`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 the figure above, 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

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

```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