it verifies signatures. It can be anything, but it’s suggested to use the host name of your cache (e.g. with a suffix denoting the number of the key (to be incremented every time you need to revoke a key).

  2. The file name where the secret key is to be stored.

  3. The file name where the public key is to be stored.

For an example, see the manual page for nix-push.

Chapter 20. Utilities

This section lists utilities that you can use when you work with Nix.


nix-channel — manage Nix channels


nix-channel { --add url [name] | --remove name | --list | --update [names...] | --rollback [generation] }


A Nix channel is mechanism that allows you to automatically stay up-to-date with a set of pre-built Nix expressions. A Nix channel is just a URL that points to a place containing both a set of Nix expressions and a pointer to a binary cache. See also Chapter 11, Channels.

This command has the following operations:

--add url [name]

Adds a channel named name with URL url to the list of subscribed channels. If name is omitted, it defaults to the last component of url, with the suffixes -stable or -unstable removed.

--remove name

Removes the channel named name from the list of subscribed channels.


Prints the names and URLs of all subscribed channels on standard output.

--update [names…]

Downloads the Nix expressions of all subscribed channels (or only those included in names if specified), makes them the default for nix-env operations (by symlinking them from the directory ~/.nix-defexpr), and performs a nix-pull on the manifests of all channels to make pre-built binaries available.

--rollback [generation]

Reverts the previous call to nix-channel --update. Optionally, you can specify a specific channel generation number to restore.

Note that --add does not automatically perform an update.

The list of subscribed channels is stored in ~/.nix-channels.


To subscribe to the Nixpkgs channel and install the GNU Hello package:

$ nix-channel --add
$ nix-channel --update
$ nix-env -iA nixpkgs.hello

You can revert channel updates using --rollback:

$ nix-instantiate --eval -E '(import <nixpkgs> {}).lib.nixpkgsVersion'

$ nix-channel --rollback
switching from generation 483 to 482

$ nix-instantiate --eval -E '(import <nixpkgs> {}).lib.nixpkgsVersion'



nix-channel uses a nix-env profile to keep track of previous versions of the subscribed channels. Every time you run nix-channel --update, a new channel generation (that is, a symlink to the channel Nix expressions in the Nix store) is created. This enables nix-channel --rollback to revert to previous versions.


This is a symlink to /nix/var/nix/profiles/per-user/username/channels. It ensures that nix-env can find your channels. In a multi-user installation, you may also have ~/.nix-defexpr/channels_root, which links to the channels of the root user.

Channel format

A channel URL should point to a directory containing the following files:


A tarball containing Nix expressions and files referenced by them (such as build scripts and patches). At top-level, the tarball should contain a single directory. That directory must contain a file default.nix that serves as the channel’s “entry point”.


A file containing the URL to a binary cache (such as Nix will automatically check this cache for pre-built binaries, if the user has sufficient rights to add binary caches. For instance, in a multi-user Nix setup, the binary caches provided by the channels of the root user are used automatically, but caches corresponding to the channels of non-root users are ignored. Binary caches can be created and maintained using nix-push.


(Deprecated in favour of binary caches.) A manifest as created by nix-push. Only used if binary-cache-url is not present or if the nix.conf option force-manifest is set.


nix-collect-garbage — delete unreachable store paths


nix-collect-garbage [--delete-old] [-d] [--delete-older-than period] [ --print-roots | --print-live | --print-dead | --delete ] [--max-freed bytes] [--dry-run]


The command nix-collect-garbage is mostly an alias of nix-store --gc, that is, it deletes all unreachable paths in the Nix store to clean up your system. However, it provides two additional options: -d (--delete-old), which deletes all old generations of all profiles in /nix/var/nix/profiles by invoking nix-env --delete-generations old on all profiles (of course, this makes rollbacks to previous configurations impossible); and --delete-older-than period, where period is a value such as 30d, which deletes all generations older than the specified number of days in all profiles in /nix/var/nix/profiles (except for the generations that were active at that point in time).


To delete from the Nix store everything that is not used by the current generations of each profile, do

$ nix-collect-garbage -d


nix-copy-closure — copy a closure to or from a remote machine via SSH


nix-copy-closure [ --to | --from ] [--sign] [--gzip] [--include-outputs] [--use-substitutes] [-s] [-v] user@machine paths


nix-copy-closure gives you an easy and efficient way to exchange software between machines. Given one or more Nix store paths on the local machine, nix-copy-closure computes the closure of those paths (i.e. all their dependencies in the Nix store), and copies all paths in the closure to the remote machine via the ssh (Secure Shell) command. With the --from, the direction is reversed: the closure of paths on a remote machine is copied to the Nix store on the local machine.

This command is efficient because it only sends the store paths that are missing on the target machine.

Since nix-copy-closure calls ssh, you may be asked to type in the appropriate password or passphrase. In fact, you may be asked twice because nix-copy-closure currently connects twice to the remote machine, first to get the set of paths missing on the target machine, and second to send the dump of those paths. If this bothers you, use ssh-agent.



Copy the closure of paths from the local Nix store to the Nix store on machine. This is the default.


Copy the closure of paths from the Nix store on machine to the local Nix store.


Let the sending machine cryptographically sign the dump of each path with the key in sysconfdir/nix/signing-key.sec. If the user on the target machine does not have direct access to the Nix store (i.e., if the target machine has a multi-user Nix installation), then the target machine will check the dump against sysconfdir/nix/ before unpacking it in its Nix store. This allows secure sharing of store paths between untrusted users on two machines, provided that there is a trust relation between the Nix installations on both machines (namely, they have matching public/secret keys).


Enable compression of the SSH connection.


Also copy the outputs of store derivations included in the closure.

--use-substitutes / -s

Attempt to download missing paths on the target machine using Nix’s substitute mechanism. Any paths that cannot be substituted on the target are still copied normally from the source. This is useful, for instance, if the connection between the source and target machine is slow, but the connection between the target machine and (the default binary cache server) is fast.


Show verbose output.

Environment variables


Additional options to be passed to ssh on the command line.


Copy Firefox with all its dependencies to a remote machine:

$ nix-copy-closure --to alice@itchy.labs $(type -tP firefox)

Copy Subversion from a remote machine and then install it into a user environment:

$ nix-copy-closure --from alice@itchy.labs \
$ nix-env -i /nix/store/0dj0503hjxy5mbwlafv1rsbdiyx1gkdy-subversion-1.4.4


nix-daemon — Nix multi-user support daemon




The Nix daemon is necessary in multi-user Nix installations. It performs build actions and other operations on the Nix store on behalf of unprivileged users.


nix-hash — compute the cryptographic hash of a path


nix-hash [--flat] [--base32] [--truncate] [--type hashAlgo] path...

nix-hash --to-base16 hash...

nix-hash --to-base32 hash...


The command nix-hash computes the cryptographic hash of the contents of each path and prints it on standard output. By default, it computes an MD5 hash, but other hash algorithms are available as well. The hash is printed in hexadecimal.

The hash is computed over a serialisation of each path: a dump of the file system tree rooted at the path. This allows directories and symlinks to be hashed as well as regular files. The dump is in the NAR format produced by nix-store --dump. Thus, nix-hash path yields the same cryptographic hash as nix-store --dump path | md5sum.



Print the cryptographic hash of the contents of each regular file path. That is, do not compute the hash over the dump of path. The result is identical to that produced by the GNU commands md5sum and sha1sum.


Print the hash in a base-32 representation rather than hexadecimal. This base-32 representation is more compact and can be used in Nix expressions (such as in calls to fetchurl).


Truncate hashes longer than 160 bits (such as SHA-256) to 160 bits.

--type hashAlgo

Use the specified cryptographic hash algorithm, which can be one of md5, sha1, and sha256.


Don’t hash anything, but convert the base-32 hash representation hash to hexadecimal.


Don’t hash anything, but convert the hexadecimal hash representation hash to base-32.


Computing hashes:

$ mkdir test
$ echo "hello" > test/world

$ nix-hash test/ (MD5 hash; default)

$ nix-store --dump test/ | md5sum (for comparison)
8179d3caeff1869b5ba1744e5a245c04  -

$ nix-hash --type sha1 test/

$ nix-hash --type sha1 --base32 test/

$ nix-hash --type sha256 --flat test/
error: reading file `test/': Is a directory

$ nix-hash --type sha256 --flat test/world

Converting between hexadecimal and base-32:

$ nix-hash --type sha1 --to-base32 e4fd8ba5f7bbeaea5ace89fe10255536cd60dab6

$ nix-hash --type sha1 --to-base16 nvd61k9nalji1zl9rrdfmsmvyyjqpzg4


nix-install-package — install a Nix Package file


nix-install-package [--non-interactive] [ { --profile | -p } path ] [--set]
{{ --url url } | { file }}


The command nix-install-package interactively installs a Nix Package file (*.nixpkg), which is a small file that contains a store path to be installed along with the URL of a binary cache. The Nix Package file is either file, or automatically downloaded from url if the --url switch is used.

nix-install-package is used in one-click installs to download and install pre-built binary packages with all necessary dependencies. nix-install-package is intended to be associated with the MIME type application/nix-package in a web browser so that it is invoked automatically when you click on *.nixpkg files. When invoked, it restarts itself in a terminal window (since otherwise it would be invisible when run from a browser), asks the user to confirm whether to install the package, and if so downloads and installs the package into the user’s current profile.

To obtain a window, nix-install-package tries to restart itself with xterm, konsole and gnome-terminal.



Do not open a new terminal window and do not ask for confirmation.

--profile, -p

Install the package into the specified profile rather than the user’s current profile.


Install the package as the profile so that the profile contains exactly the contents of the package.


To install subversion-1.4.0.nixpkg into the user’s current profile, without any prompting:

$ nix-install-package --non-interactive subversion-1.4.0.nixpkg

To install the same package from some URL into a different profile:

$ nix-install-package --non-interactive -p /nix/var/nix/profiles/eelco \

Format of nixpkg files

A Nix Package file consists of a single line with the following format:

NIXPKG1 manifestURL name system drvPath outPath

The elements are as follows:


The version of the Nix Package file.


The manifest to be pulled by nix-pull. The manifest must contain outPath.


The symbolic name and version of the package.


The platform identifier of the platform for which this binary package is intended.


The path in the Nix store of the derivation from which outPath was built. Not currently used.


The path in the Nix store of the package.


The URL of a binary cache containing the closure of outPath.

An example follows:

NIXPKG1 http://.../nixpkgs-0.10pre6622/MANIFEST subversion-1.4.0 i686-darwin \
  /nix/store/4kh60jkp...-subversion-1.4.0.drv \

(The line breaks (\) are for presentation purposes and not part of the actual file.)


nix-instantiate — instantiate store derivations from Nix expressions


nix-instantiate [ --parse | --eval [--strict] [--xml] ] [--read-write-mode] [--arg name value] [ { --attr | -A } attrPath ] [--add-root path] [--indirect] { --expr | -E } files...

nix-instantiate --find-file files...


The command nix-instantiate generates store derivations from (high-level) Nix expressions. It evaluates the Nix expressions in each of files (which defaults to ./default.nix). Each top-level expression should evaluate to a derivation, a list of derivations, or a set of derivations. The paths of the resulting store derivations are printed on standard output.

If files is the character -, then a Nix expression will be read from standard input.

See also Chapter 17, Common Options for a list of common options.


--add-root path, --indirect

See the corresponding options in nix-store.


Just parse the input files, and print their abstract syntax trees on standard output in ATerm format.


Just parse and evaluate the input files, and print the resulting values on standard output. No instantiation of store derivations takes place.


Look up the given files in Nix’s search path (as specified by the NIX_PATH environment variable). If found, print the corresponding absolute paths on standard output. For instance, if NIX_PATH is nixpkgs=/home/alice/nixpkgs, then nix-instantiate --find-file nixpkgs/default.nix will print /home/alice/nixpkgs/default.nix.


When used with --parse and --eval, print the resulting expression as an XML representation of the abstract syntax tree rather than as an ATerm. The schema is the same as that used by the toXML built-in.


When used with --parse and --eval, print the resulting expression as an JSON representation of the abstract syntax tree rather than as an ATerm.


When used with --eval, recursively evaluate list elements and attributes. Normally, such sub-expressions are left unevaluated (since the Nix expression language is lazy).


This option can cause non-termination, because lazy data structures can be infinitely large.


