Adds initial support for external commands (#14953)

Author: bcardiff

spec/primitives/external_command_spec.cr

@@ -0,0 +1,34 @@
+{% skip_file if flag?(:interpreted) %}
+
+require "../spec_helper"
+
+describe Crystal::Command do
+  it "exec external commands", tags: %w[slow] do
+    with_temp_executable "crystal-external" do |path|
+      with_tempfile "crystal-external.cr" do |source_file|
+        File.write source_file, <<-CRYSTAL
+          puts ENV["CRYSTAL"]?
+          puts PROGRAM_NAME
+          puts ARGV
+          CRYSTAL
+
+        Process.run(ENV["CRYSTAL_SPEC_COMPILER_BIN"]? || "bin/crystal", ["build", source_file, "-o", path])
+      end
+
+      File.exists?(path).should be_true
+
+      process = Process.new(ENV["CRYSTAL_SPEC_COMPILER_BIN"]? || "bin/crystal",
+        ["external", "foo", "bar"],
+        output: :pipe,
+        env: {"PATH" => {ENV["PATH"], File.dirname(path)}.join(Process::PATH_DELIMITER)}
+      )
+      output = process.output.gets_to_end
+      status = process.wait
+      status.success?.should be_true
+      lines = output.lines
+      lines[0].should match /crystal/
+      lines[1].should match /crystal-external/
+      lines[2].should eq %(["foo", "bar"])
+    end
+  end
+end

src/compiler/crystal/command.cr

@@ -130,6 +130,9 @@ class Crystal::Command
     else
       if command.ends_with?(".cr")
         error "file '#{command}' does not exist"
+      elsif external_command = Process.find_executable("crystal-#{command}")
+        options.shift
+        Process.exec(external_command, options, env: {"CRYSTAL" => Process.executable_path})
       else
         error "unknown command: #{command}"
       end