![cobra logo](https://cloud.githubusercontent.com/assets/173412/10886352/ad566232-814f-11e5-9cd0-aa101788c117.png)

Cobra is both a library for creating powerful modern CLI applications as well as a program to generate applications and command files.

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# Table of Contents

- [Overview](#overview)

- [Concepts](#concepts)

  * [Commands](#commands)

  * [Flags](#flags)

- [Installing](#installing)

- [Getting Started](#getting-started)

  * [Using the Cobra Generator](#using-the-cobra-generator)

  * [Using the Cobra Library](#using-the-cobra-library)

  * [Working with Flags](#working-with-flags)

  * [Positional and Custom Arguments](#positional-and-custom-arguments)

  * [Example](#example)

  * [Help Command](#help-command)

  * [Usage Message](#usage-message)

  * [PreRun and PostRun Hooks](#prerun-and-postrun-hooks)

  * [Suggestions when "unknown command" happens](#suggestions-when-unknown-command-happens)

  * [Generating documentation for your command](#generating-documentation-for-your-command)

  * [Generating bash completions](#generating-bash-completions)

- [Contributing](#contributing)

- [License](#license)

# Overview

Cobra is a library providing a simple interface to create powerful modern CLI

interfaces similar to git & go tools.

Cobra is also an application that will generate your application scaffolding to rapidly

develop a Cobra-based application.

Cobra provides:

* Easy subcommand-based CLIs: `app server`, `app fetch`, etc.

* Fully POSIX-compliant flags (including short & long versions)

* Nested subcommands

* Global, local and cascading flags

* Easy generation of applications & commands with `cobra init appname` & `cobra add cmdname`

* Intelligent suggestions (`app srver`... did you mean `app server`?)

* Automatic help generation for commands and flags

* Automatic help flag recognition of `-h`, `--help`, etc.

* Automatically generated bash autocomplete for your application

* Automatically generated man pages for your application

* Command aliases so you can change things without breaking them

* The flexibility to define your own help, usage, etc.

* Optional tight integration with [viper](http://github.com/spf13/viper) for 12-factor apps

# Concepts

Cobra is built on a structure of commands, arguments & flags.

**Commands** represent actions, **Args** are things and **Flags** are modifiers for those actions.

The best applications will read like sentences when used. Users will know how

to use the application because they will natively understand how to use it.

The pattern to follow is

`APPNAME VERB NOUN --ADJECTIVE.`

    or

`APPNAME COMMAND ARG --FLAG`

A few good real world examples may better illustrate this point.

In the following example, 'server' is a command, and 'port' is a flag:

    hugo server --port=1313

In this command we are telling Git to clone the url bare.

    git clone URL --bare

## Commands

Command is the central point of the application. Each interaction that

the application supports will be contained in a Command. A command can

have children commands and optionally run an action.

In the example above, 'server' is the command.

[More about cobra.Command](https://godoc.org/github.com/spf13/cobra#Command)

## Flags

A flag is a way to modify the behavior of a command. Cobra supports

fully POSIX-compliant flags as well as the Go [flag package](https://golang.org/pkg/flag/).

A Cobra command can define flags that persist through to children commands

and flags that are only available to that command.

In the example above, 'port' is the flag.

Flag functionality is provided by the [pflag

library](https://github.com/spf13/pflag), a fork of the flag standard library

which maintains the same interface while adding POSIX compliance.

# Installing

Using Cobra is easy. First, use `go get` to install the latest version

of the library. This command will install the `cobra` generator executable

along with the library and its dependencies:

    go get -u github.com/spf13/cobra/cobra

Next, include Cobra in your application:

```go

import "github.com/spf13/cobra"

```

# Getting Started

While you are welcome to provide your own organization, typically a Cobra-based

application will follow the following organizational structure:

```

  ▾ appName/

    ▾ cmd/

        add.go

        your.go

        commands.go

        here.go

      main.go

```

In a Cobra app, typically the main.go file is very bare. It serves one purpose: initializing Cobra.

```go

package main

import (

  "{pathToYourApp}/cmd"

)

func main() {

  cmd.Execute()

}

```

## Using the Cobra Generator

Cobra provides its own program that will create your application and add any

commands you want. It's the easiest way to incorporate Cobra into your application.

