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# cli.go

`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.

## 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](http://golang.org/doc/install.html).

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
```

## 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()`. 

``` go
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:

``` go
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.

## 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!

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

``` go
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 neat 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
```

### Arguments

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

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

### Flags

Setting and querying flags is simple.

``` go
...
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 also set a destination variable for a flag, to which the content will be scanned.

``` go
...
var language string
app.Flags = []cli.Flag {
  cli.StringFlag{
    Name:        "lang",
    Value:       "english",
    Usage:       "language for the greeting",
    Destination: &language,
  },
}
app.Action = func(c *cli.Context) {
  name := "someone"
  if len(c.Args()) > 0 {
    name = c.Args()[0]
  }
  if language == "spanish" {
    println("Hola", name)
  } else {
    println("Hello", name)
  }
}
...
```

See full list of flags at http://godoc.org/github.com/codegangsta/cli

#### Alternate Names

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

``` go
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.

#### Values from the Environment

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

``` go
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.

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

### Subcommands

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

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

### Bash Completion

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.

```go
...
var tasks = []string{"cook", "clean", "laundry", "eat", "sleep", "code"}
app := cli.NewApp()
app.EnableBashCompletion = true
app.Commands = []cli.Command{
  {
    Name:  "complete",
    Aliases: []string{"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)
      }
    },
  }
}
...
```

#### To Enable

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`

#### To Distribute

Copy `autocomplete/bash_autocomplete` into `/etc/bash_completion.d/` and rename
it to the name of the program you wish to add autocomplete support for (or
automatically install it there if you are distributing a package). Don't forget
to source the file to make it active in the current shell.

```
sudo cp src/bash_autocomplete /etc/bash_completion.d/<myprogram>
source /etc/bash_completion.d/<myprogram>
```

Alternatively, you can just document that users should source the generic
`autocomplete/bash_autocomplete` in their bash configuration with `$PROG` set
to the name of their program (as above).

## Contribution Guidelines

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 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.
