execline: provide tools to execlineb via execline wrapper

Introduces the `execlineb-with-builtins` flag, which when
true (default) will add all execline builtins to the PATH of
`execlineb`.

This is usually what users expect.

If the flag is set to `false`, the unpatched execline derivation is
returned instead.

pkgs/tools/misc/execline/default.nix

@@ -1,7 +1,16 @@
-{ skawarePackages }:
+{ lib, skawarePackages
+# for execlineb-with-builtins
+, coreutils, gnugrep, writeScriptBin, runCommand
+# Whether to wrap bin/execlineb to have the execline tools on its PATH.
+, execlineb-with-builtins ? true
+}:
 
 with skawarePackages;
 
+let
+  outputs = [ "bin" "lib" "dev" "doc" "out" ];
+
+  execline =
     buildPackage {
       pname = "execline";
       version = "2.5.1.0";
@@ -9,7 +18,7 @@ buildPackage {
 
       description = "A small scripting language, to be used in place of a shell in non-interactive scripts";
 
-  outputs = [ "bin" "lib" "dev" "doc" "out" ];
+      inherit outputs;
 
       # TODO: nsss support
       configureFlags = [
@@ -32,4 +41,50 @@ buildPackage {
         mv examples $doc/share/doc/execline/examples
       '';
 
+    };
+
+  # a wrapper around execlineb, which provides all execline
+  # tools on `execlineb`’s PATH.
+  execlineb-with-builtins-drv =
+    let eldir = "${execline}/bin";
+    in writeScriptBin "execlineb" ''
+      #!${eldir}/execlineb -s0
+      # appends the execlineb bin dir to PATH if not yet in PATH
+      ${eldir}/define eldir ${eldir}
+      ''${eldir}/ifelse
+      {
+        # since this is nix, we can grep for the execline drv hash in PATH
+        # to see whether it’s already in there
+        ''${eldir}/pipeline
+        { ${coreutils}/bin/printenv PATH }
+        ${gnugrep}/bin/grep --quiet "${eldir}"
       }
+      # it’s there already
+      { ''${eldir}/execlineb $@ }
+      # not there yet, add it
+      ''${eldir}/importas oldpath PATH
+      ''${eldir}/export PATH "''${eldir}:''${oldpath}"
+      ''${eldir}/execlineb $@
+    '';
+
+  # the original execline package, with bin/execlineb overwritten
+  execline-with-builtins = runCommand "my-execline"
+    (execline.drvAttrs // {
+      preferLocalBuild = true;
+      allowSubstitutes = false;
+    })
+    # copy every output and just overwrite the execlineb binary in $bin
+    ''
+      ${lib.concatMapStringsSep "\n"
+        (output: ''
+          cp -r ${execline.${output}} "''$${output}"
+          chmod --recursive +w "''$${output}"
+        '')
+        outputs}
+      install ${execlineb-with-builtins-drv}/bin/execlineb $bin/bin/execlineb
+    '';
+
+in
+  if execlineb-with-builtins
+  then execline-with-builtins
+  else execline