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

Many of the most widely used Go projects are built using Cobra including:

• Kubernetes
• Hugo
• rkt
• etcd
• Moby (former Docker)
• Docker (distribution)
• OpenShift
• Delve
• GopherJS
• CockroachDB
• Bleve
• ProjectAtomic (enterprise)
• GiantSwarm's swarm
• Nanobox/Nanopack
• rclone
• nehm

Table of Contents

• Overview
• Concepts
  • Commands
  • Flags
• Installing
• Getting Started
  • Using the Cobra Generator
  • Using the Cobra Library
  • Working with Flags
  • Positional and Custom Arguments
  • Example
  • Help Command
  • Usage Message
  • PreRun and PostRun Hooks
  • Suggestions when "unknown command" happens
  • Generating documentation for your command
  • Generating bash completions
• Contributing
• 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 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

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. 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, 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:

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.

package main

import (
  "fmt"
  "os"

  "{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 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:

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
  },
}

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

For example cmd/root.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.

package main

import (
  "fmt"
  "os"

  "{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:

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.

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.

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.

rootCmd.Flags().StringVarP(&Source, "source", "s", "", "Source directory to read from")

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.

command := cobra.Command{
  Use: "print [OPTIONS] [COMMANDS]",
  TraverseChildren: true,
}

Bind Flags with Config

You can also bind your flags with viper:

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.

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:

var cmd = &cobra.Command{
  Short: "hello",
  Args: func(cmd *cobra.Command, args []string) error {
    if len(args) < 1 {
      return errors.New("requires at least one arg")
    }
    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

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, " "))
    },
  }

  var cmdTimes = &cobra.Command{
    Use:   "times [# times] [string to echo]",
    Short: "Echo anything to the screen more times",
    Long: `echo things multiple times back to the user by providing
a count and a string.`,
    Args: cobra.MinimumNArgs(1),
    Run: func(cmd *cobra.Command, args []string) {
      for i := 0; i < echoTimes; i++ {
        fmt.Println("Echo: " + strings.Join(args, " "))
      }
    },
  }

  cmdTimes.Flags().IntVarP(&echoTimes, "times", "t", 1, "times to echo the input")

  var rootCmd = &cobra.Command{Use: "app"}
  rootCmd.AddCommand(cmdPrint, cmdEcho)
  cmdEcho.AddCommand(cmdTimes)
  rootCmd.Execute()
}

For a more complete example of a larger application, please checkout Hugo.

Help Command

Cobra automatically adds a help command to your application when you have subcommands. This will be called when a user runs 'app help'. Additionally, help will also support all other commands as input. Say, for instance, you have a command called 'create' without any additional configuration; Cobra will work when 'app help create' is called. Every command will automatically have the '--help' flag added.

Example

The following output is automatically generated by Cobra. Nothing beyond the command and flag definitions are needed.

$ cobra help

Cobra is a CLI library for Go that empowers applications.
This application is a tool to generate the needed files
to quickly create a Cobra application.

Usage:
  cobra [command]

Available Commands:
  add         Add a command to a Cobra Application
  help        Help about any command
  init        Initialize a Cobra Application

Flags:
  -a, --author string    author name for copyright attribution (default "YOUR NAME")
      --config string    config file (default is $HOME/.cobra.yaml)
  -h, --help             help for cobra
  -l, --license string   name of license for the project
      --viper            use Viper for configuration (default true)

Use "cobra [command] --help" for more information about a command.

Help is just a command like any other. There is no special logic or behavior around it. In fact, you can provide your own if you want.

Defining your own help

You can provide your own Help command or your own template for the default command to use with following functions:

cmd.SetHelpCommand(cmd *Command)
cmd.SetHelpFunc(f func(*Command, []string))
cmd.SetHelpTemplate(s string)

The latter two will also apply to any children commands.

Usage Message

When the user provides an invalid flag or invalid command, Cobra responds by showing the user the 'usage'.

Example

You may recognize this from the help above. That's because the default help embeds the usage as part of its output.

$ cobra --invalid
Error: unknown flag: --invalid
Usage:
  cobra [command]

Available Commands:
  add         Add a command to a Cobra Application
  help        Help about any command
  init        Initialize a Cobra Application

Flags:
  -a, --author string    author name for copyright attribution (default "YOUR NAME")
      --config string    config file (default is $HOME/.cobra.yaml)
  -h, --help             help for cobra
  -l, --license string   name of license for the project
      --viper            use Viper for configuration (default true)

Use "cobra [command] --help" for more information about a command.

Defining your own usage

You can provide your own usage function or template for Cobra to use. Like help, the function and template are overridable through public methods:

cmd.SetUsageFunc(f func(*Command) error)
cmd.SetUsageTemplate(s string)

Version Flag

Cobra adds a top-level '--version' flag if the Version field is set on the root command. Running an application with the '--version' flag will print the version to stdout using the version template. The template can be customized using the cmd.SetVersionTemplate(s string) function.

