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

```console
$ nix-env --install --attr nixpkgs.subversion nixpkgs.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:

```console
$ 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

```console
$ 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:

```console
$ nix-env --switch-generation 43
```

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

```console
$ 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/nix.sh` does). This makes it
easier to switch to a different profile. You can do that using the
command `nix-env --switch-profile`:

```console
$ 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](garbage-collection.md).

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

```console
$ nix-env --profile /nix/var/nix/profiles/other-profile --install --attr nixpkgs.subversion
```

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