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.

You can view the API docs here: https://godoc.org/github.com/codegangsta/cli

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 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"
  "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"
  "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!

Start by creating a directory named greet, and within it, add a file, greet.go with the following code in it:

package main

import (
  "os"
  "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{
    Name: "lang",
    Value: "english",
    Usage: "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)
  }
}
...

You can set alternate (or short) names for flags by providing a comma-delimited list for the Name. e.g.

app.Flags = []cli.Flag {
  cli.StringFlag{
    Name: "lang, l",
    Value: "english",
    Usage: "language for the greeting",
  },
}

That flag can then be set with --lang spanish or -l spanish. Note that giving two different forms of the same flag in the same command invocation is an error.

You can also have the default value set from the environment via EnvVar. e.g.

app.Flags = []cli.Flag {
  cli.StringFlag{
    Name: "lang, l",
    Value: "english",
    Usage: "language for the greeting",
    EnvVar: "APP_LANG",
  },
}

The EnvVar may also be given as a comma-delimited "cascade", where the first environment variable that resolves is used as the default.

app.Flags = []cli.Flag {
  cli.StringFlag{
    Name: "lang, l",
    Value: "english",
    Usage: "language for the greeting",
    EnvVar: "LEGACY_COMPAT_LANG,APP_LANG,LANG",
  },
}

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().First())
    },
  },
  {
    Name:      "complete",
    ShortName: "c",
    Usage:     "complete a task on the list",
    Action: func(c *cli.Context) {
      println("completed task: ", c.Args().First())
    },
  },
  {
    Name:      "template",
    ShortName: "r",
    Usage:     "options for task templates",
    Subcommands: []cli.Command{
      {
        Name:  "add",
        Usage: "add a new template",
        Action: func(c *cli.Context) {
            println("new task template: ", c.Args().First())
        },
      },
      {
        Name:  "remove",
        Usage: "remove an existing template",
        Action: func(c *cli.Context) {
          println("removed task template: ", c.Args().First())
        },
      },
    },
  },     
}
...

You can enable completion commands by setting the EnableBashCompletion flag on the App object. By default, this setting will only auto-complete to show an app's subcommands, but you can write your own completion methods for the App or its subcommands.

...
var tasks = []string{"cook", "clean", "laundry", "eat", "sleep", "code"}
app := cli.NewApp()
app.EnableBashCompletion = true
app.Commands = []cli.Command{
  {
    Name: "complete",
    ShortName: "c",
    Usage: "complete a task on the list",
    Action: func(c *cli.Context) {
       println("completed task: ", c.Args().First())
    },
    BashComplete: func(c *cli.Context) {
      // This will complete if no args are passed
      if len(c.Args()) > 0 {
        return
      }
      for _, t := range tasks {
        fmt.Println(t)
      }
    },
  }
}
...

Source the autocomplete/bash_autocomplete file in your .bashrc file while setting the PROG variable to the name of your program:

PROG=myprogram source /.../cli/autocomplete/bash_autocomplete

Feel free to put up a pull request to fix a bug or maybe add a feature. I will give it a code review and make sure that it does not break backwards compatibility. If I or any other collaborators agree that it is in line with the vision of the project, we will work with you to get the code into a mergeable state and merge it into the master branch.

If you are have contributed something significant to the project, I will most likely add you as a collaborator. As a collaborator you are given the ability to merge others pull requests. It is very important that new code does not break existing code, so be careful about what code you do choose to merge. If you have any questions feel free to link @codegangsta to the issue in question and we can review it together.

If you feel like you have contributed to the project but have not yet been added as a collaborator, I probably forgot to add you. Hit @codegangsta up over email and we will get it figured out.