considering stderr in UsageString

command.go

@@ -384,13 +384,22 @@ func (c *Command) Help() error {
 	return nil
 }
 
-// UsageString return usage string.
+// UsageString returns usage string.
 func (c *Command) UsageString() string {
+	// Storing normal writers
 	tmpOutput := c.outWriter
+	tmpErr := c.errWriter
+
 	bb := new(bytes.Buffer)
 	c.outWriter = bb
+	c.errWriter = bb
+
 	c.Usage()
+
+	// Setting things back to normal
 	c.outWriter = tmpOutput
+	c.errWriter = tmpErr
+
 	return bb.String()
 }
 

command_test.go

@@ -1405,6 +1405,22 @@ func TestSetIn(t *testing.T) {
 	}
 }
 
+func TestUsageStringRedirected(t *testing.T) {
+	c := &Command{}
+
+	c.usageFunc = func(cmd *Command) error {
+		cmd.Print("[stdout1]")
+		cmd.PrintErr("[stderr2]")
+		cmd.Print("[stdout3]")
+		return nil;
+	}
+
+	expected := "[stdout1][stderr2][stdout3]"
+	if 	got := c.UsageString(); got != expected {
+		t.Errorf("Expected usage string to consider both stdout and stderr")
+	}
+}
+
 func TestFlagErrorFunc(t *testing.T) {
 	c := &Command{Use: "c", Run: emptyRun}