Apropos

By default as of NixOS 21.05, apropos, whatis and man -k do not find anything when run, because the man page index cache is not generated.

To build the cache automatically, you can set this in your system configuration:

{
  documentation.man.cache.enable = true;
}

Or in older versions:

{
  documentation.man.generateCaches = true;
}

This will create an immutable cache in sync with environment.systemPackages. Since it is immutable, the mandb command (see below) will fail.

If you are using home-manager, the above will not populate the cache with the programs installed that way, instead add this to your home.nix:

{
  programs.man.generateCaches = true;
}

Some programs might set one of these options to true by default, in that case you might still need to set the other to true manually. An example of such a program is fish, which uses it to automatically create completions (see [fish_update_completions](https://fishshell.com/docs/current/cmds/fish_update_completions.html)).

To generate it manually, run:

$ sudo mkdir -p /var/cache/man/nixos
$ sudo mandb

This will need to be re-run to update the cache when new software is installed.