[Here](https://github.com/spf13/cobra/blob/master/cobra/README.md) you can find more information about it.

## Using the Cobra Library

To manually implement Cobra you need to create a bare main.go file and a rootCmd file.

You will optionally provide additional commands as you see fit.

### Create rootCmd

Cobra doesn't require any special constructors. Simply create your commands.

Ideally you place this in app/cmd/root.go:

```go

var rootCmd = &cobra.Command{

  Use:   "hugo",

  Short: "Hugo is a very fast static site generator",

  Long: `A Fast and Flexible Static Site Generator built with

                love by spf13 and friends in Go.

                Complete documentation is available at http://hugo.spf13.com`,

  Run: func(cmd *cobra.Command, args []string) {

    // Do Stuff Here

  },

}

func Execute() {

  if err := rootCmd.Execute(); err != nil {

    fmt.Println(err)

    os.Exit(1)

  }

}

```

You will additionally define flags and handle configuration in your init() function.

For example cmd/root.go:

```go

import (

  "fmt"

  "os"

  homedir "github.com/mitchellh/go-homedir"

  "github.com/spf13/cobra"

  "github.com/spf13/viper"

)

func init() {

  cobra.OnInitialize(initConfig)

  rootCmd.PersistentFlags().StringVar(&cfgFile, "config", "", "config file (default is $HOME/.cobra.yaml)")

  rootCmd.PersistentFlags().StringVarP(&projectBase, "projectbase", "b", "", "base project directory eg. github.com/spf13/")

  rootCmd.PersistentFlags().StringP("author", "a", "YOUR NAME", "Author name for copyright attribution")

  rootCmd.PersistentFlags().StringVarP(&userLicense, "license", "l", "", "Name of license for the project (can provide `licensetext` in config)")

  rootCmd.PersistentFlags().Bool("viper", true, "Use Viper for configuration")

  viper.BindPFlag("author", rootCmd.PersistentFlags().Lookup("author"))

  viper.BindPFlag("projectbase", rootCmd.PersistentFlags().Lookup("projectbase"))

  viper.BindPFlag("useViper", rootCmd.PersistentFlags().Lookup("viper"))

  viper.SetDefault("author", "NAME HERE <EMAIL ADDRESS>")

  viper.SetDefault("license", "apache")

}

func initConfig() {

  // Don't forget to read config either from cfgFile or from home directory!

  if cfgFile != "" {

    // Use config file from the flag.

    viper.SetConfigFile(cfgFile)

  } else {

    // Find home directory.

    home, err := homedir.Dir()

    if err != nil {

      fmt.Println(err)

      os.Exit(1)

    }

    // Search config in home directory with name ".cobra" (without extension).

    viper.AddConfigPath(home)

    viper.SetConfigName(".cobra")

  }

  if err := viper.ReadInConfig(); err != nil {

    fmt.Println("Can't read config:", err)

    os.Exit(1)

  }

}

```

### Create your main.go

With the root command you need to have your main function execute it.

Execute should be run on the root for clarity, though it can be called on any command.

In a Cobra app, typically the main.go file is very bare. It serves, one purpose, to initialize Cobra.

```go

package main

import (

  "{pathToYourApp}/cmd"

)

func main() {

  cmd.Execute()

}

```

### Create additional commands

Additional commands can be defined and typically are each given their own file

inside of the cmd/ directory.

If you wanted to create a version command you would create cmd/version.go and

populate it with the following:

```go

package cmd

import (

  "fmt"

  "github.com/spf13/cobra"

)

func init() {

  rootCmd.AddCommand(versionCmd)

}

var versionCmd = &cobra.Command{

  Use:   "version",

  Short: "Print the version number of Hugo",

  Long:  `All software has versions. This is Hugo's`,

  Run: func(cmd *cobra.Command, args []string) {

    fmt.Println("Hugo Static Site Generator v0.9 -- HEAD")

  },

}

```

## Working with Flags

Flags provide modifiers to control how the action command operates.

