What is @bash@ shebang

[richard-hajek]

Referenced here

home-manager/home-manager/home-manager

Line 1 in 9846abe

#!@bash@/bin/bash

Is it some kind of nix specific shebang?

[richard-hajek]

and here

home-manager/modules/programs/command-not-found/command-not-found.pl

Line 1 in 9846abe

#! @perl@/bin/perl -w

[richard-hajek]

I assume it means "find the derivation of name in the @@", but what version? Where is this behaviour defined?

[khaneliman]

Those are not a special Nix shebang syntax, and they do not mean "look up a derivation named bash".

They are template placeholders that are replaced when Home Manager builds the script.

For home-manager/home-manager, the file is copied into the output and then patched in home-manager/default.nix with:

substituteInPlace $out/bin/home-manager 
  --subst-var-by bash "${bash}" 
  ...

So #!@bash@/bin/bash becomes something like:

#!/nix/store/...-bash-.../bin/bash

The exact Bash version is whichever bash package comes from the pkgs/Nixpkgs instance used to evaluate that Home Manager package.

The command-not-found.pl case is the same idea, but it uses pkgs.replaceVars in modules/programs/command-not-found/default.nix:

cnfScript = pkgs.replaceVars ./command-not-found.pl {
  inherit (cfg) dbPath;
  perl = pkgs.perl.withPackages (p: [
    p.DBDSQLite
    p.StringShellQuote
  ]);
};

That replaces @perl@ with the store path of a Perl interpreter that has those Perl modules available, and replaces @dbPath@ with the configured package database path.

The purpose is to make the generated scripts refer to exact Nix store paths instead of relying on /usr/bin/env, /bin/sh, or whatever happens to be in the user's PATH. It also lets Home Manager inject build-time/configuration-time values into script templates.

[richard-hajek]

Got it, thank you for the detailed explanation!