When used with --eval, perform evaluation in read/write mode so nix language features that require it will still work (at the cost of needing to do instantiation of every evaluated derivation).


Instantiating store derivations from a Nix expression, and building them using nix-store:

$ nix-instantiate test.nix (instantiate)

$ nix-store -r $(nix-instantiate test.nix) (build)
/nix/store/qhqk4n8ci095g3sdp93x7rgwyh9rdvgk-perl-BerkeleyDB-0.26 (output path)

$ ls -l /nix/store/qhqk4n8ci095g3sdp93x7rgwyh9rdvgk-perl-BerkeleyDB-0.26
dr-xr-xr-x    2 eelco    users        4096 1970-01-01 01:00 lib

You can also give a Nix expression on the command line:

$ nix-instantiate -E 'with import <nixpkgs> { }; hello'

This is equivalent to:

$ nix-instantiate '<nixpkgs>' -A hello

Parsing and evaluating Nix expressions:

$ nix-instantiate --parse -E '1 + 2'
1 + 2

$ nix-instantiate --eval -E '1 + 2'

$ nix-instantiate --eval --xml -E '1 + 2'
<?xml version='1.0' encoding='utf-8'?>
  <int value="3" />

The difference between non-strict and strict evaluation:

$ nix-instantiate --eval --xml -E 'rec { x = "foo"; y = x; }'
  <attr name="x">
    <string value="foo" />
  <attr name="y">
    <unevaluated />

Note that y is left unevaluated (the XML representation doesn’t attempt to show non-normal forms).

$ nix-instantiate --eval --xml --strict -E 'rec { x = "foo"; y = x; }'
  <attr name="x">
    <string value="foo" />
  <attr name="y">
    <string value="foo" />


nix-prefetch-url — copy a file from a URL into the store and print its hash


nix-prefetch-url [--type hashAlgo] [--print-path] url [hash]


The command nix-prefetch-url downloads the file referenced by the URL url, prints its cryptographic hash, and copies it into the Nix store. The file name in the store is hash-baseName, where baseName is everything following the final slash in url.

This command is just a convenience for Nix expression writers. Often a Nix expression fetches some source distribution from the network using the fetchurl expression contained in Nixpkgs. However, fetchurl requires a cryptographic hash. If you don't know the hash, you would have to download the file first, and then fetchurl would download it again when you build your Nix expression. Since fetchurl uses the same name for the downloaded file as nix-prefetch-url, the redundant download can be avoided.

If hash is specified, then a download is not performed if the Nix store already contains a file with the same hash and base name. Otherwise, the file is downloaded, and an error if signaled if the actual hash of the file does not match the specified hash.

This command prints the hash on standard output. Additionally, if the option --print-path is used, the path of the downloaded file in the Nix store is also printed.


--type hashAlgo

Use the specified cryptographic hash algorithm, which can be one of md5, sha1, and sha256.


Print the store path of the downloaded file on standard output.


Unpack the archive (which must be a tarball or zip file) and add the result to the Nix store. The resulting hash can be used with functions such as Nixpkgs’s fetchzip or fetchFromGitHub.


Override the name of the file in the Nix store. By default, this is hash-basename, where basename is the last component of url. Overriding the name is necessary when basename contains characters that are not allowed in Nix store paths.


$ nix-prefetch-url

$ nix-prefetch-url --print-path mirror://gnu/hello/hello-2.10.tar.gz

$ nix-prefetch-url --unpack --print-path


nix-pull — register availability of pre-built binaries (deprecated)


nix-pull url



This command and the use of manifests is deprecated. It is better to use binary caches.

The command nix-pull obtains a list of pre-built store paths from the URL url, and for each of these store paths, registers a substitute derivation that downloads and unpacks it into the Nix store. This is used to speed up installations: if you attempt to install something that has already been built and stored into the network cache, Nix can transparently re-use the pre-built store paths.

The file at url must be compatible with the files created by nix-push.


$ nix-pull


nix-push — generate a binary cache


nix-push --dest dest-dir [--bzip2] [--none] [--force] [--link] [--manifest] [--manifest-path filename] [--url-prefix url] [--key-file path] paths...


The command nix-push produces a binary cache, a directory containing compressed Nix archives (NARs) plus some metadata of the closure of the specified store paths. This directory can then be made available through a web server to other Nix installations, allowing them to skip building from source and instead download binaries from the cache automatically.

nix-push performs the following actions.

  1. Each path in paths is built (using nix-store --realise).

  2. All paths in the closure of paths are determined (using nix-store --query --requisites --include-outputs). Note that since the --include-outputs flag is used, if paths includes a store derivation, you get a combined source/binary distribution (e.g., source tarballs will be included).

  3. All store paths determined in the previous step are packaged into a NAR (using nix-store --dump) and compressed using xz or bzip2. The resulting files have the extension .nar.xz or .nar.bz2. Also for each store path, Nix generates a file with extension .narinfo containing metadata such as the references, cryptographic hash and size of each path.

  4. Optionally, a single manifest file is created that contains the same metadata as the .narinfo files. This is for compatibility with Nix versions prior to 1.2 (see nix-pull for details).

  5. A file named nix-cache-info is placed in the destination directory. The existence of this file marks the directory as a binary cache.


--dest dest-dir

Set the destination directory to dir, which is created if it does not exist. This flag is required.


Compress NARs using bzip2 instead of xz. The latter compresses about 30% better on typical archives, decompresses about twice as fast, but compresses a lot slower and is not supported by Nix prior to version 1.2.


Do not compress NARs.


Overwrite .narinfo files if they already exist.


By default, NARs are generated in the Nix store and then copied to dest-dir. If this option is given, hard links are used instead. This only works if dest-dir is on the same filesystem as the Nix store.


Force the generation of a manifest suitable for use by nix-pull. The manifest is stored as dest-dir/MANIFEST.

--manifest-path filename

Like --manifest, but store the manifest in filename.

--url-prefix url

Manifests are expected to contain the absolute URLs of NARs. For generating these URLs, the prefix url is used. It defaults to file://dest-dir.

--key-file path

Sign the binary cache using the secret key stored in path. This secret key must have been created using nix-store --generate-binary-cache-key. Users of this binary cache should add the corresponding public key to the option binary-cache-public-keys in nix.conf.


To add the closure of Thunderbird to a binary cache:

$ nix-push --dest /tmp/cache $(nix-build -A thunderbird)

Assuming that /tmp/cache is exported by a web server as, you can then use this cache on another machine to speed up the installation of Thunderbird:

$ nix-build -A thunderbird --option binary-caches

Alternatively, you could add binary-caches = to nix.conf.

To also include build-time dependencies (such as source tarballs):

$ nix-push --dest /tmp/cache $(nix-instantiate -A thunderbird)

To generate a manifest suitable for nix-pull:

$ nix-push --dest /tmp/cache $(nix-build -A thunderbird) --manifest

On another machine you can then do:

$ nix-pull

to cause the binaries to be used by subsequent Nix operations.

To generate a signed binary cache, you must first generate a key pair, in this example called, storing the secret key in ./sk and the public key in ./pk:

$ nix-store --generate-binary-cache-key sk pk

$ cat sk

$ cat pk

You can then generate a binary cache signed with the secret key:

$ nix-push --dest /tmp/cache --key-file ./sk $(type -p firefox)

Users who wish to verify the integrity of binaries downloaded from your cache would add the following to their nix.conf:

binary-caches =
signed-binary-caches = *
binary-cache-public-keys =

Nix will then ignore any binary that has a missing, incorrect or unrecognised signature.

Binary cache format and operation

A binary cache with URL url only denotes a valid binary cache if the file url/nix-cache-info exists. If this file does not exist (or cannot be downloaded), the cache is ignored. If it does exist, it must be a text file containing cache properties. Here’s an example:

StoreDir: /nix/store
WantMassQuery: 1
Priority: 10

The properties that are currently supported are:


The path of the Nix store to which this binary cache applies. Binaries are not relocatable — a binary built for /nix/store won’t generally work in /home/alice/store — so to prevent binaries from being used in a wrong store, a binary cache is only used if its StoreDir matches the local Nix configuration. The default is /nix/store.


Query operations such as nix-env -qas can cause thousands of cache queries, and thus thousands of HTTP requests, to determine which packages are available in binary form. While these requests are small, not every server may appreciate a potential onslaught of queries. If WantMassQuery is set to 0 (default), “mass queries” such as nix-env -qas will skip this cache. Thus a package may appear not to have a binary substitute. However, the binary will still be used when you actually install the package. If WantMassQuery is set to 1, mass queries will use this cache.


Each binary cache has a priority (defaulting to 50). Binary caches are checked for binaries in order of ascending priority; thus a higher number denotes a lower priority. The binary cache has priority 40.

Every time Nix needs to build some store path p, it will check each configured binary cache to see if it has a NAR file for p, until it finds one. If no cache has a NAR, Nix will fall back to building the path from source (if applicable). To see if a cache with URL url has a binary for p, Nix fetches url/h, where h is the hash part of p. Thus, if we have a cache and we want to obtain the store path


then Nix will attempt to fetch

(Commands such as nix-env -qas will issue an HTTP HEAD request, since it only needs to know if the .narinfo file exists.) The .narinfo file is a simple text file that looks like this:

StorePath: /nix/store/a8922c0h87iilxzzvwn2hmv8x210aqb9-glibc-2.7
URL: nar/0zzjpdz46mdn74v09m053yczlz4am038g8r74iy8w43gx8801h70.nar.bz2
Compression: bzip2
FileHash: sha256:0zzjpdz46mdn74v09m053yczlz4am038g8r74iy8w43gx8801h70
FileSize: 24473768
NarHash: sha256:0s491y1h9hxj5ghiizlxk7ax6jwbha00zwn7lpyd5xg5bhf60vzg
NarSize: 109521136
References: 2ma2k0ys8knh4an48n28vigcmc2z8773-linux-headers- ...
Deriver: 7akyyc87ka32xwmqza9dvyg5pwx3j212-glibc-2.7.drv

The fields are as follows:


The full store path, including the name part (e.g., glibc-2.7). It must match the requested store path.


The URL of the NAR, relative to the binary cache URL.


The compression method; either xz or bzip2.


The SHA-256 hash of the compressed NAR.


The size of the compressed NAR.


The SHA-256 hash of the uncompressed NAR. This is equal to the hash of the store path as returned by nix-store -q --hash p.


The size of the uncompressed NAR.


The references of the store path, without the Nix store prefix.


The deriver of the store path, without the Nix store prefix. This field is optional.


The Nix platform type of this binary, if known. This field is optional.


A signature of the the form key-name:sig, where key-name is the symbolic name of the key pair used to sign and verify the cache (e.g., and sig is the actual signature, computed over the StorePath, NarHash, NarSize and References fields using the Ed25519 public-key signature system.

Thus, in our example, after recursively ensuring that the references exist (e.g., /nix/store/2ma2k0ys8knh4an48n28vigcmc2z8773-linux-headers-, Nix will fetch

and decompress and unpack it to /nix/store/a8922c0h87iilxzzvwn2hmv8x210aqb9-glibc-2.7.

Chapter 21. Files

This section lists configuration files that you can use when you work with Nix.


nix.conf — Nix configuration file


A number of persistent settings of Nix are stored in the file sysconfdir/nix/nix.conf. This file is a list of name = value pairs, one per line. Comments start with a # character. Here is an example configuration file:

gc-keep-outputs = true       # Nice for developers
gc-keep-derivations = true   # Idem
env-keep-derivations = false

You can override settings using the --option flag, e.g. --option gc-keep-outputs false.

The following settings are currently available:


If true, the garbage collector will keep the outputs of non-garbage derivations. If false (default), outputs will be deleted unless they are GC roots themselves (or reachable from other roots).

In general, outputs must be registered as roots separately. However, even if the output of a derivation is registered as a root, the collector will still delete store paths that are used only at build time (e.g., the C compiler, or source tarballs downloaded from the network). To prevent it from doing so, set this option to true.


If true (default), the garbage collector will keep the derivations from which non-garbage store paths were built. If false, they will be deleted unless explicitly registered as a root (or reachable from other roots).

Keeping derivation around is useful for querying and traceability (e.g., it allows you to ask with what dependencies or options a store path was built), so by default this option is on. Turn it off to save a bit of disk space (or a lot if gc-keep-outputs is also turned on).


