cli.go is simple, fast, and fun package for building command line apps in Go. The goal is to enable developers to write fast and distributable command line applications in an expressive way.

Command line apps are usually so tiny that there is absolutely no reason why your code should not be self-documenting. Things like generating help text and parsing command flags/options should not hinder productivity when writing a command line app.

This is where cli.go comes into play. cli.go makes command line programming fun, organized, and expressive!

Make sure you have the a working Go environment (go 1.1 is required). See the install instructions.

To install cli.go, simply run:

$ go get github.com/codegangsta/cli

Make sure your PATH includes to the $GOPATH/bin directory so your commands can be easily used:

export PATH=$PATH:$GOPATH/bin

One of the philosophies behind cli.go is that an API should be playful and full of discovery. So a cli.go app can be as little as one line of code in main().

package main

import "os"
import "github.com/codegangsta/cli"

func main() {
  cli.NewApp().Run(os.Args)
}

This app will run and show help text, but is not very useful. Let's give an action to execute and some help documentation:

package main

import "os"
import "github.com/codegangsta/cli"

func main() {
  app := cli.NewApp()
  app.Name = "boom"
  app.Usage = "make an explosive entrance"
  app.Action = func(c *cli.Context) {
    println("boom! I say!")
  }
  
  app.Run(os.Args)
}

Running this already gives you a ton of functionality, plus support for things like subcommands and flags, which are covered below.

Being a programmer can be a lonely job. Thankfully by the power of automation that is not the case! Let's create a greeter app to fend off our demons of loneliness!

/* greet.go */
package main

import "os"
import "github.com/codegangsta/cli"

func main() {
  app := cli.NewApp()
  app.Name = "greet"
  app.Usage = "fight the loneliness!"
  app.Action = func(c *cli.Context) {
    println("Hello friend!")
  }
  
  app.Run(os.Args)
}

Install our command to the $GOPATH/bin directory:

$ go install

Finally run our new command:

$ greet
Hello friend!

cli.go also generates some bitchass help text:

$ greet help
NAME:
    greet - fight the loneliness!

USAGE:
    greet [global options] command [command options] [arguments...]

VERSION:
    0.0.0

COMMANDS:
    help, h  Shows a list of commands or help for one command

GLOBAL OPTIONS
    --version	Shows version information

You can lookup arguments by calling the Args function on cli.Context.

...
app.Action = func(c *cli.Context) {
  println("Hello", c.Args()[0])
}
...

Setting and querying flags is simple.

...
app.Flags = []cli.Flag {
  cli.StringFlag{"lang", "english", "language for the greeting"},
}
app.Action = func(c *cli.Context) {
  name := "someone"
  if len(c.Args()) > 0 {
    name = c.Args()[0]
  }
  if c.String("lang") == "spanish" {
    println("Hola", name)
  } else {
    println("Hello", name)
  }
}
...

Subcommands can be defined for a more git-like command line app.

...
app.Commands = []cli.Command{
  {
    Name:      "add",
    ShortName: "a",
    Usage:     "add a task to the list",
    Action: func(c *cli.Context) {
      println("added task: ", c.Args()[0])
    },
  },
  {
    Name:      "complete",
    ShortName: "c",
    Usage:     "complete a task on the list",
    Action: func(c *cli.Context) {
      println("completed task: ", c.Args()[0])
    },
  },
}
...

cli.go is written by none other than the Code Gangsta