Commandeer

Commandeer sets up command line flags based on struct fields and tags.

Do you...

• like to develop Go apps as libraries with tiny main packages?
• get frustrated keeping your flags up to date as your code evolves?
• feel irked by the overlap between comments on struct fields and help strings for flags?
• hate switching between your app's main and library packages?

You might like Commandeer. See the godoc for detailed usage, or just...

Try It!

Here's how it works, define your app like so:

package myapp

import "fmt"

type Main struct {
	Num     int    `help:"How many does it take?"`
	Vehicle string `help:"What did they get?"`
}

func NewMain() *Main { return &Main{Num: 5, Vehicle: "jeep"} }

func (m *Main) Run() error {
	if m.Num < 2 || m.Vehicle == "" {
		return fmt.Errorf("Need more gophers and/or vehicles.")
	}
	fmt.Printf("%d gophers stole my %s!\n", m.Num, m.Vehicle)
	return nil
}

and your main package:

package main

import (
	"fmt"

	"github.com/jaffee/commandeer"
	"github.com/jaffee/commandeer/examples/myapp"
)

func main() {
	err := commandeer.Run(myapp.NewMain())
	if err != nil {
		fmt.Println(err)
	}
}

Now...

$ ./myapp -h
Usage of ./myapp:
  -num int
    	How many does it take? (default 5)
  -vehicle string
    	What did they get? (default "jeep")

$ ./myapp
5 gophers stole my jeep!
$ ./myapp -num 3 -vehicle horse
3 gophers stole my horse!

Notice that Commandeer set up the default values for each flag based on the values in the struct passed to Run.

Commandeer is set up for minimal dependency pollution - it uses only stdlib dependencies and is a few hundred lines of code itself. You need only import it from a tiny main package (as in the example), and shouldn't need to reference it anywhere else.

If you aren't allergic to external dependencies, you can also try github.com/jaffee/commandeer/cobrafy which pulls in the excellent Cobra and pflag packages giving you GNU/POSIX style flags and some other nice features should you care to use them. See the godoc, or the myapp-cobrafy example.

Features

In addition to the help struct tag, you can use flag to override the computed flag name, e.g.

type Main struct {
	Num     int    `flag:"number"`
}

You can also use flag:"-" to explicitly ignore fields from being used as flags, e.g.

type Main struct {
	Num     int    `flag:"-"`
}

Nested structs are supported, by default the field names will be joined with "." to create the flag name, e.g.

type Main struct {
	Vehicle struct {
		Color string
		Weight int
	}
}

produces:

  -vehicle.color string
    	
  -vehicle.weight int

If you wish to avoid this prefix behavior (e.g. if you have an embedded struct field and you want to elevate its fields to the top level) you can use flag:"!embed", e.g.

type Main struct {
	Vehicle struct {
		Color string
		Weight int
	} `flag:"!embed"`
}

which will produce:

  -color string
    	
  -weight int

Contributing

Yes please!

For small stuff, feel free to submit a PR directly. For larger things, especially API changes, it's best to make an issue first so it can be discussed.

Similar projects

• https://github.com/anacrolix/tagflag
• https://github.com/octago/sflags