If false (default), derivations are not stored in Nix user environments. That is, the derivation any build-time-only dependencies may be garbage-collected.

If true, when you add a Nix derivation to a user environment, the path of the derivation is stored in the user environment. Thus, the derivation will not be garbage-collected until the user environment generation is deleted (nix-env --delete-generations). To prevent build-time-only dependencies from being collected, you should also turn on gc-keep-outputs.

The difference between this option and gc-keep-derivations is that this one is “sticky”: it applies to any user environment created while this option was enabled, while gc-keep-derivations only applies at the moment the garbage collector is run.


This option defines the maximum number of jobs that Nix will try to build in parallel. The default is 1. You should generally set it to the number of CPUs in your system (e.g., 2 on an Athlon 64 X2). It can be overridden using the --max-jobs (-j) command line switch.


Sets the value of the NIX_BUILD_CORES environment variable in the invocation of builders. Builders can use this variable at their discretion to control the maximum amount of parallelism. For instance, in Nixpkgs, if the derivation attribute enableParallelBuilding is set to true, the builder passes the -jN flag to GNU Make. It can be overridden using the --cores command line switch and defaults to 1. The value 0 means that the builder should use all available CPU cores in the system.


This option defines the maximum number of seconds that a builder can go without producing any data on standard output or standard error. This is useful (for instance in an automated build system) to catch builds that are stuck in an infinite loop, or to catch remote builds that are hanging due to network problems. It can be overridden using the --max-silent-time command line switch.

The value 0 means that there is no timeout. This is also the default.


This option defines the maximum number of seconds that a builder can run. This is useful (for instance in an automated build system) to catch builds that are stuck in an infinite loop but keep writing to their standard output or standard error. It can be overridden using the --timeout command line switch.

The value 0 means that there is no timeout. This is also the default.


This option defines the maximum number of bytes that a builder can write to its stdout/stderr. If the builder exceeds this limit, it’s killed. A value of 0 (the default) means that there is no limit.


This options specifies the Unix group containing the Nix build user accounts. In multi-user Nix installations, builds should not be performed by the Nix account since that would allow users to arbitrarily modify the Nix store and database by supplying specially crafted builders; and they cannot be performed by the calling user since that would allow him/her to influence the build result.

Therefore, if this option is non-empty and specifies a valid group, builds will be performed under the user accounts that are a member of the group specified here (as listed in /etc/group). Those user accounts should not be used for any other purpose!

Nix will never run two builds under the same user account at the same time. This is to prevent an obvious security hole: a malicious user writing a Nix expression that modifies the build result of a legitimate Nix expression being built by another user. Therefore it is good to have as many Nix build user accounts as you can spare. (Remember: uids are cheap.)

The build users should have permission to create files in the Nix store, but not delete them. Therefore, /nix/store should be owned by the Nix account, its group should be the group specified here, and its mode should be 1775.

If the build users group is empty, builds will be performed under the uid of the Nix process (that is, the uid of the caller if NIX_REMOTE is empty, the uid under which the Nix daemon runs if NIX_REMOTE is daemon). Obviously, this should not be used in multi-user settings with untrusted users.


If set to true, builds will be performed in a sandboxed environment, i.e., they’re isolated from the normal file system hierarchy and will only see their dependencies in the Nix store, the temporary build directory, private versions of /proc, /dev, /dev/shm and /dev/pts (on Linux), and the paths configured with the build-sandbox-paths option. This is useful to prevent undeclared dependencies on files in directories such as /usr/bin. In addition, on Linux, builds run in private PID, mount, network, IPC and UTS namespaces to isolate them from other processes in the system (except that fixed-output derivations do not run in private network namespace to ensure they can access the network).

Currently, sandboxing only work on Linux and Mac OS X. The use of a sandbox requires that Nix is run as root (so you should use the “build users” feature to perform the actual builds under different users than root).

If this option is set to relaxed, then fixed-output derivations and derivations that have the __noChroot attribute set to true do not run in sandboxes.

The default is false.


A list of paths bind-mounted into Nix sandbox environments. You can use the syntax target=source to mount a path in a different location in the sandbox; for instance, /bin=/nix-bin will mount the path /nix-bin as /bin inside the sandbox. If source is followed by ?, then it is not an error if source does not exist; for example, /dev/nvidiactl? specifies that /dev/nvidiactl will only be mounted in the sandbox if it exists in the host filesystem.

Depending on how Nix was built, the default value for this option may be empty or provide /bin/sh as a bind-mount of bash.


A list of additional paths appended to build-sandbox-paths. Useful if you want to extend its default value.


If set to true (default), Nix will use binary substitutes if available. This option can be disabled to force building from source.


If set to true, Nix will fall back to building from source if a binary substitute fails. This is equivalent to the --fallback flag. The default is false.


If set to true, Nix will “cache” build failures, meaning that it will remember (in its database) that a derivation previously failed. If you then try to build the derivation again, Nix will immediately fail rather than perform the build again. Failures in fixed-output derivations (such as fetchurl calls) are never cached. The “failed” status of a derivation can be cleared using nix-store --clear-failed-paths. By default, failure caching is disabled.


If set to true (the default), Nix will write the build log of a derivation (i.e. the standard output and error of its builder) to the directory /nix/var/log/nix/drvs. The build log can be retrieved using the command nix-store -l path.


If set to true (the default), build logs written to /nix/var/log/nix/drvs will be compressed on the fly using bzip2. Otherwise, they will not be compressed.


If set to true (the default), Nix will check the binary caches specified by binary-caches and related options to obtain binary substitutes.


A list of URLs of binary caches, separated by whitespace. The default is


A list of names of files that will be read to obtain additional binary cache URLs. The default is /nix/var/nix/profiles/per-user/username/channels/binary-caches/*. Note that when you’re using the Nix daemon, username is always equal to root, so Nix will only use the binary caches provided by the channels installed by root. Do not set this option to read files created by untrusted users!


A list of URLs of binary caches, separated by whitespace. These are not used by default, but can be enabled by users of the Nix daemon by specifying --option binary-caches urls on the command line. Unprivileged users are only allowed to pass a subset of the URLs listed in binary-caches and trusted-binary-caches.


Additional binary caches appended to those specified in binary-caches and binary-caches-files. When used by unprivileged users, untrusted binary caches (i.e. those not listed in trusted-binary-caches) are silently ignored.


If set to *, Nix will only download binaries if they are signed using one of the keys listed in binary-cache-public-keys.


A whitespace-separated list of public keys corresponding to the secret keys trusted to sign binary caches. For example:


The maximum number of parallel HTTP connections used by the binary cache substituter to get NAR info files. This number should be high to minimise latency. It defaults to 25.


Whether HTTPS binary caches are required to have a certificate that can be verified. Defaults to true.


If this option is set to false (default) and a Nix channel provides both a manifest and a binary cache, only the binary cache will be used. If set to true, the manifest will be fetched as well. This is useful if you want to use binary patches (which are currently not supported by binary caches).


If set to an absolute path to a netrc file, Nix will use the HTTP authentication credentials in this file when trying to download from a remote host through HTTP or HTTPS. Defaults to $NIX_CONF_DIR/netrc.

The netrc file consists of a list of accounts in the following format:

machine my-machine
login my-username
password my-password

For the exact syntax, see the curl documentation.


This option specifies the canonical Nix system name of the current installation, such as i686-linux or powerpc-darwin. Nix can only build derivations whose system attribute equals the value specified here. In general, it never makes sense to modify this value from its default, since you can use it to ‘lie’ about the platform you are building on (e.g., perform a Mac OS build on a Linux machine; the result would obviously be wrong). It only makes sense if the Nix binaries can run on multiple platforms, e.g., ‘universal binaries’ that run on powerpc-darwin and i686-darwin.

It defaults to the canonical Nix system name detected by configure at build time.


If set to true, changes to the Nix store metadata (in /nix/var/nix/db) are synchronously flushed to disk. This improves robustness in case of system crashes, but reduces performance. The default is true.


If set to true, Nix automatically detects files in the store that have identical contents, and replaces them with hard links to a single copy. This saves disk space. If set to false (the default), you can still run nix-store --optimise to get rid of duplicate files.


The timeout (in seconds) for establishing connections in the binary cache substituter. It corresponds to curl’s --connect-timeout option.


A list of URL prefixes (such as from which nix-store -l will try to fetch build logs if they’re not available locally.


A list of names of users (separated by whitespace) that have additional rights when connecting to the Nix daemon, such as the ability to specify additional binary caches, or to import unsigned NARs. You can also specify groups by prefixing them with @; for instance, @wheel means all users in the wheel group. The default is root.


The users listed here have the ability to compromise the security of a multi-user Nix store. For instance, they could install Trojan horses subsequently executed by other users. So you should consider carefully whether to add users to this list.


A list of names of users (separated by whitespace) that are allowed to connect to the Nix daemon. As with the trusted-users option, you can specify groups by prefixing them with @. Also, you can allow all users by specifying *. The default is *.

Note that trusted users are always allowed to connect.


If set to true, the Nix evaluator will not allow access to any files outside of the Nix search path (as set via the NIX_PATH environment variable or the -I option). The default is false.


If set, the path to a program that can set extra derivation-specific settings for this system. This is used for settings that can't be captured by the derivation model itself and are too variable between different versions of the same system to be hard-coded into nix.

The hook is passed the derivation path and, if sandboxes are enabled, the sandbox directory. It can then modify the sandbox and send a series of commands to modify various settings to stdout. The currently recognized commands are:


Pass a list of files and directories to be included in the sandbox for this build. One entry per line, terminated by an empty line. Entries have the same format as build-sandbox-paths.


How many times to repeat builds to check whether they are deterministic. The default value is 0. If the value is non-zero, every build is repeated the specified number of times. If the contents of any of the runs differs from the previous ones, the build is rejected and the resulting store paths are not registered as “valid” in Nix’s database.

Appendix A. Troubleshooting

This section provides solutions for some common problems. See the Nix bug tracker for a list of currently known issues.

A.1. Collisions in nix-env

Symptom: when installing or upgrading, you get an error message such as

$ nix-env -i docbook-xml
adding /nix/store/s5hyxgm62gk2...-docbook-xml-4.2
collision between `/nix/store/s5hyxgm62gk2...-docbook-xml-4.2/xml/dtd/docbook/calstblx.dtd'
  and `/nix/store/06h377hr4b33...-docbook-xml-4.3/xml/dtd/docbook/calstblx.dtd'
  at /nix/store/ line 62.

The cause is that two installed packages in the user environment have overlapping filenames (e.g., xml/dtd/docbook/calstblx.dtd. This usually happens when you accidentally try to install two versions of the same package. For instance, in the example above, the Nix Packages collection contains two versions of docbook-xml, so nix-env -i will try to install both. The default user environment builder has no way to way to resolve such conflicts, so it just gives up.

Solution: remove one of the offending packages from the user environment (if already installed) using nix-env -e, or specify exactly which version should be installed (e.g., nix-env -i docbook-xml-4.2).

A.2. Too many links Error in the Nix store

Symptom: when building something, you get an error message such as

mkdir: cannot create directory `/nix/store/name': Too many links

This is usually because you have more than 32,000 subdirectories in /nix/store, as can be seen using ls -l:

$ ls -ld /nix/store
drwxrwxrwt 32000 nix nix 4620288 Sep 8 15:08 store

The ext2 file system is limited to an inode link count of 32,000 (each subdirectory increasing the count by one). Furthermore, the st_nlink field of the stat system call is a 16-bit value.

This only happens on very large Nix installations (such as build machines).

Quick solution: run the garbage collector. You may want to use the --max-links option.

Real solution: put the Nix store on a file system that supports more than 32,000 subdirectories per directory, such as ext4. (This doesn’t solve the st_nlink limit, but ext4 lies to the kernel by reporting a link count of 1 if it exceeds the limit.)

Appendix B. Glossary


A description of a build action. The result of a derivation is a store object. Derivations are typically specified in Nix expressions using the derivation primitive. These are translated into low-level store derivations (implicitly by nix-env and nix-build, or explicitly by nix-instantiate).


The location in the file system where store objects live. Typically /nix/store.

store path

The location in the file system of a store object, i.e., an immediate child of the Nix store directory.

store object

A file that is an immediate child of the Nix store directory. These can be regular files, but also entire directory trees. Store objects can be sources (objects copied from outside of the store), derivation outputs (objects produced by running a build action), or derivations (files describing a build action).