### Assign flags to a command

Since the flags are defined and used in different locations, we need to

define a variable outside with the correct scope to assign the flag to

work with.

```go

var Verbose bool

var Source string

```

There are two different approaches to assign a flag.

### Persistent Flags

A flag can be 'persistent' meaning that this flag will be available to the

command it's assigned to as well as every command under that command. For

global flags, assign a flag as a persistent flag on the root.

```go

rootCmd.PersistentFlags().BoolVarP(&Verbose, "verbose", "v", false, "verbose output")

```

### Local Flags

A flag can also be assigned locally which will only apply to that specific command.

```go

```

### Local Flag on Parent Commands

By default Cobra only parses local flags on the target command, any local flags on

parent commands are ignored. By enabling `Command.TraverseChildren` Cobra will

parse local flags on each command before executing the target command.

```go

command := cobra.Command{

  Use: "print [OPTIONS] [COMMANDS]",

  TraverseChildren: true,

}

```

### Bind Flags with Config

You can also bind your flags with [viper](https://github.com/spf13/viper):

```go

var author string

func init() {

  rootCmd.PersistentFlags().StringVar(&author, "author", "YOUR NAME", "Author name for copyright attribution")

  viper.BindPFlag("author", rootCmd.PersistentFlags().Lookup("author"))

}

```

In this example the persistent flag `author` is bound with `viper`.

**Note**, that the variable `author` will not be set to the value from config,

when the `--author` flag is not provided by user.

More in [viper documentation](https://github.com/spf13/viper#working-with-flags).

### Required flags

Flags are optional by default. If instead you wish your command to report an error

when a flag has not been set, mark it as required:

```go

rootCmd.Flags().StringVarP(&Region, "region", "r", "", "AWS region (required)")

rootCmd.MarkFlagRequired("region")

```

## Positional and Custom Arguments

Validation of positional arguments can be specified using the `Args` field

of `Command`.

The following validators are built in:

- `NoArgs` - the command will report an error if there are any positional args.

- `ArbitraryArgs` - the command will accept any args.

- `OnlyValidArgs` - the command will report an error if there are any positional args that are not in the `ValidArgs` field of `Command`.

- `MinimumNArgs(int)` - the command will report an error if there are not at least N positional args.

- `MaximumNArgs(int)` - the command will report an error if there are more than N positional args.

- `ExactArgs(int)` - the command will report an error if there are not exactly N positional args.

- `RangeArgs(min, max)` - the command will report an error if the number of args is not between the minimum and maximum number of expected args.

An example of setting the custom validator:

```go

var cmd = &cobra.Command{

  Short: "hello",

  Args: func(cmd *cobra.Command, args []string) error {

    if len(args) < 1 {

    }

    if myapp.IsValidColor(args[0]) {

      return nil

    }

    return fmt.Errorf("invalid color specified: %s", args[0])

  },

  Run: func(cmd *cobra.Command, args []string) {

    fmt.Println("Hello, World!")

  },

}

```

## Example

In the example below, we have defined three commands. Two are at the top level

and one (cmdTimes) is a child of one of the top commands. In this case the root

is not executable meaning that a subcommand is required. This is accomplished

by not providing a 'Run' for the 'rootCmd'.

We have only defined one flag for a single command.

More documentation about flags is available at https://github.com/spf13/pflag

```go

package main

import (

  "fmt"

  "strings"

  "github.com/spf13/cobra"

)

func main() {

  var echoTimes int

  var cmdPrint = &cobra.Command{

    Use:   "print [string to print]",

    Short: "Print anything to the screen",

    Long: `print is for printing anything back to the screen.

For many years people have printed back to the screen.`,

    Args: cobra.MinimumNArgs(1),

    Run: func(cmd *cobra.Command, args []string) {

      fmt.Println("Print: " + strings.Join(args, " "))

    },

  }

  var cmdEcho = &cobra.Command{

    Use:   "echo [string to echo]",

    Short: "Echo anything to the screen",

    Long: `echo is for echoing anything back.

Echo works a lot like print, except it has a child command.`,

    Args: cobra.MinimumNArgs(1),

    Run: func(cmd *cobra.Command, args []string) {

      fmt.Println("Print: " + strings.Join(args, " "))