PreRun and PostRun Hooks

It is possible to run functions before or after the main Run function of your command. The PersistentPreRun and PreRun functions will be executed before Run. PersistentPostRun and PostRun will be executed after Run. The Persistent*Run functions will be inherited by children if they do not declare their own. These functions are run in the following order:

• PersistentPreRun
• PreRun
• Run
• PostRun
• PersistentPostRun

An example of two commands which use all of these features is below. When the subcommand is executed, it will run the root command's PersistentPreRun but not the root command's PersistentPostRun:

package main

import (
  "fmt"

  "github.com/spf13/cobra"
)

func main() {

  var rootCmd = &cobra.Command{
    Use:   "root [sub]",
    Short: "My root command",
    PersistentPreRun: func(cmd *cobra.Command, args []string) {
      fmt.Printf("Inside rootCmd PersistentPreRun with args: %v\n", args)
    },
    PreRun: func(cmd *cobra.Command, args []string) {
      fmt.Printf("Inside rootCmd PreRun with args: %v\n", args)
    },
    Run: func(cmd *cobra.Command, args []string) {
      fmt.Printf("Inside rootCmd Run with args: %v\n", args)
    },
    PostRun: func(cmd *cobra.Command, args []string) {
      fmt.Printf("Inside rootCmd PostRun with args: %v\n", args)
    },
    PersistentPostRun: func(cmd *cobra.Command, args []string) {
      fmt.Printf("Inside rootCmd PersistentPostRun with args: %v\n", args)
    },
  }

  var subCmd = &cobra.Command{
    Use:   "sub [no options!]",
    Short: "My subcommand",
    PreRun: func(cmd *cobra.Command, args []string) {
      fmt.Printf("Inside subCmd PreRun with args: %v\n", args)
    },
    Run: func(cmd *cobra.Command, args []string) {
      fmt.Printf("Inside subCmd Run with args: %v\n", args)
    },
    PostRun: func(cmd *cobra.Command, args []string) {
      fmt.Printf("Inside subCmd PostRun with args: %v\n", args)
    },
    PersistentPostRun: func(cmd *cobra.Command, args []string) {
      fmt.Printf("Inside subCmd PersistentPostRun with args: %v\n", args)
    },
  }

  rootCmd.AddCommand(subCmd)

  rootCmd.SetArgs([]string{""})
  rootCmd.Execute()
  fmt.Println()
  rootCmd.SetArgs([]string{"sub", "arg1", "arg2"})
  rootCmd.Execute()
}

Output:

Inside rootCmd PersistentPreRun with args: []
Inside rootCmd PreRun with args: []
Inside rootCmd Run with args: []
Inside rootCmd PostRun with args: []
Inside rootCmd PersistentPostRun with args: []

Inside rootCmd PersistentPreRun with args: [arg1 arg2]
Inside subCmd PreRun with args: [arg1 arg2]
Inside subCmd Run with args: [arg1 arg2]
Inside subCmd PostRun with args: [arg1 arg2]
Inside subCmd PersistentPostRun with args: [arg1 arg2]

Suggestions when "unknown command" happens

Cobra will print automatic suggestions when "unknown command" errors happen. This allows Cobra to behave similarly to the git command when a typo happens. For example:

$ hugo srever
Error: unknown command "srever" for "hugo"

Did you mean this?
        server

Run 'hugo --help' for usage.

Suggestions are automatic based on every subcommand registered and use an implementation of Levenshtein distance. Every registered command that matches a minimum distance of 2 (ignoring case) will be displayed as a suggestion.

If you need to disable suggestions or tweak the string distance in your command, use:

command.DisableSuggestions = true

or

command.SuggestionsMinimumDistance = 1

You can also explicitly set names for which a given command will be suggested using the SuggestFor attribute. This allows suggestions for strings that are not close in terms of string distance, but makes sense in your set of commands and for some which you don't want aliases. Example:

$ kubectl remove
Error: unknown command "remove" for "kubectl"

Did you mean this?
        delete

Run 'kubectl help' for usage.

Generating documentation for your command

Cobra can generate documentation based on subcommands, flags, etc. in the following formats:

• Markdown
• ReStructured Text
• Man Page

Generating bash completions

Cobra can generate a bash-completion file. If you add more information to your command, these completions can be amazingly powerful and flexible. Read more about it in Bash Completions.

Contributing

1. Fork it
2. Download your fork to your PC (git clone https://github.com/your_username/cobra && cd cobra)
3. Create your feature branch (git checkout -b my-new-feature)
4. Make changes and add them (git add .)
5. Commit your changes (git commit -m 'Add some feature')
6. Push to the branch (git push origin my-new-feature)
7. Create new pull request

License

Cobra is released under the Apache 2.0 license. See LICENSE.txt