A substitute is a command invocation stored in the Nix database that describes how to build a store object, bypassing normal the build mechanism (i.e., derivations). Typically, the substitute builds the store object by downloading a pre-built version of the store object from some server.


The assumption that equal Nix derivations when run always produce the same output. This cannot be guaranteed in general (e.g., a builder can rely on external inputs such as the network or the system time) but the Nix model assumes it.

Nix expression

A high-level description of software packages and compositions thereof. Deploying software using Nix entails writing Nix expressions for your packages. Nix expressions are translated to derivations that are stored in the Nix store. These derivations can then be built.


A store path P is said to have a reference to a store path Q if the store object at P contains the path Q somewhere. This implies than an execution involving P potentially needs Q to be present. The references of a store path are the set of store paths to which it has a reference.


The closure of a store path is the set of store paths that are directly or indirectly “reachable” from that store path; that is, it’s the closure of the path under the references relation. For instance, if the store object at path P contains a reference to path Q, then Q is in the closure of P. For correct deployment it is necessary to deploy whole closures, since otherwise at runtime files could be missing. The command nix-store -qR prints out closures of store paths.

output path

A store path produced by a derivation.


The deriver of an output path is the store derivation that built it.


A store path is considered valid if it exists in the file system, is listed in the Nix database as being valid, and if all paths in its closure are also valid.

user environment

An automatically generated store object that consists of a set of symlinks to “active” applications, i.e., other store paths. These are generated automatically by nix-env. See Chapter 9, Profiles.


A symlink to the current user environment of a user, e.g., /nix/var/nix/profiles/default.


A Nix ARchive. This is a serialisation of a path in the Nix store. It can contain regular files, directories and symbolic links. NARs are generated and unpacked using nix-store --dump and nix-store --restore.

Appendix C. Hacking

This section provides some notes on how to hack on Nix. To get the latest version of Nix from GitHub:

$ git clone git://
$ cd nix

To build it and its dependencies:

$ nix-build release.nix -A build.x86_64-linux

To build all dependencies and start a shell in which all environment variables are set up so that those dependencies can be found:

$ ./dev-shell

To build Nix itself in this shell:

[nix-shell]$ ./
[nix-shell]$ configurePhase
[nix-shell]$ make

To test it:

[nix-shell]$ make install
[nix-shell]$ make installcheck

Appendix D. Nix Release Notes

D.1. Release 1.11 (2016-01-19)

This is primarily a bug fix release. It also has a number of new features:

  • nix-prefetch-url can now download URLs specified in a Nix expression. For example,

    $ nix-prefetch-url -A hello.src

    will prefetch the file specified by the fetchurl call in the attribute hello.src from the Nix expression in the current directory, and print the cryptographic hash of the resulting file on stdout. This differs from nix-build -A hello.src in that it doesn't verify the hash, and is thus useful when you’re updating a Nix expression.

    You can also prefetch the result of functions that unpack a tarball, such as fetchFromGitHub. For example:

    $ nix-prefetch-url --unpack

    or from a Nix expression:

    $ nix-prefetch-url -A nix-repl.src

  • The builtin function <nix/fetchurl.nix> now supports downloading and unpacking NARs. This removes the need to have multiple downloads in the Nixpkgs stdenv bootstrap process (like a separate busybox binary for Linux, or curl/mkdir/sh/bzip2 for Darwin). Now all those files can be combined into a single NAR, optionally compressed using xz.

  • Nix now supports SHA-512 hashes for verifying fixed-output derivations, and in builtins.hashString.

  • The new flag --option build-repeat N will cause every build to be executed N+1 times. If the build output differs between any round, the build is rejected, and the output paths are not registered as valid. This is primarily useful to verify build determinism. (We already had a --check option to repeat a previously succeeded build. However, with --check, non-deterministic builds are registered in the DB. Preventing that is useful for Hydra to ensure that non-deterministic builds don't end up getting published to the binary cache.)

  • The options --check and --option build-repeat N, if they detect a difference between two runs of the same derivation and -K is given, will make the output of the other run available under store-path-check. This makes it easier to investigate the non-determinism using tools like diffoscope, e.g.,

    $ nix-build pkgs/stdenv/linux -A stage1.pkgs.zlib --check -K
    error: derivation ‘/nix/store/l54i8wlw2265…-zlib-1.2.8.drv’ may not
    be deterministic: output ‘/nix/store/11a27shh6n2i…-zlib-1.2.8’
    differs from ‘/nix/store/11a27shh6n2i…-zlib-1.2.8-check’
    $ diffoscope /nix/store/11a27shh6n2i…-zlib-1.2.8 /nix/store/11a27shh6n2i…-zlib-1.2.8-check
    ├── lib/libz.a
    │   ├── metadata
    │   │ @@ -1,15 +1,15 @@
    │   │ -rw-r--r-- 30001/30000   3096 Jan 12 15:20 2016 adler32.o
    │   │ +rw-r--r-- 30001/30000   3096 Jan 12 15:28 2016 adler32.o

  • Improved FreeBSD support.

  • nix-env -qa --xml --meta now prints license information.

  • The maximum number of parallel TCP connections that the binary cache substituter will use has been decreased from 150 to 25. This should prevent upsetting some broken NAT routers, and also improves performance.

  • All "chroot"-containing strings got renamed to "sandbox". In particular, some Nix options got renamed, but the old names are still accepted as lower-priority aliases.

This release has contributions from Anders Claesson, Anthony Cowley, Bjørn Forsman, Brian McKenna, Danny Wilson, davidak, Eelco Dolstra, Fabian Schmitthenner, FrankHB, Ilya Novoselov, janus, Jim Garrison, John Ericson, Jude Taylor, Ludovic Courtès, Manuel Jacob, Mathnerd314, Pascal Wittmann, Peter Simons, Philip Potter, Preston Bennes, Rommel M. Martinez, Sander van der Burg, Shea Levy, Tim Cuthbertson, Tuomas Tynkkynen, Utku Demir and Vladimír Čunát.

D.2. Release 1.10 (2015-09-03)

This is primarily a bug fix release. It also has a number of new features:

  • A number of builtin functions have been added to reduce Nixpkgs/NixOS evaluation time and memory consumption: all, any, concatStringsSep, foldl’, genList, replaceStrings, sort.

  • The garbage collector is more robust when the disk is full.

  • Nix supports a new API for building derivations that doesn’t require a .drv file to be present on disk; it only requires an in-memory representation of the derivation. This is used by the Hydra continuous build system to make remote builds more efficient.

  • The function <nix/fetchurl.nix> now uses a builtin builder (i.e. it doesn’t require starting an external process; the download is performed by Nix itself). This ensures that derivation paths don’t change when Nix is upgraded, and obviates the need for ugly hacks to support chroot execution.

  • --version -v now prints some configuration information, in particular what compile-time optional features are enabled, and the paths of various directories.

  • Build users have their supplementary groups set correctly.

This release has contributions from Eelco Dolstra, Guillaume Maudoux, Iwan Aucamp, Jaka Hudoklin, Kirill Elagin, Ludovic Courtès, Manolis Ragkousis, Nicolas B. Pierron and Shea Levy.

D.3. Release 1.9 (2015-06-12)

In addition to the usual bug fixes, this release has the following new features:

  • Signed binary cache support. You can enable signature checking by adding the following to nix.conf:

    signed-binary-caches = *
    binary-cache-public-keys =

    This will prevent Nix from downloading any binary from the cache that is not signed by one of the keys listed in binary-cache-public-keys.

    Signature checking is only supported if you built Nix with the libsodium package.

    Note that while Nix has had experimental support for signed binary caches since version 1.7, this release changes the signature format in a backwards-incompatible way.

  • Automatic downloading of Nix expression tarballs. In various places, you can now specify the URL of a tarball containing Nix expressions (such as Nixpkgs), which will be downloaded and unpacked automatically. For example:

    • In nix-env:

      $ nix-env -f -iA firefox

      This installs Firefox from the latest tested and built revision of the NixOS 14.12 channel.

    • In nix-build and nix-shell:

      $ nix-build -A hello

      This builds GNU Hello from the latest revision of the Nixpkgs master branch.

    • In the Nix search path (as specified via NIX_PATH or -I). For example, to start a shell containing the Pan package from a specific version of Nixpkgs:

      $ nix-shell -p pan -I nixpkgs=

    • In nixos-rebuild (on NixOS):

      $ nixos-rebuild test -I nixpkgs=

    • In Nix expressions, via the new builtin function fetchTarball:

      with import (fetchTarball {}; …

      (This is not allowed in restricted mode.)

  • nix-shell improvements:

    • nix-shell now has a flag --run to execute a command in the nix-shell environment, e.g. nix-shell --run make. This is like the existing --command flag, except that it uses a non-interactive shell (ensuring that hitting Ctrl-C won’t drop you into the child shell).

    • nix-shell can now be used as a #!-interpreter. This allows you to write scripts that dynamically fetch their own dependencies. For example, here is a Haskell script that, when invoked, first downloads GHC and the Haskell packages on which it depends:

      #! /usr/bin/env nix-shell
      #! nix-shell -i runghc -p haskellPackages.ghc haskellPackages.HTTP
      import Network.HTTP
      main = do
        resp <- Network.HTTP.simpleHTTP (getRequest "")
        body <- getResponseBody resp
        print (take 100 body)

      Of course, the dependencies are cached in the Nix store, so the second invocation of this script will be much faster.

  • Chroot improvements:

    • Chroot builds are now supported on Mac OS X (using its sandbox mechanism).

    • If chroots are enabled, they are now used for all derivations, including fixed-output derivations (such as fetchurl). The latter do have network access, but can no longer access the host filesystem. If you need the old behaviour, you can set the option build-use-chroot to relaxed.

    • On Linux, if chroots are enabled, builds are performed in a private PID namespace once again. (This functionality was lost in Nix 1.8.)

    • Store paths listed in build-chroot-dirs are now automatically expanded to their closure. For instance, if you want /nix/store/…-bash/bin/sh mounted in your chroot as /bin/sh, you only need to say build-chroot-dirs = /bin/sh=/nix/store/…-bash/bin/sh; it is no longer necessary to specify the dependencies of Bash.

  • The new derivation attribute passAsFile allows you to specify that the contents of derivation attributes should be passed via files rather than environment variables. This is useful if you need to pass very long strings that exceed the size limit of the environment. The Nixpkgs function writeTextFile uses this.

  • You can now use ~ in Nix file names to refer to your home directory, e.g. import ~/.nixpkgs/config.nix.

  • Nix has a new option restrict-eval that allows limiting what paths the Nix evaluator has access to. By passing --option restrict-eval true to Nix, the evaluator will throw an exception if an attempt is made to access any file outside of the Nix search path. This is primarily intended for Hydra to ensure that a Hydra jobset only refers to its declared inputs (and is therefore reproducible).

  • nix-env now only creates a new “generation” symlink in /nix/var/nix/profiles if something actually changed.

  • The environment variable NIX_PAGER can now be set to override PAGER. You can set it to cat to disable paging for Nix commands only.

  • Failing <...> lookups now show position information.

  • Improved Boehm GC use: we disabled scanning for interior pointers, which should reduce the “Repeated allocation of very large block” warnings and associated retention of memory.

This release has contributions from aszlig, Benjamin Staffin, Charles Strahan, Christian Theune, Daniel Hahler, Danylo Hlynskyi Daniel Peebles, Dan Peebles, Domen Kožar, Eelco Dolstra, Harald van Dijk, Hoang Xuan Phu, Jaka Hudoklin, Jeff Ramnani, j-keck, Linquize, Luca Bruno, Michael Merickel, Oliver Dunkl, Rob Vermaas, Rok Garbas, Shea Levy, Tobias Geerinckx-Rice and William A. Kennington III.

D.4. Release 1.8 (2014-12-14)

  • Breaking change: to address a race condition, the remote build hook mechanism now uses nix-store --serve on the remote machine. This requires build slaves to be updated to Nix 1.8.

  • Nix now uses HTTPS instead of HTTP to access the default binary cache,

  • nix-env selectors are now regular expressions. For instance, you can do

    $ nix-env -qa '.*zip.*'

    to query all packages with a name containing zip.

