humanlog: support force color mode with --color=yes

For piping the output of humanlog to 'less -SR' and still getting nice
colors.

cmd/humanlog/main.go

@@ -72,6 +72,12 @@ func newApp() *cli.App {
 		Value: humanlog.DefaultOptions.TruncateLength,
 	}
 
+	colorFlag := cli.StringFlag{
+		Name:  "color",
+		Usage: "specify color mode: auto, on/force, off",
+		Value: "auto",
+	}
+
 	lightBg := cli.BoolFlag{
 		Name:  "light-bg",
 		Usage: "use black as the base foreground color (for terminals with light backgrounds)",
@@ -119,7 +125,7 @@ func newApp() *cli.App {
 	app.Version = Version
 	app.Usage = "reads structured logs from stdin, makes them pretty on stdout!"
 
-	app.Flags = []cli.Flag{skipFlag, keepFlag, sortLongest, skipUnchanged, truncates, truncateLength, lightBg, timeFormat, ignoreInterrupts, messageFieldsFlag, timeFieldsFlag, levelFieldsFlag}
+	app.Flags = []cli.Flag{skipFlag, keepFlag, sortLongest, skipUnchanged, truncates, truncateLength, colorFlag, lightBg, timeFormat, ignoreInterrupts, messageFieldsFlag, timeFieldsFlag, levelFieldsFlag}
 
 	app.Action = func(c *cli.Context) error {
 
@@ -130,6 +136,10 @@ func newApp() *cli.App {
 		opts.TruncateLength = c.Int(truncateLength.Name)
 		opts.LightBg = c.BoolT(lightBg.Name)
 		opts.TimeFormat = c.String(timeFormat.Name)
+		var err error
+		if opts.ColorFlag, err = humanlog.GrokColorMode(c.String(colorFlag.Name)); err != nil {
+			fatalf(c, "bad --%s value: %s", colorFlag.Name, err.Error())
+		}
 
 		switch {
 		case c.IsSet(skipFlag.Name) && c.IsSet(keepFlag.Name):
@@ -156,6 +166,8 @@ func newApp() *cli.App {
 			signal.Ignore(os.Interrupt)
 		}
 
+		opts.ColorFlag.Apply()
+
 		log.Print("reading stdin...")
 		if err := humanlog.Scanner(os.Stdin, colorable.NewColorableStdout(), opts); err != nil {
 			log.Fatalf("scanning caught an error: %v", err)

handler.go

@@ -1,6 +1,8 @@
 package humanlog
 
 import (
+	"fmt"
+	"strings"
 	"time"
 
 	"github.com/fatih/color"
@@ -18,6 +20,7 @@ var DefaultOptions = &HandlerOptions{
 	SortLongest:    true,
 	SkipUnchanged:  true,
 	Truncates:      true,
+	ColorFlag:      ColorModeAuto,
 	LightBg:        false,
 	TruncateLength: 15,
 	TimeFormat:     time.Stamp,
@@ -43,6 +46,39 @@ var DefaultOptions = &HandlerOptions{
 	UnknownLevelColor:     color.New(color.FgMagenta),
 }
 
+type ColorMode int
+
+const (
+	ColorModeOff ColorMode = iota
+	ColorModeOn
+	ColorModeAuto
+)
+
+func GrokColorMode(colorMode string) (ColorMode, error) {
+	switch strings.ToLower(colorMode) {
+	case "on", "always", "force", "true", "yes", "1":
+		return ColorModeOn, nil
+	case "off", "never", "false", "no", "0":
+		return ColorModeOff, nil
+	case "auto", "tty", "maybe", "":
+		return ColorModeAuto, nil
+	default:
+		return ColorModeAuto, fmt.Errorf("'%s' is not a color mode (try 'on', 'off' or 'auto')", colorMode)
+	}
+}
+
+func (colorMode ColorMode) Apply() {
+	switch colorMode {
+	case ColorModeOff:
+		color.NoColor = true
+	case ColorModeOn:
+		color.NoColor = false
+	default:
+		// 'Auto' default is applied as a global variable initializer function, so nothing
+		// to do here.
+	}
+}
+
 type HandlerOptions struct {
 	Skip map[string]struct{}
 	Keep map[string]struct{}
@@ -55,6 +91,7 @@ type HandlerOptions struct {
 	SkipUnchanged  bool
 	Truncates      bool
 	LightBg        bool
+	ColorFlag      ColorMode
 	TruncateLength int
 	TimeFormat     string