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
• Docker (distribution)
• OpenShift
• Delve
• GopherJS
• CockroachDB
• Bleve
• ProjectAtomic (enterprise)
• Parse (CLI)

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 sub-command based CLIs: app server, app fetch, etc.
• Fully posix compliant flags (including short & long versions)
• Nested sub commands
• Global, local and cascading flags
• Easy generation of applications & commands with cobra create appname & cobra add cmdname
• Intelligent suggestions (app srver.. did you mean app server)
• Automatic help generation for commands and flags
• Automatic detailed help for app help [command]
• 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 flexibilty to define your own help, usage, etc
• Optional tight integration with viper for 12 factor apps

Cobra has an exceptionally clean interface and simple design without needless constructors or initialization methods.

Applications built with Cobra commands are designed to be as user friendly as possible. Flags can be placed before or after the command (as long as a confusing space isn’t provided). Both short and long flags can be used. A command need not even be fully typed. The shortest unambiguous string will suffice. Help is automatically generated and available for the application or for a specific command using either the help command or the --help flag.

Concepts

Cobra is built on a structure of commands & flags.

Commands represent actions and Flags are modifiers for those actions.

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

hugo server --port=1313

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

A Command has the following structure:

type Command struct {
    Use string // The one-line usage message.
    Short string // The short description shown in the 'help' output.
    Long string // The long message shown in the 'help <this-command>' output.
    Run func(cmd *Command, args []string) // Run runs the command.
}

Flags

A Flag is a way to modify the behavior of an 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.

Usage

Cobra works by creating a set of commands and then organizing them into a tree. The tree defines the structure of the application.

Once each command is defined with it's corresponding flags, then the tree is assigned to the commander which is finally executed.

Installing

Using Cobra is easy. First use go get to install the latest version of the library.

$ go get github.com/spf13/cobra

Next include cobra in your application.

import "github.com/spf13/cobra"

Create the root command

The root command represents your binary itself.

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

var HugoCmd = &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
	},
}

Create additional commands

Additional commands can be defined.

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")
	},
}

Attach command to its parent

In this example we are attaching it to the root, but commands can be attached at any level.

HugoCmd.AddCommand(versionCmd)

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.

HugoCmd.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.

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

Remove a command from its parent

Removing a command is not a common action in simple programs but it allows 3rd parties to customize an existing command tree.

In this example, we remove the existing VersionCmd command of an existing root command, and we replace it by our own version.

mainlib.RootCmd.RemoveCommand(mainlib.VersionCmd)
mainlib.RootCmd.AddCommand(versionCmd)

Once all commands and flags are defined, Execute the commands

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

HugoCmd.Execute()

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.
            `,
		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.
            `,
		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.`,
		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

The 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.

> hugo help

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

Usage:
  hugo [flags]
  hugo [command]

Available Commands:
  server          :: Hugo runs it's own a webserver to render the files
  version         :: Print the version number of Hugo
  check           :: Check content in the source directory
  benchmark       :: Benchmark hugo by building a site a number of times
  help [command]  :: Help about any command

 Available Flags:
  -b, --base-url="": hostname (and path) to the root eg. http://spf13.com/
  -D, --build-drafts=false: include content marked as draft
      --config="": config file (default is path/config.yaml|json|toml)
  -d, --destination="": filesystem path to write files to
  -s, --source="": filesystem path to read files relative from
      --stepAnalysis=false: display memory and timing of different steps of the program
      --uglyurls=false: if true, use /filename.html instead of /filename/
  -v, --verbose=false: verbose output
  -w, --watch=false: watch filesystem for changes and recreate as needed

Use "hugo help [command]" for more information about that 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 you own template for the default command to use.

The default help command is

func (c *Command) initHelp() {
	if c.helpCommand == nil {
		c.helpCommand = &Command{
			Use:   "help [command]",
			Short: "Help about any command",
			Long: `Help provides help for any command in the application.
        Simply type ` + c.Name() + ` help [path to command] for full details.`,
			Run: c.HelpFunc(),
		}
	}
	c.AddCommand(c.helpCommand)
}

You can provide your own command, function or template through the following methods.

command.SetHelpCommand(cmd *Command)

command.SetHelpFunc(f func(*Command, []string))

command.SetHelpTemplate(s string)

The latter two will also apply to any children commands.