  • nix-store --read-log can now fetch remote build logs. If a build log is not available locally, then ‘nix-store -l’ will now try to download it from the servers listed in the ‘log-servers’ option in nix.conf. For instance, if you have the configuration option

    log-servers =

    then it will try to get logs from name of the store path. This allows you to do things like:

    $ nix-store -l $(which xterm)

    and get a log even if xterm wasn't built locally.

  • New builtin functions: attrValues, deepSeq, fromJSON, readDir, seq.

  • nix-instantiate --eval now has a --json flag to print the resulting value in JSON format.

  • nix-copy-closure now uses nix-store --serve on the remote side to send or receive closures. This fixes a race condition between nix-copy-closure and the garbage collector.

  • Derivations can specify the new special attribute allowedRequisites, which has a similar meaning to allowedReferences. But instead of only enforcing to explicitly specify the immediate references, it requires the derivation to specify all the dependencies recursively (hence the name, requisites) that are used by the resulting output.

  • On Mac OS X, Nix now handles case collisions when importing closures from case-sensitive file systems. This is mostly useful for running NixOps on Mac OS X.

  • The Nix daemon has new configuration options allowed-users (specifying the users and groups that are allowed to connect to the daemon) and trusted-users (specifying the users and groups that can perform privileged operations like specifying untrusted binary caches).

  • The configuration option build-max-jobs now defaults to the number of available CPU cores.

  • Build users are now used by default when Nix is invoked as root. This prevents builds from accidentally running as root.

  • Nix now includes systemd units and Upstart jobs.

  • Speed improvements to nix-store --optimise.

  • Language change: the == operator now ignores string contexts (the “dependencies” of a string).

  • Nix now filters out Nix-specific ANSI escape sequences on standard error. They are supposed to be invisible, but some terminals show them anyway.

  • Various commands now automatically pipe their output into the pager as specified by the PAGER environment variable.

  • Several improvements to reduce memory consumption in the evaluator.

This release has contributions from Adam Szkoda, Aristid Breitkreuz, Bob van der Linden, Charles Strahan, darealshinji, Eelco Dolstra, Gergely Risko, Joel Taylor, Ludovic Courtès, Marko Durkovic, Mikey Ariel, Paul Colomiets, Ricardo M. Correia, Ricky Elrod, Robert Helgesson, Rob Vermaas, Russell O'Connor, Shea Levy, Shell Turner, Sönke Hahn, Steve Purcell, Vladimír Čunát and Wout Mertens.

D.5. Release 1.7 (2014-04-11)

In addition to the usual bug fixes, this release has the following new features:

  • Antiquotation is now allowed inside of quoted attribute names (e.g. set."${foo}"). In the case where the attribute name is just a single antiquotation, the quotes can be dropped (e.g. the above example can be written set.${foo}). If an attribute name inside of a set declaration evaluates to null (e.g. { ${null} = false; }), then that attribute is not added to the set.

  • Experimental support for cryptographically signed binary caches. See the commit for details.

  • An experimental new substituter, download-via-ssh, that fetches binaries from remote machines via SSH. Specifying the flags --option use-ssh-substituter true --option ssh-substituter-hosts user@hostname will cause Nix to download binaries from the specified machine, if it has them.

  • nix-store -r and nix-build have a new flag, --check, that builds a previously built derivation again, and prints an error message if the output is not exactly the same. This helps to verify whether a derivation is truly deterministic. For example:

    $ nix-build '<nixpkgs>' -A patchelf
    $ nix-build '<nixpkgs>' -A patchelf --check
    error: derivation `/nix/store/1ipvxs…-patchelf-0.6' may not be deterministic:
      hash mismatch in output `/nix/store/4pc1dm…-patchelf-0.6.drv'

  • The nix-instantiate flags --eval-only and --parse-only have been renamed to --eval and --parse, respectively.

  • nix-instantiate, nix-build and nix-shell now have a flag --expr (or -E) that allows you to specify the expression to be evaluated as a command line argument. For instance, nix-instantiate --eval -E '1 + 2' will print 3.

  • nix-shell improvements:

    • It has a new flag, --packages (or -p), that sets up a build environment containing the specified packages from Nixpkgs. For example, the command

      $ nix-shell -p sqlite xorg.libX11 hello

      will start a shell in which the given packages are present.

    • It now uses shell.nix as the default expression, falling back to default.nix if the former doesn’t exist. This makes it convenient to have a shell.nix in your project to set up a nice development environment.

    • It evaluates the derivation attribute shellHook, if set. Since stdenv does not normally execute this hook, it allows you to do nix-shell-specific setup.

    • It preserves the user’s timezone setting.

  • In chroots, Nix now sets up a /dev containing only a minimal set of devices (such as /dev/null). Note that it only does this if you don’t have /dev listed in your build-chroot-dirs setting; otherwise, it will bind-mount the /dev from outside the chroot.

    Similarly, if you don’t have /dev/pts listed in build-chroot-dirs, Nix will mount a private devpts filesystem on the chroot’s /dev/pts.

  • New built-in function: builtins.toJSON, which returns a JSON representation of a value.

  • nix-env -q has a new flag --json to print a JSON representation of the installed or available packages.

  • nix-env now supports meta attributes with more complex values, such as attribute sets.

  • The -A flag now allows attribute names with dots in them, e.g.

    $ nix-instantiate --eval '<nixos>' -A 'config.systemd.units."nscd.service".text'

  • The --max-freed option to nix-store --gc now accepts a unit specifier. For example, nix-store --gc --max-freed 1G will free up to 1 gigabyte of disk space.

  • nix-collect-garbage has a new flag --delete-older-than Nd, which deletes all user environment generations older than N days. Likewise, nix-env --delete-generations accepts a Nd age limit.

  • Nix now heuristically detects whether a build failure was due to a disk-full condition. In that case, the build is not flagged as “permanently failed”. This is mostly useful for Hydra, which needs to distinguish between permanent and transient build failures.

  • There is a new symbol __curPos that expands to an attribute set containing its file name and line and column numbers, e.g. { file = "foo.nix"; line = 10; column = 5; }. There also is a new builtin function, unsafeGetAttrPos, that returns the position of an attribute. This is used by Nixpkgs to provide location information in error messages, e.g.

    $ nix-build '<nixpkgs>' -A libreoffice --argstr system x86_64-darwin
    error: the package ‘libreoffice-’ in ‘.../applications/office/libreoffice/default.nix:263’
      is not supported on ‘x86_64-darwin’

  • The garbage collector is now more concurrent with other Nix processes because it releases certain locks earlier.

  • The binary tarball installer has been improved. You can now install Nix by running:

    $ bash <(curl

  • More evaluation errors include position information. For instance, selecting a missing attribute will print something like

    error: attribute `nixUnstabl' missing, at /etc/nixos/configurations/misc/eelco/mandark.nix:216:15

  • The command nix-setuid-helper is gone.

  • Nix no longer uses Automake, but instead has a non-recursive, GNU Make-based build system.

  • All installed libraries now have the prefix libnix. In particular, this gets rid of libutil, which could clash with libraries with the same name from other packages.

  • Nix now requires a compiler that supports C++11.

This release has contributions from Danny Wilson, Domen Kožar, Eelco Dolstra, Ian-Woo Kim, Ludovic Courtès, Maxim Ivanov, Petr Rockai, Ricardo M. Correia and Shea Levy.

D.6. Release 1.6.1 (2013-10-28)

This is primarily a bug fix release. Changes of interest are:

  • Nix 1.6 accidentally changed the semantics of antiquoted paths in strings, such as "${/foo}/bar". This release reverts to the Nix 1.5.3 behaviour.

  • Previously, Nix optimised expressions such as "${expr}" to expr. Thus it neither checked whether expr could be coerced to a string, nor applied such coercions. This meant that "${123}" evaluatued to 123, and "${./foo}" evaluated to ./foo (even though "${./foo} " evaluates to "/nix/store/hash-foo "). Nix now checks the type of antiquoted expressions and applies coercions.

  • Nix now shows the exact position of undefined variables. In particular, undefined variable errors in a with previously didn't show any position information, so this makes it a lot easier to fix such errors.

  • Undefined variables are now treated consistently. Previously, the tryEval function would catch undefined variables inside a with but not outside. Now tryEval never catches undefined variables.

  • Bash completion in nix-shell now works correctly.

  • Stack traces are less verbose: they no longer show calls to builtin functions and only show a single line for each derivation on the call stack.

  • New built-in function: builtins.typeOf, which returns the type of its argument as a string.

D.7. Release 1.6 (2013-09-10)

In addition to the usual bug fixes, this release has several new features:

  • The command nix-build --run-env has been renamed to nix-shell.

  • nix-shell now sources $stdenv/setup inside the interactive shell, rather than in a parent shell. This ensures that shell functions defined by stdenv can be used in the interactive shell.

  • nix-shell has a new flag --pure to clear the environment, so you get an environment that more closely corresponds to the “real” Nix build.

  • nix-shell now sets the shell prompt (PS1) to ensure that Nix shells are distinguishable from your regular shells.

  • nix-env no longer requires a * argument to match all packages, so nix-env -qa is equivalent to nix-env -qa '*'.

  • nix-env -i has a new flag --remove-all (-r) to remove all previous packages from the profile. This makes it easier to do declarative package management similar to NixOS’s environment.systemPackages. For instance, if you have a specification my-packages.nix like this:

    with import <nixpkgs> {};
    [ thunderbird

    then after any change to this file, you can run:

    $ nix-env -f my-packages.nix -ir

    to update your profile to match the specification.

  • The ‘with’ language construct is now more lazy. It only evaluates its argument if a variable might actually refer to an attribute in the argument. For instance, this now works:

      pkgs = with pkgs; { foo = "old"; bar = foo; } // overrides;
      overrides = { foo = "new"; };

    This evaluates to "new", while previously it gave an “infinite recursion” error.

  • Nix now has proper integer arithmetic operators. For instance, you can write x + y instead of builtins.add x y, or x < y instead of builtins.lessThan x y. The comparison operators also work on strings.

  • On 64-bit systems, Nix integers are now 64 bits rather than 32 bits.

  • When using the Nix daemon, the nix-daemon worker process now runs on the same CPU as the client, on systems that support setting CPU affinity. This gives a significant speedup on some systems.

  • If a stack overflow occurs in the Nix evaluator, you now get a proper error message (rather than “Segmentation fault”) on some systems.

  • In addition to directories, you can now bind-mount regular files in chroots through the (now misnamed) option build-chroot-dirs.

This release has contributions from Domen Kožar, Eelco Dolstra, Florian Friesdorf, Gergely Risko, Ivan Kozik, Ludovic Courtès and Shea Levy.

D.8. Release 1.5.2 (2013-05-13)

This is primarily a bug fix release. It has contributions from Eelco Dolstra, Lluís Batlle i Rossell and Shea Levy.

D.9. Release 1.5 (2013-02-27)

This is a brown paper bag release to fix a regression introduced by the hard link security fix in 1.4.

D.10. Release 1.4 (2013-02-26)

This release fixes a security bug in multi-user operation. It was possible for derivations to cause the mode of files outside of the Nix store to be changed to 444 (read-only but world-readable) by creating hard links to those files (details).

There are also the following improvements:

  • New built-in function: builtins.hashString.

  • Build logs are now stored in /nix/var/log/nix/drvs/XX/, where XX is the first two characters of the derivation. This is useful on machines that keep a lot of build logs (such as Hydra servers).

  • The function corepkgs/fetchurl can now make the downloaded file executable. This will allow getting rid of all bootstrap binaries in the Nixpkgs source tree.

  • Language change: The expression "${./path} ..." now evaluates to a string instead of a path.

D.11. Release 1.3 (2013-01-04)

This is primarily a bug fix release. When this version is first run on Linux, it removes any immutable bits from the Nix store and increases the schema version of the Nix store. (The previous release removed support for setting the immutable bit; this release clears any remaining immutable bits to make certain operations more efficient.)

This release has contributions from Eelco Dolstra and Stuart Pernsteiner.

D.12. Release 1.2 (2012-12-06)

This release has the following improvements and changes:

  • Nix has a new binary substituter mechanism: the binary cache. A binary cache contains pre-built binaries of Nix packages. Whenever Nix wants to build a missing Nix store path, it will check a set of binary caches to see if any of them has a pre-built binary of that path. The configuration setting binary-caches contains a list of URLs of binary caches. For instance, doing

    $ nix-env -i thunderbird --option binary-caches

    will install Thunderbird and its dependencies, using the available pre-built binaries in The main advantage over the old “manifest”-based method of getting pre-built binaries is that you don’t have to worry about your manifest being in sync with the Nix expressions you’re installing from; i.e., you don’t need to run nix-pull to update your manifest. It’s also more scalable because you don’t need to redownload a giant manifest file every time.

