cli.go

cli.go is a 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.

Note: This package provides a subset of the functionality provided by the original cli.go package created by Codegangsta. It simplies the API, reformats the help output, and adds some minor functionality.

Overview

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!

Installation

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/davelaursen/cli

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

export PATH=$PATH:$GOPATH/bin

Getting Started

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/davelaursen/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/davelaursen/cli"
)

func main() {
  app := cli.NewApp()
  app.Name = "boom"
  app.Summary = "make an explosive entrance"
  app.Usage = "boom [help]"
  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 flags, which are covered below.

Example

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"
  "github.com/davelaursen/cli"
)

func main() {
  app := cli.NewApp()
  app.Name = "greet"
  app.Summary = "fight the loneliness!" 
  app.Usage = "greet [help]"
  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
greet, v0.0.0 - fight the loneliness!

USAGE:
   greet [help]

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

OPTIONS:
   --version  print the version

Arguments

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

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

Flags

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)
  }
}
...

Alternate Names

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",
  },
}

Values from the Environment

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",
  },
}

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.