2020-07-22 21:17:48 +00:00
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# Profiles
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Profiles and user environments are Nix’s mechanism for implementing the
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ability to allow different users to have different configurations, and
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to do atomic upgrades and rollbacks. To understand how they work, it’s
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useful to know a bit about how Nix works. In Nix, packages are stored in
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unique locations in the *Nix store* (typically, `/nix/store`). For
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instance, a particular version of the Subversion package might be stored
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in a directory
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`/nix/store/dpmvp969yhdqs7lm2r1a3gng7pyq6vy4-subversion-1.1.3/`, while
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another version might be stored in
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`/nix/store/5mq2jcn36ldlmh93yj1n8s9c95pj7c5s-subversion-1.1.2`. The long
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strings prefixed to the directory names are cryptographic hashes\[1\] of
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*all* inputs involved in building the package — sources, dependencies,
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compiler flags, and so on. So if two packages differ in any way, they
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end up in different locations in the file system, so they don’t
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interfere with each other. [figure\_title](#fig-user-environments) shows
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a part of a typical Nix store.
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Of course, you wouldn’t want to type
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$ /nix/store/dpmvp969yhdq...-subversion-1.1.3/bin/svn
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every time you want to run Subversion. Of course we could set up the
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2020-07-23 08:44:54 +00:00
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`PATH` environment variable to include the `bin` directory of every
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2020-07-22 21:17:48 +00:00
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package we want to use, but this is not very convenient since changing
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2020-07-23 08:44:54 +00:00
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`PATH` doesn’t take effect for already existing processes. The solution
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2020-07-22 21:17:48 +00:00
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Nix uses is to create directory trees of symlinks to *activated*
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packages. These are called *user environments* and they are packages
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themselves (though automatically generated by `nix-env`), so they too
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reside in the Nix store. For instance, in
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[figure\_title](#fig-user-environments) the user environment
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`/nix/store/0c1p5z4kda11...-user-env` contains a symlink to just
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Subversion 1.1.2 (arrows in the figure indicate symlinks). This would be
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what we would obtain if we had done
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$ nix-env -i subversion
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on a set of Nix expressions that contained Subversion 1.1.2.
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This doesn’t in itself solve the problem, of course; you wouldn’t want
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to type `/nix/store/0c1p5z4kda11...-user-env/bin/svn` either. That’s why
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there are symlinks outside of the store that point to the user
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environments in the store; for instance, the symlinks `default-42-link`
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and `default-43-link` in the example. These are called *generations*
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since every time you perform a `nix-env` operation, a new user
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environment is generated based on the current one. For instance,
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generation 43 was created from generation 42 when we did
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$ nix-env -i subversion firefox
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on a set of Nix expressions that contained Firefox and a new version of
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Subversion.
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Generations are grouped together into *profiles* so that different users
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don’t interfere with each other if they don’t want to. For example:
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$ ls -l /nix/var/nix/profiles/
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...
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lrwxrwxrwx 1 eelco ... default-42-link -> /nix/store/0c1p5z4kda11...-user-env
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lrwxrwxrwx 1 eelco ... default-43-link -> /nix/store/3aw2pdyx2jfc...-user-env
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lrwxrwxrwx 1 eelco ... default -> default-43-link
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This shows a profile called `default`. The file `default` itself is
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actually a symlink that points to the current generation. When we do a
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`nix-env` operation, a new user environment and generation link are
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created based on the current one, and finally the `default` symlink is
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made to point at the new generation. This last step is atomic on Unix,
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which explains how we can do atomic upgrades. (Note that the
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building/installing of new packages doesn’t interfere in any way with
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old packages, since they are stored in different locations in the Nix
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store.)
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If you find that you want to undo a `nix-env` operation, you can just do
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$ nix-env --rollback
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which will just make the current generation link point at the previous
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link. E.g., `default` would be made to point at `default-42-link`. You
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can also switch to a specific generation:
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$ nix-env --switch-generation 43
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which in this example would roll forward to generation 43 again. You can
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also see all available generations:
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$ nix-env --list-generations
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You generally wouldn’t have `/nix/var/nix/profiles/some-profile/bin` in
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your `PATH`. Rather, there is a symlink `~/.nix-profile` that points to
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your current profile. This means that you should put
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`~/.nix-profile/bin` in your `PATH` (and indeed, that’s what the
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initialisation script `/nix/etc/profile.d/nix.sh` does). This makes it
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easier to switch to a different profile. You can do that using the
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command `nix-env --switch-profile`:
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$ nix-env --switch-profile /nix/var/nix/profiles/my-profile
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$ nix-env --switch-profile /nix/var/nix/profiles/default
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These commands switch to the `my-profile` and default profile,
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respectively. If the profile doesn’t exist, it will be created
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automatically. You should be careful about storing a profile in another
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location than the `profiles` directory, since otherwise it might not be
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used as a root of the garbage collector (see
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[???](#sec-garbage-collection)).
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All `nix-env` operations work on the profile pointed to by
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`~/.nix-profile`, but you can override this using the `--profile` option
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(abbreviation `-p`):
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$ nix-env -p /nix/var/nix/profiles/other-profile -i subversion
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This will *not* change the `~/.nix-profile` symlink.
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1. 160-bit truncations of SHA-256 hashes encoded in a base-32 notation,
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to be precise.
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