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 generate it manually (and again to update it when new software is installed), run:

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

To enable automatically building the immutable cache, you can set:

documentation.man.generateCaches = true;

Note that this will rebuild the cache every time you change anything in environment.systemPackages, and will make the mandb command fail.