nixpkgs/doc/packages/build-support.md

Build Support

pkgs.substitute

pkgs.substitute is a wrapper around the substitute Bash function in the standard environment. It replaces strings in src as specified by the substitutions argument.

:::{.example #ex-pkgs-substitute}

Usage of pkgs.substitute

In a build script, the line:

substitute $infile $outfile --replace-fail @foo@ ${foopkg}/bin/foo

is equivalent to:

{ substitute, foopkg }:
substitute {
  src = ./sourcefile.txt;
  substitutions = [
    "--replace"
    "@foo@"
    "${foopkg}/bin/foo"
  ];
}

:::

pkgs.substituteAll

pkgs.substituteAll substitutes all instances of @varName@ (@s included) in file src with the value of the corresponding environment variable. As this uses the [substituteAll] (#fun-substitute) function, its limitations regarding variable names that will or will not be replaced also apply here.

:::{.example #ex-pkgs-substituteAll}

Usage of pkgs.substituteAll

If say-goodbye.sh contains the following:

#! @bash@/bin/bash

echo @unchanged@
@hello@/bin/hello --greeting @greeting@

the following derivation will make substitutions to @bash@, @hello@, and @greeting@:

{
  substituteAll,
  bash,
  hello,
}:
substituteAll {
  src = ./say-goodbye.sh;
  env = {
    inherit bash hello;
    greeting = "goodbye";
  };
}

such that $out will result in something like the following:

#! /nix/store/s30jrpgav677fpc9yvkqsib70xfmx7xi-bash-5.2p26/bin/bash

echo @unchanged@
/nix/store/566f5isbvw014h7knmzmxa5l6hshx43k-hello-2.12.1/bin/hello --greeting goodbye

:::

pkgs.substituteAllFiles

pkgs.substituteAllFiles replaces @varName@ with the value of the environment variable varName. It expects src to be a directory and requires a files argument that specifies which files will be subject to replacements; only these files will be placed in $out.

As it also uses the substituteAll function, it is subject to the same limitations on environment variables as discussed in pkgs.substituteAll.

:::{.example #ex-pkgs-substitute-all-files}

Usage of pkgs.substituteAllFiles

If the current directory contains {foo,bar,baz}.txt and the following default.nix

{ substituteAllFiles }:
substituteAllFiles {
  src = ./.;
  files = [
    "foo.txt"
    "bar.txt"
  ];
  hello = "there";
}

in the resulting derivation, every instance of @hello@ will be replaced with there in $out/foo.txt and $out/bar.txt; baz.txtwill not be processed nor will it appear in$out`. :::