Added Shell::Basic#say_error, which is like #say but prints to stderr.

* Allows for printing ANSI colored error messages to stderr.

Author: postmodern

lib/thor/shell/basic.rb

@@ -103,6 +103,23 @@ def say(message = "", color = nil, force_new_line = (message.to_s !~ /( |\t)\Z/)
         stdout.flush
       end
 
+      # Say (print) an error to the user. If the sentence ends with a whitespace
+      # or tab character, a new line is not appended (print + flush). Otherwise
+      # are passed straight to puts (behavior got from Highline).
+      #
+      # ==== Example
+      # say_error("error: something went wrong")
+      #
+      def say_error(message = "", color = nil, force_new_line = (message.to_s !~ /( |\t)\Z/))
+        return if quiet?
+
+        buffer = prepare_message(message, *color)
+        buffer << "\n" if force_new_line && !message.to_s.end_with?("\n")
+
+        stderr.print(buffer)
+        stderr.flush
+      end
+
       # Say a status with the given color and appends the message. Since this
       # method is used frequently by actions, it allows nil or false to be given
       # in log_status, avoiding the message from being shown. If a Symbol is

spec/shell/basic_spec.rb

@@ -190,6 +190,48 @@ def shell
     end
   end
 
+  describe "#say_error" do
+    it "prints a message to the user" do
+      expect($stderr).to receive(:print).with("Running...\n")
+      shell.say_error("Running...")
+    end
+
+    it "prints a message to the user without new line if it ends with a whitespace" do
+      expect($stderr).to receive(:print).with("Running... ")
+      shell.say_error("Running... ")
+    end
+
+    it "does not use a new line with whitespace+newline embedded" do
+      expect($stderr).to receive(:print).with("It's \nRunning...\n")
+      shell.say_error("It's \nRunning...")
+    end
+
+    it "prints a message to the user without new line" do
+      expect($stderr).to receive(:print).with("Running...")
+      shell.say_error("Running...", nil, false)
+    end
+
+    it "coerces everything to a string before printing" do
+      expect($stderr).to receive(:print).with("this_is_not_a_string\n")
+      shell.say_error(:this_is_not_a_string, nil, true)
+    end
+
+    it "does not print a message if muted" do
+      expect($stderr).not_to receive(:print)
+      shell.mute do
+        shell.say_error("Running...")
+      end
+    end
+
+    it "does not print a message if base is set to quiet" do
+      shell.base = MyCounter.new [1, 2]
+      expect(shell.base).to receive(:options).and_return(:quiet => true)
+
+      expect($stderr).not_to receive(:print)
+      shell.say_error("Running...")
+    end
+  end
+
   describe "#print_wrapped" do
     let(:message) do
       "Creates a back-up of the given folder by compressing it in a .tar.gz\n"\