Usage

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 it's output.

Usage:
  hugo [flags]
  hugo [command]

Available Commands:
  server          Hugo runs it's own a webserver to render the files
  version         Print the version number of Hugo
  check           Check content in the source directory
  benchmark       Benchmark hugo by building a site a number of times
  help [command]  Help about any command

 Available Flags:
  -b, --base-url="": hostname (and path) to the root eg. http://spf13.com/
  -D, --build-drafts=false: include content marked as draft
      --config="": config file (default is path/config.yaml|json|toml)
  -d, --destination="": filesystem path to write files to
  -s, --source="": filesystem path to read files relative from
      --stepAnalysis=false: display memory and timing of different steps of the program
      --uglyurls=false: if true, use /filename.html instead of /filename/
  -v, --verbose=false: verbose output
  -w, --watch=false: watch filesystem for changes and recreate as needed

Defining your own usage

You can provide your own usage function or template for cobra to use.

The default usage function is

	return func(c *Command) error {
		err := tmpl(c.Out(), c.UsageTemplate(), c)
		return err
	}

Like help the function and template are over ridable through public methods.

command.SetUsageFunc(f func(*Command) error)

command.SetUsageTemplate(s string)

PreRun or 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 inherrited by children if they do not declare their own. These function are run in the following order:

• PersistentPreRun
• PreRun
• Run
• PostRun
• PersistentPostRun

And example of two commands which use all of these features is below. When the subcommand in 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 sub command",
		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.Print("\n")
	rootCmd.SetArgs([]string{"sub", "arg1", "arg2"})
	_ = rootCmd.Execute()
}

Alternative Error Handling

Cobra also has functions where the return signature is an error. This allows for errors to bubble up to the top, providing a way to handle the errors in one location. The current list of functions that return an error is:

• PersistentPreRunE
• PreRunE
• RunE
• PostRunE
• PersistentPostRunE

Example Usage using RunE:

package main

import (
	"errors"
	"log"

	"github.com/spf13/cobra"
)

func main() {
	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`,
		RunE: func(cmd *cobra.Command, args []string) error {
			// Do Stuff Here
			return errors.New("some random error")
		},
	}

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

Suggestions when "unknown command" happens

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

$ hugo srever
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:

$ hugo delete
unknown command "delete" for "hugo"

Did you mean this?
  remove

Run 'hugo --help' for usage.

Generating markdown formatted documentation for your command

Cobra can generate a markdown formatted document based on the subcommands, flags, etc. A simple example of how to do this for your command can be found in Markdown Docs

Generating man pages for your command

Cobra can generate a man page based on the subcommands, flags, etc. A simple example of how to do this for your command can be found in Man Docs

Generating bash completions for your command

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

Debugging

Cobra provides a ‘DebugFlags’ method on a command which when called will print out everything Cobra knows about the flags for each command

Example

command.DebugFlags()

Release Notes

• 0.9.0 June 17, 2014
  • flags can appears anywhere in the args (provided they are unambiguous)
  • --help prints usage screen for app or command
  • Prefix matching for commands
  • Cleaner looking help and usage output
  • Extensive test suite
• 0.8.0 Nov 5, 2013
  • Reworked interface to remove commander completely
  • Command now primary structure
  • No initialization needed
  • Usage & Help templates & functions definable at any level
  • Updated Readme
• 0.7.0 Sept 24, 2013
  • Needs more eyes
  • Test suite
  • Support for automatic error messages
  • Support for help command
  • Support for printing to any io.Writer instead of os.Stderr
  • Support for persistent flags which cascade down tree
  • Ready for integration into Hugo
• 0.1.0 Sept 3, 2013
  • Implement first draft

Extensions

Libraries for extending Cobra:

• cmdns: Enables name spacing a command's immediate children. It provides an alternative way to structure sub commands, similar to heroku apps:create and ovrclk clusters:launch.

ToDo

• Launch proper documentation site

Contributing

1. Fork it
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create new Pull Request

Contributors

Names in no particular order:

• spf13

License

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