[Nix-dev] some basic questions from a new NixOS user

[Nicolas Pierron]

Hi Linus,

On Thu, Nov 24, 2011 at 22:44, Linus Arver <linusarver at gmail.com> wrote:
> On Thu, Nov 24, 2011 at 07:00:29PM +0100, Nicolas Pierron wrote:
>> Hi Linus,
>>
>> On Thu, Nov 24, 2011 at 07:24, Linus Arver <linusarver at gmail.com> wrote:
>> > I'm new to this list.
>>
>> Great.
>>
>> > 1a. I've added a new user account with "useradd," per the manual, and
>> > have done a couple installations of software with `nix-env -i'. This is
>> > great, but I'm curious --- what is the preferred way to install
>> > software? The wiki page at http://nixos.org/wiki/Install/remove_software
>> > says that there are two ways to install software --- system-wide (by
>> > editing environment.systemPackages), and per-user (by just doing nix-env
>> > -i [package]). My understanding is that in order to get the advertised
>> > advantages of NixOS's rollback functionality, you have to do system-wide
>> > installs because the whole "nixos-rebuild" command only looks at
>> > /etc/nixos/configuration.nix. I.e., there is no way to rollback the
>> > system on a per-user basis. So, do most people on here have a very long
>> > environment.systemPackages definition in their configuration.nix,
>> > defining all system-wide packages?
>>
>> Using either per-user install or system-wide install depends on what
>> you want to do.  Personally I use both.
>>
>> NixOS is build on top of Nix and relies on the nix-env command to
>> update the system profile and also to do rollbacks (have a look at the
>> end of nix-env man page).  One advantage of per-user profiles is that
>> the root account does not see (in its default PATH) the user installed
>> software and also the opposite.  This difference is not a big security
>> protection but it helps to separate the administrative tools from the
>> commonly used tools.
>>
>> On my case, I am using user profiles to install software that I don't
>> want to see under the root user (inkscape, gimp, firefox, …) and I am
>> also using user-profiles (not the default ~/.nix-profile) to test
>> compilation with different tool-chains (x64-gcc45, x86-gcc46, …), in
>> which case I reset the environment and rebuild it from scratch with
>> only this user profile.
>>
>
> Thanks, this cleared up a lot of things. I just learned from reading the
> Nix manual that you can create new profiles by just doing
>
>    nix-env --switch-profile /nix/var/nix/profiles/foo
>
> and it will create "foo" if it doesn't exist already. I'm having trouble
> understanding the hierarchy of folders/files in the profiles folder,
> though. For my system, there appear to be 3 distinct profiles
>
>    1. /nix/var/nix/profiles/default (3 generations)
>    2. /nix/var/nix/profiles/system (7 generations)
>    3. /nix/var/nix/profiles/per-user/l/profile (11 generations)
>    4. /nix/var/nix/profiles/per-user/root (no files)
>
> I can tell that #3 is what I'm currently using (if I do "nix-env
> --list-generations" I get 11 generatios). If I do a "su" to change into
> root and then do "nix-env --list-generations" I get 3 generations, so
> that would correspond to #1. So my question is --- why does #4 exist? Is
> it to house new, custom-created profiles for the root user? Perhaps
> these different profiles should be documented in the Nix manual...

And easier way to find your current profile is to use

$ ls -l ~/.nix-profile

>> > 1b. If I wanted zsh, git, and rxvt-unicode to be available system-wide,
>> > can I do:
>> >
>> >    environment.systemPackages = [
>> >        pkgs.zsh
>> >        pkgs.git
>> >        pkgs.rxvt-unicode
>> >    ];
>> >
>> > in my configuration.nix? There does not seem to be any documentation
>> > regarding the syntax for this.
>>
>> Sure, you can do it, except that the papckages names are extracted
>> from /etc/nixos/nixpkgs/pkgs/top-level/all-packages.nix .  Thus you
>> should not use the package name, but the key listed in this file.  So
>> you will need to replace
>>
>> >        pkgs.rxvt-unicode
>>
>> by    pkgs.rxvt_unicode
>>
>
> Thanks, that makes sense --- I was getting errors with the former form.
> Is the path
> "/etc/nixos/nixpkgs/pkgs/top-level/all-packages.nix" documented
> somewhere so I can learn more about it?

