dream2nix/v1/nix/modules/flake-parts/writers.nix

{
  perSystem = {
    config,
    lib,
    pkgs,
    ...
  }: {
    options.writers = {
      writePureShellScript = lib.mkOption {
        type = lib.types.functionTo lib.types.anything;
      };
      writePureShellScriptBin = lib.mkOption {
        type = lib.types.functionTo lib.types.anything;
      };
    };

    /*
      create a script that runs in a `pure` environment, in the sense that:
      - PATH only contains exactly the packages passed via the PATH arg
      - NIX_PATH is set to the path of the current `pkgs`
      - TMPDIR is set up and cleaned up even if the script fails
      - all environment variables are unset, except:
      - the ones listed in `keepVars` below
      - ones listed via the KEEP_VARS variable
      - the behavior is similar to `nix-shell --pure`
    */
    config.writers =
      let
        mkScript = PATH: script: ''
          #!${pkgs.bash}/bin/bash
          set -Eeuo pipefail

          export PATH="${lib.makeBinPath PATH}"
          export NIX_PATH=nixpkgs=${pkgs.path}

          export TMPDIR=$(${pkgs.coreutils}/bin/mktemp -d)

          trap "${pkgs.coreutils}/bin/chmod -R +w '$TMPDIR'; ${pkgs.coreutils}/bin/rm -rf '$TMPDIR'" EXIT

          if [ -z "''${IMPURE:-}" ]; then
            ${cleanEnv}
          fi

          ${script}
        '';

        # list taken from nix source: src/nix-build/nix-build.cc
        keepVars = lib.concatStringsSep " " [
          "HOME"
          "XDG_RUNTIME_DIR"
          "USER"
          "LOGNAME"
          "DISPLAY"
          "WAYLAND_DISPLAY"
          "WAYLAND_SOCKET"
          "PATH"
          "TERM"
          "IN_NIX_SHELL"
          "NIX_SHELL_PRESERVE_PROMPT"
          "TZ"
          "PAGER"
          "NIX_BUILD_SHELL"
          "SHLVL"
          "http_proxy"
          "https_proxy"
          "ftp_proxy"
          "all_proxy"
          "no_proxy"

          # We want to keep out own variables as well
          "IMPURE"
          "KEEP_VARS"
          "NIX_PATH"
          "TMPDIR"
        ];

        cleanEnv = ''

      KEEP_VARS="''${KEEP_VARS:-}"

      unsetVars=$(
        ${pkgs.coreutils}/bin/comm \
          <(${pkgs.gawk}/bin/awk 'BEGIN{for(v in ENVIRON) print v}' | ${pkgs.coreutils}/bin/cut -d = -f 1 | ${pkgs.coreutils}/bin/sort) \
          <(echo "${keepVars} $KEEP_VARS" | ${pkgs.coreutils}/bin/tr " " "\n" | ${pkgs.coreutils}/bin/sort) \
          -2 \
          -3
      )

      unset $unsetVars
    '';
      in
      {
        writePureShellScript = PATH: script:
          pkgs.writeScript "script.sh" (mkScript PATH script);

        writePureShellScriptBin = binName: PATH: script:
          pkgs.writeScriptBin binName (mkScript PATH script);
      };
  };
}