nix: Add command baseDir to parse --expr relative to shebang script

src/libutil/args.cc

@@ -276,6 +276,7 @@ void Args::parseCmdline(const Strings & _cmdline, bool allowShebang)
                     cmdline.push_back(word);
                 }
                 cmdline.push_back(script);
+                commandBaseDir = dirOf(script);
                 for (auto pos = savedArgs.begin(); pos != savedArgs.end();pos++)
                     cmdline.push_back(*pos);
             }
@@ -336,6 +337,14 @@ void Args::parseCmdline(const Strings & _cmdline, bool allowShebang)
         d.completer(*completions, d.n, d.prefix);
 }
 
+Path Args::getCommandBaseDir() const
+{
+    if (parent)
+        return parent->getCommandBaseDir();
+    else
+        return commandBaseDir;
+}
+
 bool Args::processFlag(Strings::iterator & pos, Strings::iterator end)
 {
     assert(pos != end);

src/libutil/args.hh

@@ -24,6 +24,16 @@ class AddCompletions;
 
 class Args
 {
+    /**
+     * @brief The command's "working directory", but only set when top level.
+     *
+     * Use getCommandBaseDir() to get the directory regardless of whether this
+     * is a top-level command or subcommand.
+     *
+     * @see getCommandBaseDir()
+     */
+    Path commandBaseDir = ".";
+
 public:
 
     /**
@@ -44,6 +54,16 @@ public:
      */
     virtual std::string doc() { return ""; }
 
+    /**
+     * @brief Get the base directory for the command.
+     *
+     * @return Generally the working directory, but in case of a shebang
+     *         interpreter, returns the directory of the script.
+     *
+     * This only returns the correct value after parseCmdline() has run.
+     */
+    Path getCommandBaseDir() const;
+
 protected:
 
     /**