Not directly, but multiple reference are made to it in Nixpkgs documentation.

http://nixos.org/nixpkgs/docs.html

>> > 2. If I do "nix-env -i" as the root user, then the installation is only
>> > for the root user, correct?
>>
>> I am no longer sure since I haven't reinstalled NixOS since a while,
>> but I think you have a default profile which is used by all users
>> including root.  So you may want to switch to another profile such as
>> $NIX_USER_PROFILE_DIR .
>
> I don't understand --- I thought switching profiles is done with the
> "nix-env --switch-profile" command?

It is, but you can manualy change to another profile if the current
link is not what you want.

> My original question was formulated
> when I did not have a good idea about profiles --- and maybe it doesn't
> make much sense in light of my discoveries above regarding
> /nix/var/nix/profiles.

As a user (root or l), when you type "nix-env" command, it will update
the profile which is pointed by the symbolic link ~/.nix-profile ---
unless you used the "-p" option.
The profile /nix/var/nix/profiles/default is loaded by all users
inside /etc/bashrc script.  So if any user update the
default profile, the modification would be visible by all.

>> Don't hesitate to give us your feedback if you think that some steps
>> are not clear and took you time to figure out.  I'll update the wiki
>> page to fix one mistake and to separate the uninstall from the
>> removal.
>
> Thanks! The biggest problem for me is the lack of practical
> documentation --- right now the NixOS docs are concerned more about the
> technical advantages of NixOS and do not really help actual users trying
> out the system. Of course, this will improve as more users install
> NixOS, but it's the same chicken-and-egg problem that all Linux distros
> have to face.
>
> Anyway, my #1 goal right now is to get Xmonad installed and started on
> boot. I don't want to bother with logging in manually with SLIM --- I
> just want to stare at a blank rxvt-unicode terminal from Xmonad when I
> start up. In my Arch Linux boxes, I have a /etc/inittab which deals with
> how all of this is handled (automatic logging in, startx, etc.), but I'm
> not so sure how to translate this setup into NixOS.

In NixOS, you have to set options inside your
/etc/nixos/configuration.nix file.  Based on your goals I guess you
will be interested by the following list of options:

# enable Xmonad
services.xserver.windowManager.xmonad.enable

# Automatic logging
services.xserver.displayManager.auto.enable
services.xserver.displayManager.auto.user

To start rxvt, I recommend you to copy the
/etc/nixos/nixos/modules/services/x11/desktop-managers/xterm.nix nix
expression and adapt it to your shell, and enable it inside your
configuration.  This module start XTerm at the beginning of the
session.

> My #2 goal is to get firefox installed. Right now if I do "nix-env -i
> firefox", it fails because install_flash_player_10_linux.tar.gz cannot
> be downloaded from any mirror. Maybe this is only affecting i686
> platforms.

In the mean time you can change the options to avoid installing the
flash player.  You can copy the following into ~/.nixpkgs/config.nix .
 This file would be loaded by all-packages.nix.

{
  firefox = {
    enableAdobeFlash = false;
  };
}

> Curiously, zsh does not give me any problems in this regard. It's not
> really a problem for me, because I use zsh whenever possible (and will
> set it to my login shell as soon as I figure out chsh for my regular
> user...)

chsh does not work if you reference your user inside NixOS
configuration file, the reason is that each time you boot, NixOS
activation script will update /etc/passwd file with the list of users
and shells.  But you can update your shell inside your
configuration.nix file.  Have a look at one module of my
configurations:

https://svn.nixos.org/repos/nix/configurations/trunk/misc/nicolas.b.pierron/common/user.nix

-- 
Nicolas Pierron
http://www.linkedin.com/in/nicolasbpierron - http://nbp.name/