How to set startup script in Fish shell using home-manager?

[TLATER]

@Fineness9214

options.programs.fish.interactiveShellInit = mkOption {
        type = types.lines;
        default = "fnm env --use-on-cd | source";
        description = ''
          Shell script code called during interactive fish shell
          initialisation.
        '';
      };

That’s how you would create a completely new option. You’d also have to make mkOption available from nixpkgs’ lib to do this.

But this option already exists: Redirecting…

Just set it:

programs.fish.interactiveShellIinit = ''
  fnm env --use-on-cd | source
'';