check-if-cobra-flag-was-set.md

Check If Cobra Flag Was Set

When using Cobra to define a CLI, we can specify a flag for a command like so:

var Seed int64
myCmd.PersistentFlags().Int64VarP(&Seed, "seed", "", -1, "set a seed")

This --seed flag has a default of -1. If the flag isn't specified, then when we access that flag's value, we'll get -1.

But how do we differentiate between the default -1 and someone passing -1 to the --seed flag when running the program?

In the command definition, we can look at the flags and see, by name, if specific ones were changed by user input rather than being the defaults.

myCommand := &cobra.Command{
	// coommand setup ...
	Run: func(cmd *cobra.Command, args []string) {
		if cmd.Flags().Changed("seed") {
			seed, err := cmd.Flags().GetInt64("seed")
			if err != nil {
				fmt.Println("Seed flag is missing from `cmdFlags()`")
				os.Exit(1)
			}

			fmt.Printf("Seed was set to %d\n", seed)
		} else {
			fmt.Println("Seed was not set")
		}
	}
}

If we don't want to rely on the default and instead want to specify some other behavior when the flag is not manually set by the user, we can detect that scenario like this.