lib/shell: add library of convenience functions

This library holds a few convenience functions for generating shell
code.

modules/lib/default.nix

@@ -15,4 +15,6 @@
         entryAfter = d.dagEntryAfter;
         entryBefore = d.dagEntryBefore;
       };
+
+  shell = import ./shell.nix { inherit lib; };
 }

modules/lib/shell.nix

@@ -0,0 +1,11 @@
+{ lib }:
+
+rec {
+  # Produces a Bourne shell like variable export statement.
+  export = n: v: "export ${n}=\"${toString v}\"";
+
+  # Given an attribute set containing shell variable names and their
+  # assignment, this function produces a string containing an export
+  # statement for each set entry.
+  exportAll = vars: lib.concatStringsSep "\n" (lib.mapAttrsToList export vars);
+}