An example of a command application

This small example shows how to use commandpackage.

In the main file (see main.go) create a new command to be used as the application, in the initialization add commands to them using Àdd method. And in the mainfunction just call Main method:

// App is the current application
// (i.e. the root command).
var app = &command.Command{
  Usage: "myapp <command> [<argument>...]",
  Short: "a demonstration application for Command package",
}

// Usually commands are added in an init function.
func init() {
  // add commands
  app.Add(hello)
}

// Usually Main is reduced to run Main method of the root command.
func main() {
  app.Main()
}

The Main method will exit the program if any error happens. If you want more control, you can use Execute method directly.

The best practice is to use a file for each command (they might even be in their own package). Assign a Run function to the command, and if there are flags to be defined, define a SetFlag function (using method 'Flags' to retrieve the command's FlagSet). Document all the arguments and flags in the Long field of the command:

var hello = &command.Command{
  Usage: "hello [--utf8] [--message <message>]",
  Short: "print a greeting message",
  Long: `
Command hello prints a greeting "hello, world" message.

Flags are:

  -utf8
    Show an utf message.

  --message <message>
    Use the indicated message instead of "world" message.
  `,
  Run: func(c *command.Command, args []string) error {
    if utf {
      fmt.Fprintf(c.Stdout(), "hello, 世界\n")
      return nil
    }
    fmt.Fprintf(c.Stdout(), "hello, %s\n", msg)
    return nil
  },
  SetFlags: func(c *command.Command) {
    // Define flag usage in the Long field
    c.Flags().BoolVar(&utf, "utf8", false, "")
    c.Flags().StringVar(&msg, "message", "world", "")
  },
}

Any command can have descendant commands (see cmd.go file). While it is preferable to restrict command addition to the main application command, it is useful in some cases that descendant a command has its own descendants (for an example of an application that use that kind of design see go mod command of go tool [please note that go tool does not use command package]).

If possible, provide documentation commands (called topics) to provide additional help information (see help.go).