config: Added CommandPreProc (allows shell vars)

app.go

@@ -236,6 +236,15 @@ func (a *App) addCommand(cmd *Command, addHelpFlag bool) {
 
 // RunCommand runs a single command.
 func (a *App) RunCommand(args []string) error {
+	if a.config.CommandPreProc != nil {
+		var err error
+
+		args, err = a.config.CommandPreProc(args)
+		if err != nil {
+			return fmt.Errorf("command preprocessor failed - %w", err)
+		}
+	}
+
 	// Parse the arguments string and obtain the command path to the root,
 	// and the command flags.
 	cmds, fg, args, err := a.commands.parse(args, a.flagMap, false)

config.go

@@ -74,8 +74,23 @@ type Config struct {
 	HelpSubCommands       bool
 	HelpHeadlineColor     *color.Color
 
-	// Override default iterrupt handler
+	// Override default iterrupt handler.
 	InterruptHandler func(a *App, count int)
+
+	// CommandPreProc is an optional function that allows callers
+	// to modify the command string and its arguments prior to
+	// looking up the command code and executing it. This field
+	// is useful for implementing variable replacement using
+	// functionality like os.Expand (ps: "PreProc" here means
+	// "preprocessor").
+	//
+	// This function receives the list of strings typed by the user
+	// and returns the new list of strings to use in the RunCommand
+	// method. An optional error can be returned as well which will
+	// stop the command from being executed.
+	//
+	// This field is ignored if it is nil.
+	CommandPreProc func(args []string) ([]string, error)
 }
 
 // SetDefaults sets the default values if not set.