Using the literalExample function would make sense here. Otherwise it will put the store path into the manual.

@Mic92 when I tested this, the expression did not get expanded to a store path in the manpages. (also see #34469)

This is from my man configuration.nix

services.xserver.xautolock.locker
           The script to use when automatically locking the computer.

           Type: string

           Default: "\${pkgs.xlockmore}/bin/xlock"

           Example: "\${pkgs.i3lock}/bin/i3lock -i /path/to/img"

           Declared by:
               <nixpkgs/nixos/modules/services/x11/xautolock.nix>

(note the escaped '$', that's a part of what the above mentioned PR is about)

Ok. The store path does not get expanded.

Can we take @rnhmjoj 's suggestion in #34371 ? i.e. use something like if [[ -x ${cfg."${option}"} ]]
builtins.substring 0 1 cfg."${option}" == "/" won't work if someone want locker = "reboot".

Yes, I think this should be done using a shell script.

I agree that using "reboot" would not work just like that. Though one could easily specify an absolute path:

services.xserver.xautlock.locker = "${pkgs.systemd}/bin/reboot";

Also this way it can be done using just nix itself and the path can be found by using readlink $(which someCommand) for example. I'm still open to suggestions :)

Maybe there are some users already happily use xautolock service by setting locker = "poweroff". If we only allow absolute path, this PR will force them to change their configuration.nix.
Also there is no downside if we uses if [[ -x ${cfg."${option}"} ]] to check if the value is an executable or not, right?
However, I think you are right and this is just a subtle improvement. The service will still work fine without the executable testing technique.

@sifmelcara as far as I know this would not work just like that, as the -x flag checks if the following 'file' exists and is executable (see man bash). "poweroff" itself is neither a file nor executable in that context and therefore the condition would evaluate to false:

if [[ -x poweroff ]]; then echo "it works"; else echo "it doesn't work"; fi

On the other hand, this would work if poweroff was replaced with "$(which poweroff)" in the condition.
I think that changing the module to expect absolute paths wouldn't be much of a problem as the message in the assertion should give enough information on why the evaluation failed for someone who has something like "poweroff" in the config.

Oh you are definitely right. Anyway, thank you for your great works!