    A Nix channel can provide a binary cache URL that will be used automatically if you subscribe to that channel. If you use the Nixpkgs or NixOS channels ( you automatically get the cache

    Binary caches are created using nix-push. For details on the operation and format of binary caches, see the nix-push manpage. More details are provided in this nix-dev posting.

  • Multiple output support should now be usable. A derivation can declare that it wants to produce multiple store paths by saying something like

    outputs = [ "lib" "headers" "doc" ];

    This will cause Nix to pass the intended store path of each output to the builder through the environment variables lib, headers and doc. Other packages can refer to a specific output by referring to pkg.output, e.g.

    buildInputs = [ pkg.lib pkg.headers ];

    If you install a package with multiple outputs using nix-env, each output path will be symlinked into the user environment.

  • Dashes are now valid as part of identifiers and attribute names.

  • The new operation nix-store --repair-path allows corrupted or missing store paths to be repaired by redownloading them. nix-store --verify --check-contents --repair will scan and repair all paths in the Nix store. Similarly, nix-env, nix-build, nix-instantiate and nix-store --realise have a --repair flag to detect and fix bad paths by rebuilding or redownloading them.

  • Nix no longer sets the immutable bit on files in the Nix store. Instead, the recommended way to guard the Nix store against accidental modification on Linux is to make it a read-only bind mount, like this:

    $ mount --bind /nix/store /nix/store
    $ mount -o remount,ro,bind /nix/store

    Nix will automatically make /nix/store writable as needed (using a private mount namespace) to allow modifications.

  • Store optimisation (replacing identical files in the store with hard links) can now be done automatically every time a path is added to the store. This is enabled by setting the configuration option auto-optimise-store to true (disabled by default).

  • Nix now supports xz compression for NARs in addition to bzip2. It compresses about 30% better on typical archives and decompresses about twice as fast.

  • Basic Nix expression evaluation profiling: setting the environment variable NIX_COUNT_CALLS to 1 will cause Nix to print how many times each primop or function was executed.

  • New primops: concatLists, elem, elemAt and filter.

  • The command nix-copy-closure has a new flag --use-substitutes (-s) to download missing paths on the target machine using the substitute mechanism.

  • The command nix-worker has been renamed to nix-daemon. Support for running the Nix worker in “slave” mode has been removed.

  • The --help flag of every Nix command now invokes man.

  • Chroot builds are now supported on systemd machines.

This release has contributions from Eelco Dolstra, Florian Friesdorf, Mats Erik Andersson and Shea Levy.

D.13. Release 1.1 (2012-07-18)

This release has the following improvements:

  • On Linux, when doing a chroot build, Nix now uses various namespace features provided by the Linux kernel to improve build isolation. Namely:

    • The private network namespace ensures that builders cannot talk to the outside world (or vice versa): each build only sees a private loopback interface. This also means that two concurrent builds can listen on the same port (e.g. as part of a test) without conflicting with each other.

    • The PID namespace causes each build to start as PID 1. Processes outside of the chroot are not visible to those on the inside. On the other hand, processes inside the chroot are visible from the outside (though with different PIDs).

    • The IPC namespace prevents the builder from communicating with outside processes using SysV IPC mechanisms (shared memory, message queues, semaphores). It also ensures that all IPC objects are destroyed when the builder exits.

    • The UTS namespace ensures that builders see a hostname of localhost rather than the actual hostname.

    • The private mount namespace was already used by Nix to ensure that the bind-mounts used to set up the chroot are cleaned up automatically.

  • Build logs are now compressed using bzip2. The command nix-store -l decompresses them on the fly. This can be disabled by setting the option build-compress-log to false.

  • The creation of build logs in /nix/var/log/nix/drvs can be disabled by setting the new option build-keep-log to false. This is useful, for instance, for Hydra build machines.

  • Nix now reserves some space in /nix/var/nix/db/reserved to ensure that the garbage collector can run successfully if the disk is full. This is necessary because SQLite transactions fail if the disk is full.

  • Added a basic fetchurl function. This is not intended to replace the fetchurl in Nixpkgs, but is useful for bootstrapping; e.g., it will allow us to get rid of the bootstrap binaries in the Nixpkgs source tree and download them instead. You can use it by doing import <nix/fetchurl.nix> { url = url; sha256 = "hash"; }. (Shea Levy)

  • Improved RPM spec file. (Michel Alexandre Salim)

  • Support for on-demand socket-based activation in the Nix daemon with systemd.

  • Added a manpage for nix.conf(5).

  • When using the Nix daemon, the -s flag in nix-env -qa is now much faster.

D.14. Release 1.0 (2012-05-11)

There have been numerous improvements and bug fixes since the previous release. Here are the most significant:

  • Nix can now optionally use the Boehm garbage collector. This significantly reduces the Nix evaluator’s memory footprint, especially when evaluating large NixOS system configurations. It can be enabled using the --enable-gc configure option.

  • Nix now uses SQLite for its database. This is faster and more flexible than the old ad hoc format. SQLite is also used to cache the manifests in /nix/var/nix/manifests, resulting in a significant speedup.

  • Nix now has an search path for expressions. The search path is set using the environment variable NIX_PATH and the -I command line option. In Nix expressions, paths between angle brackets are used to specify files that must be looked up in the search path. For instance, the expression <nixpkgs/default.nix> looks for a file nixpkgs/default.nix relative to every element in the search path.

  • The new command nix-build --run-env builds all dependencies of a derivation, then starts a shell in an environment containing all variables from the derivation. This is useful for reproducing the environment of a derivation for development.

  • The new command nix-store --verify-path verifies that the contents of a store path have not changed.

  • The new command nix-store --print-env prints out the environment of a derivation in a format that can be evaluated by a shell.

  • Attribute names can now be arbitrary strings. For instance, you can write { "foo-1.2" = …; "bla bla" = …; }."bla bla".

  • Attribute selection can now provide a default value using the or operator. For instance, the expression x.y.z or e evaluates to the attribute x.y.z if it exists, and e otherwise.

  • The right-hand side of the ? operator can now be an attribute path, e.g., attrs ? a.b.c.

  • On Linux, Nix will now make files in the Nix store immutable on filesystems that support it. This prevents accidental modification of files in the store by the root user.

  • Nix has preliminary support for derivations with multiple outputs. This is useful because it allows parts of a package to be deployed and garbage-collected separately. For instance, development parts of a package such as header files or static libraries would typically not be part of the closure of an application, resulting in reduced disk usage and installation time.

  • The Nix store garbage collector is faster and holds the global lock for a shorter amount of time.

  • The option --timeout (corresponding to the configuration setting build-timeout) allows you to set an absolute timeout on builds — if a build runs for more than the given number of seconds, it is terminated. This is useful for recovering automatically from builds that are stuck in an infinite loop but keep producing output, and for which --max-silent-time is ineffective.

  • Nix development has moved to GitHub (

D.15. Release 0.16 (2010-08-17)

This release has the following improvements:

  • The Nix expression evaluator is now much faster in most cases: typically, 3 to 8 times compared to the old implementation. It also uses less memory. It no longer depends on the ATerm library.

  • Support for configurable parallelism inside builders. Build scripts have always had the ability to perform multiple build actions in parallel (for instance, by running make -j 2), but this was not desirable because the number of actions to be performed in parallel was not configurable. Nix now has an option --cores N as well as a configuration setting build-cores = N that causes the environment variable NIX_BUILD_CORES to be set to N when the builder is invoked. The builder can use this at its discretion to perform a parallel build, e.g., by calling make -j N. In Nixpkgs, this can be enabled on a per-package basis by setting the derivation attribute enableParallelBuilding to true.

  • nix-store -q now supports XML output through the --xml flag.

  • Several bug fixes.

D.16. Release 0.15 (2010-03-17)

This is a bug-fix release. Among other things, it fixes building on Mac OS X (Snow Leopard), and improves the contents of /etc/passwd and /etc/group in chroot builds.

D.17. Release 0.14 (2010-02-04)

This release has the following improvements:

  • The garbage collector now starts deleting garbage much faster than before. It no longer determines liveness of all paths in the store, but does so on demand.

  • Added a new operation, nix-store --query --roots, that shows the garbage collector roots that directly or indirectly point to the given store paths.

  • Removed support for converting Berkeley DB-based Nix databases to the new schema.

  • Removed the --use-atime and --max-atime garbage collector options. They were not very useful in practice.

  • On Windows, Nix now requires Cygwin 1.7.x.

  • A few bug fixes.

D.18. Release 0.13 (2009-11-05)

This is primarily a bug fix release. It has some new features:

  • Syntactic sugar for writing nested attribute sets. Instead of

      foo = {
        bar = 123;
        xyzzy = true;
      a = { b = { c = "d"; }; };

    you can write

    { = 123;
      foo.xyzzy = true;
      a.b.c = "d";

    This is useful, for instance, in NixOS configuration files.

  • Support for Nix channels generated by Hydra, the Nix-based continuous build system. (Hydra generates NAR archives on the fly, so the size and hash of these archives isn’t known in advance.)

  • Support i686-linux builds directly on x86_64-linux Nix installations. This is implemented using the personality() syscall, which causes uname to return i686 in child processes.

  • Various improvements to the chroot support. Building in a chroot works quite well now.

  • Nix no longer blocks if it tries to build a path and another process is already building the same path. Instead it tries to build another buildable path first. This improves parallelism.

  • Support for large (> 4 GiB) files in NAR archives.

  • Various (performance) improvements to the remote build mechanism.

  • New primops: builtins.addErrorContext (to add a string to stack traces — useful for debugging), builtins.isBool, builtins.isString, builtins.isInt, builtins.intersectAttrs.

  • OpenSolaris support (Sander van der Burg).

  • Stack traces are no longer displayed unless the --show-trace option is used.

  • The scoping rules for inherit (e) ... in recursive attribute sets have changed. The expression e can now refer to the attributes defined in the containing set.

D.19. Release 0.12 (2008-11-20)

  • Nix no longer uses Berkeley DB to store Nix store metadata. The principal advantages of the new storage scheme are: it works properly over decent implementations of NFS (allowing Nix stores to be shared between multiple machines); no recovery is needed when a Nix process crashes; no write access is needed for read-only operations; no more running out of Berkeley DB locks on certain operations.

    You still need to compile Nix with Berkeley DB support if you want Nix to automatically convert your old Nix store to the new schema. If you don’t need this, you can build Nix with the configure option --disable-old-db-compat.

    After the automatic conversion to the new schema, you can delete the old Berkeley DB files:

    $ cd /nix/var/nix/db
    $ rm __db* log.* derivers references referrers reserved validpaths DB_CONFIG

    The new metadata is stored in the directories /nix/var/nix/db/info and /nix/var/nix/db/referrer. Though the metadata is stored in human-readable plain-text files, they are not intended to be human-editable, as Nix is rather strict about the format.

    The new storage schema may or may not require less disk space than the Berkeley DB environment, mostly depending on the cluster size of your file system. With 1 KiB clusters (which seems to be the ext3 default nowadays) it usually takes up much less space.

  • There is a new substituter that copies paths directly from other (remote) Nix stores mounted somewhere in the filesystem. For instance, you can speed up an installation by mounting some remote Nix store that already has the packages in question via NFS or sshfs. The environment variable NIX_OTHER_STORES specifies the locations of the remote Nix directories, e.g. /mnt/remote-fs/nix.

  • New nix-store operations --dump-db and --load-db to dump and reload the Nix database.

  • The garbage collector has a number of new options to allow only some of the garbage to be deleted. The option --max-freed N tells the collector to stop after at least N bytes have been deleted. The option --max-links N tells it to stop after the link count on /nix/store has dropped below N. This is useful for very large Nix stores on filesystems with a 32000 subdirectories limit (like ext3). The option --use-atime causes store paths to be deleted in order of ascending last access time. This allows non-recently used stuff to be deleted. The option --max-atime time specifies an upper limit to the last accessed time of paths that may be deleted. For instance,

        $ nix-store --gc -v --max-atime $(date +%s -d "2 months ago")

    deletes everything that hasn’t been accessed in two months.

