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. Note that it will not be populated with programs from home-manager. Since it is immutable, the mandb command (see below) will fail.

Some programs might set this option to true by default. For example, this is the case of 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.