Add support for --version flag (#584)

command.go

@@ -75,6 +75,11 @@ type Command struct {
 	// group commands.
 	Annotations map[string]string
 
+	// Version defines the version for this command. If this value is non-empty and the command does not
+	// define a "version" flag, a "version" boolean flag will be added to the command and, if specified,
+	// will print content of the "Version" variable.
+	Version string
+
 	// The *Run functions are executed in the following order:
 	//   * PersistentPreRun()
 	//   * PreRun()
@@ -177,6 +182,8 @@ type Command struct {
 	// helpCommand is command with usage 'help'. If it's not defined by user,
 	// cobra uses default help command.
 	helpCommand *Command
+	// versionTemplate is the version template defined by user.
+	versionTemplate string
 }
 
 // SetArgs sets arguments for the command. It is set to os.Args[1:] by default, if desired, can be overridden
@@ -222,6 +229,11 @@ func (c *Command) SetHelpTemplate(s string) {
 	c.helpTemplate = s
 }
 
+// SetVersionTemplate sets version template to be used. Application can use it to set custom template.
+func (c *Command) SetVersionTemplate(s string) {
+	c.versionTemplate = s
+}
+
 // SetGlobalNormalizationFunc sets a normalization function to all flag sets and also to child commands.
 // The user should not have a cyclic dependency on commands.
 func (c *Command) SetGlobalNormalizationFunc(n func(f *flag.FlagSet, name string) flag.NormalizedName) {
@@ -411,6 +423,19 @@ func (c *Command) HelpTemplate() string {
 {{end}}{{if or .Runnable .HasSubCommands}}{{.UsageString}}{{end}}`
 }
 
+// VersionTemplate return version template for the command.
+func (c *Command) VersionTemplate() string {
+	if c.versionTemplate != "" {
+		return c.versionTemplate
+	}
+
+	if c.HasParent() {
+		return c.parent.VersionTemplate()
+	}
+	return `{{with .Name}}{{printf "%s " .}}{{end}}{{printf "version %s" .Version}}
+`
+}
+
 func hasNoOptDefVal(name string, fs *flag.FlagSet) bool {
 	flag := fs.Lookup(name)
 	if flag == nil {
@@ -640,9 +665,10 @@ func (c *Command) execute(a []string) (err error) {
 		c.Printf("Command %q is deprecated, %s\n", c.Name(), c.Deprecated)
 	}
 
-	// initialize help flag as the last point possible to allow for user
+	// initialize help and version flag at the last point possible to allow for user
 	// overriding
 	c.InitDefaultHelpFlag()
+	c.InitDefaultVersionFlag()
 
 	err = c.ParseFlags(a)
 	if err != nil {
@@ -663,6 +689,22 @@ func (c *Command) execute(a []string) (err error) {
 		return flag.ErrHelp
 	}
 
+	// for back-compat, only add version flag behavior if version is defined
+	if c.Version != "" {
+		versionVal, err := c.Flags().GetBool("version")
+		if err != nil {
+			c.Println("\"version\" flag declared as non-bool. Please correct your code")
+			return err
+		}
+		if versionVal {
+			err := tmpl(c.OutOrStdout(), c.VersionTemplate(), c)
+			if err != nil {
+				c.Println(err)
+			}
+			return err
+		}
+	}
+
 	c.preRun()
 
 	argWoFlags := c.Flags().Args()
@@ -848,6 +890,27 @@ func (c *Command) InitDefaultHelpFlag() {
 	}
 }
 
+// InitDefaultVersionFlag adds default version flag to c.
+// It is called automatically by executing the c.
+// If c already has a version flag, it will do nothing.
+// If c.Version is empty, it will do nothing.
+func (c *Command) InitDefaultVersionFlag() {
+	if c.Version == "" {
+		return
+	}
+
+	c.mergePersistentFlags()
+	if c.Flags().Lookup("version") == nil {
+		usage := "version for "
+		if c.Name() == "" {
+			usage += "this command"
+		} else {
+			usage += c.Name()
+		}
+		c.Flags().Bool("version", false, usage)
+	}
+}
+
 // InitDefaultHelpCmd adds default help command to c.
 // It is called automatically by executing the c or by calling help and usage.
 // If c already has help command or c has no subcommands, it will do nothing.