  • nix-env now uses optimistic profile locking when performing an operation like installing or upgrading, instead of setting an exclusive lock on the profile. This allows multiple nix-env -i / -u / -e operations on the same profile in parallel. If a nix-env operation sees at the end that the profile was changed in the meantime by another process, it will just restart. This is generally cheap because the build results are still in the Nix store.

  • The option --dry-run is now supported by nix-store -r and nix-build.

  • The information previously shown by --dry-run (i.e., which derivations will be built and which paths will be substituted) is now always shown by nix-env, nix-store -r and nix-build. The total download size of substitutable paths is now also shown. For instance, a build will show something like

    the following derivations will be built:
    the following paths will be downloaded/copied (30.02 MiB):

  • Language features:

    • @-patterns as in Haskell. For instance, in a function definition

      f = args @ {x, y, z}: ...;

      args refers to the argument as a whole, which is further pattern-matched against the attribute set pattern {x, y, z}.

    • ...” (ellipsis) patterns. An attribute set pattern can now say ... at the end of the attribute name list to specify that the function takes at least the listed attributes, while ignoring additional attributes. For instance,

      {stdenv, fetchurl, fuse, ...}: ...

      defines a function that accepts any attribute set that includes at least the three listed attributes.

    • New primops: builtins.parseDrvName (split a package name string like "nix-0.12pre12876" into its name and version components, e.g. "nix" and "0.12pre12876"), builtins.compareVersions (compare two version strings using the same algorithm that nix-env uses), builtins.length (efficiently compute the length of a list), builtins.mul (integer multiplication), builtins.div (integer division).

  • nix-prefetch-url now supports mirror:// URLs, provided that the environment variable NIXPKGS_ALL points at a Nixpkgs tree.

  • Removed the commands nix-pack-closure and nix-unpack-closure. You can do almost the same thing but much more efficiently by doing nix-store --export $(nix-store -qR paths) > closure and nix-store --import < closure.

  • Lots of bug fixes, including a big performance bug in the handling of with-expressions.

D.20. Release 0.11 (2007-12-31)

Nix 0.11 has many improvements over the previous stable release. The most important improvement is secure multi-user support. It also features many usability enhancements and language extensions, many of them prompted by NixOS, the purely functional Linux distribution based on Nix. Here is an (incomplete) list:

  • Secure multi-user support. A single Nix store can now be shared between multiple (possible untrusted) users. This is an important feature for NixOS, where it allows non-root users to install software. The old setuid method for sharing a store between multiple users has been removed. Details for setting up a multi-user store can be found in the manual.

  • The new command nix-copy-closure gives you an easy and efficient way to exchange software between machines. It copies the missing parts of the closure of a set of store path to or from a remote machine via ssh.

  • A new kind of string literal: strings between double single-quotes ('') have indentation “intelligently” removed. This allows large strings (such as shell scripts or configuration file fragments in NixOS) to cleanly follow the indentation of the surrounding expression. It also requires much less escaping, since '' is less common in most languages than ".

  • nix-env --set modifies the current generation of a profile so that it contains exactly the specified derivation, and nothing else. For example, nix-env -p /nix/var/nix/profiles/browser --set firefox lets the profile named browser contain just Firefox.

  • nix-env now maintains meta-information about installed packages in profiles. The meta-information is the contents of the meta attribute of derivations, such as description or homepage. The command nix-env -q --xml --meta shows all meta-information.

  • nix-env now uses the meta.priority attribute of derivations to resolve filename collisions between packages. Lower priority values denote a higher priority. For instance, the GCC wrapper package and the Binutils package in Nixpkgs both have a file bin/ld, so previously if you tried to install both you would get a collision. Now, on the other hand, the GCC wrapper declares a higher priority than Binutils, so the former’s bin/ld is symlinked in the user environment.

  • nix-env -i / -u: instead of breaking package ties by version, break them by priority and version number. That is, if there are multiple packages with the same name, then pick the package with the highest priority, and only use the version if there are multiple packages with the same priority.

    This makes it possible to mark specific versions/variant in Nixpkgs more or less desirable than others. A typical example would be a beta version of some package (e.g., gcc-4.2.0rc1) which should not be installed even though it is the highest version, except when it is explicitly selected (e.g., nix-env -i gcc-4.2.0rc1).

  • nix-env --set-flag allows meta attributes of installed packages to be modified. There are several attributes that can be usefully modified, because they affect the behaviour of nix-env or the user environment build script:

    • meta.priority can be changed to resolve filename clashes (see above).

    • meta.keep can be set to true to prevent the package from being upgraded or replaced. Useful if you want to hang on to an older version of a package.

    • can be set to false to “disable” the package. That is, no symlinks will be generated to the files of the package, but it remains part of the profile (so it won’t be garbage-collected). Set it back to true to re-enable the package.

  • nix-env -q now has a flag --prebuilt-only (-b) that causes nix-env to show only those derivations whose output is already in the Nix store or that can be substituted (i.e., downloaded from somewhere). In other words, it shows the packages that can be installed “quickly”, i.e., don’t need to be built from source. The -b flag is also available in nix-env -i and nix-env -u to filter out derivations for which no pre-built binary is available.

  • The new option --argstr (in nix-env, nix-instantiate and nix-build) is like --arg, except that the value is a string. For example, --argstr system i686-linux is equivalent to --arg system \"i686-linux\" (note that --argstr prevents annoying quoting around shell arguments).

  • nix-store has a new operation --read-log (-l) paths that shows the build log of the given paths.

  • Nix now uses Berkeley DB 4.5. The database is upgraded automatically, but you should be careful not to use old versions of Nix that still use Berkeley DB 4.4.

  • The option --max-silent-time (corresponding to the configuration setting build-max-silent-time) allows you to set a timeout on builds — if a build produces no output on stdout or stderr for the given number of seconds, it is terminated. This is useful for recovering automatically from builds that are stuck in an infinite loop.

  • nix-channel: each subscribed channel is its own attribute in the top-level expression generated for the channel. This allows disambiguation (e.g. nix-env -i -A nixpkgs_unstable.firefox).

  • The substitutes table has been removed from the database. This makes operations such as nix-pull and nix-channel --update much, much faster.

  • nix-pull now supports bzip2-compressed manifests. This speeds up channels.

  • nix-prefetch-url now has a limited form of caching. This is used by nix-channel to prevent unnecessary downloads when the channel hasn’t changed.

  • nix-prefetch-url now by default computes the SHA-256 hash of the file instead of the MD5 hash. In calls to fetchurl you should pass the sha256 attribute instead of md5. You can pass either a hexadecimal or a base-32 encoding of the hash.

  • Nix can now perform builds in an automatically generated “chroot”. This prevents a builder from accessing stuff outside of the Nix store, and thus helps ensure purity. This is an experimental feature.

  • The new command nix-store --optimise reduces Nix store disk space usage by finding identical files in the store and hard-linking them to each other. It typically reduces the size of the store by something like 25-35%.

  • ~/.nix-defexpr can now be a directory, in which case the Nix expressions in that directory are combined into an attribute set, with the file names used as the names of the attributes. The command nix-env --import (which set the ~/.nix-defexpr symlink) is removed.

  • Derivations can specify the new special attribute allowedReferences to enforce that the references in the output of a derivation are a subset of a declared set of paths. For example, if allowedReferences is an empty list, then the output must not have any references. This is used in NixOS to check that generated files such as initial ramdisks for booting Linux don’t have any dependencies.

  • The new attribute exportReferencesGraph allows builders access to the references graph of their inputs. This is used in NixOS for tasks such as generating ISO-9660 images that contain a Nix store populated with the closure of certain paths.

  • Fixed-output derivations (like fetchurl) can define the attribute impureEnvVars to allow external environment variables to be passed to builders. This is used in Nixpkgs to support proxy configuration, among other things.

  • Several new built-in functions: builtins.attrNames, builtins.filterSource, builtins.isAttrs, builtins.isFunction, builtins.listToAttrs, builtins.stringLength, builtins.sub, builtins.substring, throw, builtins.trace, builtins.readFile.

D.21. Release 0.10.1 (2006-10-11)

This release fixes two somewhat obscure bugs that occur when evaluating Nix expressions that are stored inside the Nix store (NIX-67). These do not affect most users.

D.22. Release 0.10 (2006-10-06)


This version of Nix uses Berkeley DB 4.4 instead of 4.3. The database is upgraded automatically, but you should be careful not to use old versions of Nix that still use Berkeley DB 4.3. In particular, if you use a Nix installed through Nix, you should run

$ nix-store --clear-substitutes



Also, the database schema has changed slighted to fix a performance issue (see below). When you run any Nix 0.10 command for the first time, the database will be upgraded automatically. This is irreversible.

  • nix-env usability improvements:

    • An option --compare-versions (or -c) has been added to nix-env --query to allow you to compare installed versions of packages to available versions, or vice versa. An easy way to see if you are up to date with what’s in your subscribed channels is nix-env -qc \*.

    • nix-env --query now takes as arguments a list of package names about which to show information, just like --install, etc.: for example, nix-env -q gcc. Note that to show all derivations, you need to specify \*.

    • nix-env -i pkgname will now install the highest available version of pkgname, rather than installing all available versions (which would probably give collisions) (NIX-31).

    • nix-env (-i|-u) --dry-run now shows exactly which missing paths will be built or substituted.

    • nix-env -qa --description shows human-readable descriptions of packages, provided that they have a meta.description attribute (which most packages in Nixpkgs don’t have yet).

  • New language features:

    • Reference scanning (which happens after each build) is much faster and takes a constant amount of memory.

    • String interpolation. Expressions like

      "--with-freetype2-library=" + freetype + "/lib"

      can now be written as


      You can write arbitrary expressions within ${...}, not just identifiers.

    • Multi-line string literals.

    • String concatenations can now involve derivations, as in the example "--with-freetype2-library=" + freetype + "/lib". This was not previously possible because we need to register that a derivation that uses such a string is dependent on freetype. The evaluator now properly propagates this information. Consequently, the subpath operator (~) has been deprecated.

    • Default values of function arguments can now refer to other function arguments; that is, all arguments are in scope in the default values (NIX-45).

    • Lots of new built-in primitives, such as functions for list manipulation and integer arithmetic. See the manual for a complete list. All primops are now available in the set builtins, allowing one to test for the availability of primop in a backwards-compatible way.

    • Real let-expressions: let x = ...; ... z = ...; in ....

  • New commands nix-pack-closure and nix-unpack-closure than can be used to easily transfer a store path with all its dependencies to another machine. Very convenient whenever you have some package on your machine and you want to copy it somewhere else.

  • XML support:

    • nix-env -q --xml prints the installed or available packages in an XML representation for easy processing by other tools.

    • nix-instantiate --eval-only --xml prints an XML representation of the resulting term. (The new flag --strict forces ‘deep’ evaluation of the result, i.e., list elements and attributes are evaluated recursively.)

    • In Nix expressions, the primop builtins.toXML converts a term to an XML representation. This is primarily useful for passing structured information to builders.

  • You can now unambiguously specify which derivation to build or install in nix-env, nix-instantiate and nix-build using the --attr / -A flags, which takes an attribute name as argument. (Unlike symbolic package names such as subversion-1.4.0, attribute names in an attribute set are unique.) For instance, a quick way to perform a test build of a package in Nixpkgs is nix-build pkgs/top-level/all-packages.nix -A foo. nix-env -q --attr shows the attribute names corresponding to each derivation.

  • If the top-level Nix expression used by nix-env, nix-instantiate or nix-build evaluates to a function whose arguments all have default values, the function will be called automatically. Also, the new command-line switch --arg name value can be used to specify function arguments on the command line.

  • nix-install-package --url URL allows a package to be installed directly from the given URL.

  • Nix now works behind an HTTP proxy server; just set the standard environment variables http_proxy, https_proxy, ftp_proxy or all_proxy appropriately. Functions such as fetchurl in Nixpkgs also respect these variables.

  • nix-build -o symlink allows the symlink to the build result to be named something other than result.

  • Platform support:

    • Support for 64-bit platforms, provided a suitably patched ATerm library is used. Also, files larger than 2 GiB are now supported.

    • Added support for Cygwin (Windows, i686-cygwin), Mac OS X on Intel (i686-darwin) and Linux on PowerPC (powerpc-linux).

    • Users of SMP and multicore machines will appreciate that the number of builds to be performed in parallel can now be specified in the configuration file in the build-max-jobs setting.

  • Garbage collector improvements:

    • Open files (such as running programs) are now used as roots of the garbage collector. This prevents programs that have been uninstalled from being garbage collected while they are still running. The script that detects these additional runtime roots ( is inherently system-specific, but it should work on Linux and on all platforms that have the lsof utility.

    • nix-store --gc (a.k.a. nix-collect-garbage) prints out the number of bytes freed on standard output. nix-store --gc --print-dead shows how many bytes would be freed by an actual garbage collection.

    • nix-collect-garbage -d removes all old generations of all profiles before calling the actual garbage collector (nix-store --gc). This is an easy way to get rid of all old packages in the Nix store.

    • nix-store now has an operation --delete to delete specific paths from the Nix store. It won’t delete reachable (non-garbage) paths unless --ignore-liveness is specified.

  • Berkeley DB 4.4’s process registry feature is used to recover from crashed Nix processes.

  • A performance issue has been fixed with the referer table, which stores the inverse of the references table (i.e., it tells you what store paths refer to a given path). Maintaining this table could take a quadratic amount of time, as well as a quadratic amount of Berkeley DB log file space (in particular when running the garbage collector) (NIX-23).

  • Nix now catches the TERM and HUP signals in addition to the INT signal. So you can now do a killall nix-store without triggering a database recovery.

  • bsdiff updated to version 4.3.

  • Substantial performance improvements in expression evaluation and nix-env -qa, all thanks to Valgrind. Memory use has been reduced by a factor 8 or so. Big speedup by memoisation of path hashing.

  • Lots of bug fixes, notably:

    • Make sure that the garbage collector can run successfully when the disk is full (NIX-18).

    • nix-env now locks the profile to prevent races between concurrent nix-env operations on the same profile (NIX-7).

    • Removed misleading messages from nix-env -i (e.g., installing `foo' followed by uninstalling `foo') (NIX-17).

  • Nix source distributions are a lot smaller now since we no longer include a full copy of the Berkeley DB source distribution (but only the bits we need).

  • Header files are now installed so that external programs can use the Nix libraries.

D.23. Release 0.9.2 (2005-09-21)

This bug fix release fixes two problems on Mac OS X:

  • If Nix was linked against statically linked versions of the ATerm or Berkeley DB library, there would be dynamic link errors at runtime.

  • nix-pull and nix-push intermittently failed due to race conditions involving pipes and child processes with error messages such as open2: open(GLOB(0x180b2e4), >&=9) failed: Bad file descriptor at /nix/bin/nix-pull line 77 (issue NIX-14).

D.24. Release 0.9.1 (2005-09-20)

This bug fix release addresses a problem with the ATerm library when the --with-aterm flag in configure was not used.

D.25. Release 0.9 (2005-09-16)

NOTE: this version of Nix uses Berkeley DB 4.3 instead of 4.2. The database is upgraded automatically, but you should be careful not to use old versions of Nix that still use Berkeley DB 4.2. In particular, if you use a Nix installed through Nix, you should run

$ nix-store --clear-substitutes


  • Unpacking of patch sequences is much faster now since we no longer do redundant unpacking and repacking of intermediate paths.

  • Nix now uses Berkeley DB 4.3.

  • The derivation primitive is lazier. Attributes of dependent derivations can mutually refer to each other (as long as there are no data dependencies on the outPath and drvPath attributes computed by derivation).

    For example, the expression derivation attrs now evaluates to (essentially)

    attrs // {
      type = "derivation";
      outPath = derivation! attrs;
      drvPath = derivation! attrs;

    where derivation! is a primop that does the actual derivation instantiation (i.e., it does what derivation used to do). The advantage is that it allows commands such as nix-env -qa and nix-env -i to be much faster since they no longer need to instantiate all derivations, just the name attribute.

    Also, it allows derivations to cyclically reference each other, for example,

    webServer = derivation {
      hostName = "";
      services = [svnService];
    svnService = derivation {
      hostName = webServer.hostName;

    Previously, this would yield a black hole (infinite recursion).

  • nix-build now defaults to using ./default.nix if no Nix expression is specified.

  • nix-instantiate, when applied to a Nix expression that evaluates to a function, will call the function automatically if all its arguments have defaults.

  • Nix now uses libtool to build dynamic libraries. This reduces the size of executables.

  • A new list concatenation operator ++. For example, [1 2 3] ++ [4 5 6] evaluates to [1 2 3 4 5 6].

  • Some currently undocumented primops to support low-level build management using Nix (i.e., using Nix as a Make replacement). See the commit messages for r3578 and r3580.

  • Various bug fixes and performance improvements.

D.26. Release 0.8.1 (2005-04-13)

This is a bug fix release.

  • Patch downloading was broken.

  • The garbage collector would not delete paths that had references from invalid (but substitutable) paths.

D.27. Release 0.8 (2005-04-11)

NOTE: the hashing scheme in Nix 0.8 changed (as detailed below). As a result, nix-pull manifests and channels built for Nix 0.7 and below will now work anymore. However, the Nix expression language has not changed, so you can still build from source. Also, existing user environments continue to work. Nix 0.8 will automatically upgrade the database schema of previous installations when it is first run.

If you get the error message

you have an old-style manifest `/nix/var/nix/manifests/[...]'; please
delete it

you should delete previously downloaded manifests:

$ rm /nix/var/nix/manifests/*

If nix-channel gives the error message

manifest `[channel]/MANIFEST'
is too old (i.e., for Nix <= 0.7)

then you should unsubscribe from the offending channel (nix-channel --remove URL; leave out /MANIFEST), and subscribe to the same URL, with channels replaced by channels-v3 (e.g.,

Nix 0.8 has the following improvements:

  • The cryptographic hashes used in store paths are now 160 bits long, but encoded in base-32 so that they are still only 32 characters long (e.g., /nix/store/csw87wag8bqlqk7ipllbwypb14xainap-atk-1.9.0). (This is actually a 160 bit truncation of a SHA-256 hash.)

  • Big cleanups and simplifications of the basic store semantics. The notion of “closure store expressions” is gone (and so is the notion of “successors”); the file system references of a store path are now just stored in the database.

    For instance, given any store path, you can query its closure:

    $ nix-store -qR $(which firefox)
    ... lots of paths ...

    Also, Nix now remembers for each store path the derivation that built it (the “deriver”):

    $ nix-store -qR $(which firefox)

    So to see the build-time dependencies, you can do

    $ nix-store -qR $(nix-store -qd $(which firefox))

    or, in a nicer format:

    $ nix-store -q --tree $(nix-store -qd $(which firefox))

    File system references are also stored in reverse. For instance, you can query all paths that directly or indirectly use a certain Glibc:

    $ nix-store -q --referrers-closure \

  • The concept of fixed-output derivations has been formalised. Previously, functions such as fetchurl in Nixpkgs used a hack (namely, explicitly specifying a store path hash) to prevent changes to, say, the URL of the file from propagating upwards through the dependency graph, causing rebuilds of everything. This can now be done cleanly by specifying the outputHash and outputHashAlgo attributes. Nix itself checks that the content of the output has the specified hash. (This is important for maintaining certain invariants necessary for future work on secure shared stores.)

  • One-click installation :-) It is now possible to install any top-level component in Nixpkgs directly, through the web — see, e.g., All you have to do is associate /nix/bin/nix-install-package with the MIME type application/nix-package (or the extension .nixpkg), and clicking on a package link will cause it to be installed, with all appropriate dependencies. If you just want to install some specific application, this is easier than subscribing to a channel.

  • nix-store -r PATHS now builds all the derivations PATHS in parallel. Previously it did them sequentially (though exploiting possible parallelism between subderivations). This is nice for build farms.

  • nix-channel has new operations --list and --remove.

  • New ways of installing components into user environments:

    • Copy from another user environment:

      $ nix-env -i --from-profile .../other-profile firefox

    • Install a store derivation directly (bypassing the Nix expression language entirely):

      $ nix-env -i /nix/store/z58v41v21xd3...-aterm-2.3.1.drv

      (This is used to implement nix-install-package, which is therefore immune to evolution in the Nix expression language.)

    • Install an already built store path directly:

      $ nix-env -i /nix/store/hsyj5pbn0d9i...-aterm-2.3.1

    • Install the result of a Nix expression specified as a command-line argument:

      $ nix-env -f .../i686-linux.nix -i -E 'x: x.firefoxWrapper'

      The difference with the normal installation mode is that -E does not use the name attributes of derivations. Therefore, this can be used to disambiguate multiple derivations with the same name.

  • A hash of the contents of a store path is now stored in the database after a successful build. This allows you to check whether store paths have been tampered with: nix-store --verify --check-contents.

  • Implemented a concurrent garbage collector. It is now always safe to run the garbage collector, even if other Nix operations are happening simultaneously.

    However, there can still be GC races if you use nix-instantiate and nix-store --realise directly to build things. To prevent races, use the --add-root flag of those commands.

  • The garbage collector now finally deletes paths in the right order (i.e., topologically sorted under the “references” relation), thus making it safe to interrupt the collector without risking a store that violates the closure invariant.

  • Likewise, the substitute mechanism now downloads files in the right order, thus preserving the closure invariant at all times.

  • The result of nix-build is now registered as a root of the garbage collector. If the ./result link is deleted, the GC root disappears automatically.

  • The behaviour of the garbage collector can be changed globally by setting options in /nix/etc/nix/nix.conf.

    • gc-keep-derivations specifies whether deriver links should be followed when searching for live paths.

    • gc-keep-outputs specifies whether outputs of derivations should be followed when searching for live paths.

    • env-keep-derivations specifies whether user environments should store the paths of derivations when they are added (thus keeping the derivations alive).

  • New nix-env query flags --drv-path and --out-path.

  • fetchurl allows SHA-1 and SHA-256 in addition to MD5. Just specify the attribute sha1 or sha256 instead of md5.

  • Manual updates.

D.28. Release 0.7 (2005-01-12)

  • Binary patching. When upgrading components using pre-built binaries (through nix-pull / nix-channel), Nix can automatically download and apply binary patches to already installed components instead of full downloads. Patching is “smart”: if there is a sequence of patches to an installed component, Nix will use it. Patches are currently generated automatically between Nixpkgs (pre-)releases.

  • Simplifications to the substitute mechanism.

  • Nix-pull now stores downloaded manifests in /nix/var/nix/manifests.

  • Metadata on files in the Nix store is canonicalised after builds: the last-modified timestamp is set to 0 (00:00:00 1/1/1970), the mode is set to 0444 or 0555 (readable and possibly executable by all; setuid/setgid bits are dropped), and the group is set to the default. This ensures that the result of a build and an installation through a substitute is the same; and that timestamp dependencies are revealed.

D.29. Release 0.6 (2004-11-14)

  • Rewrite of the normalisation engine.

    • Multiple builds can now be performed in parallel (option -j).

    • Distributed builds. Nix can now call a shell script to forward builds to Nix installations on remote machines, which may or may not be of the same platform type.

    • Option --fallback allows recovery from broken substitutes.

    • Option --keep-going causes building of other (unaffected) derivations to continue if one failed.

  • Improvements to the garbage collector (i.e., it should actually work now).

  • Setuid Nix installations allow a Nix store to be shared among multiple users.

  • Substitute registration is much faster now.

  • A utility nix-build to build a Nix expression and create a symlink to the result int the current directory; useful for testing Nix derivations.

  • Manual updates.

  • nix-env changes:

    • Derivations for other platforms are filtered out (which can be overridden using --system-filter).

    • --install by default now uninstall previous derivations with the same name.

    • --upgrade allows upgrading to a specific version.

    • New operation --delete-generations to remove profile generations (necessary for effective garbage collection).

    • Nicer output (sorted, columnised).

  • More sensible verbosity levels all around (builder output is now shown always, unless -Q is given).

  • Nix expression language changes:

    • New language construct: with E1; E2 brings all attributes defined in the attribute set E1 in scope in E2.

    • Added a map function.

    • Various new operators (e.g., string concatenation).

  • Expression evaluation is much faster.

  • An Emacs mode for editing Nix expressions (with syntax highlighting and indentation) has been added.

  • Many bug fixes.

D.30. Release 0.5 and earlier

Please refer to the Subversion commit log messages.