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Published: Sep 28, 2021 License: Apache-2.0 Imports: 7 Imported by: 110


ffcli go.dev reference

ffcli stands for flags-first command line interface, and provides an opinionated way to build CLIs.


Popular CLI frameworks like spf13/cobra, urfave/cli, or alecthomas/kingpin tend to have extremely large APIs, to support a large number of "table stakes" features.

This package is intended to be a lightweight alternative to those packages. In contrast to them, the API surface area of package ffcli is very small, with the immediate goal of being intuitive and productive, and the long-term goal of supporting commandline applications that are substantially easier to understand and maintain.

To support these goals, the package is concerned only with the core mechanics of defining a command tree, parsing flags, and selecting a command to run. It does not intend to be a one-stop-shop for everything your commandline application needs. Features like tab completion or colorized output are orthogonal to command tree parsing, and should be easy to provide on top of ffcli.

Finally, this package follows in the philosophy of its parent package ff, or "flags-first". Flags, and more specifically the Go stdlib flag.FlagSet, should be the primary mechanism of getting configuration from the execution environment into your program. The affordances provided by package ff, including environment variable and config file parsing, are also available in package ffcli. Support for other flag packages is a non-goal.


  • Absolute minimum usable API
  • Prefer using existing language features/patterns/abstractions whenever possible
  • Enable integration-style testing of CLIs with mockable dependencies
  • No global state


  • All conceivably useful features
  • Integration with flag packages other than package flag and ff


The core of the package is the ffcli.Command. Here is the simplest possible example of an ffcli program.

import (


func main() {
	root := &ffcli.Command{
		Exec: func(ctx context.Context, args []string) error {
			println("hello world")
			return nil

	root.ParseAndRun(context.Background(), os.Args[1:])

Most CLIs use flags and arguments to control behavior. Here is a command which takes a string to repeat as an argument, and the number of times to repeat it as a flag.

fs := flag.NewFlagSet("repeat", flag.ExitOnError)
n := fs.Int("n", 3, "how many times to repeat")

root := &ffcli.Command{
	ShortUsage: "repeat [-n times] <arg>",
	ShortHelp:  "Repeatedly print the argument to stdout.",
	FlagSet:    fs,
	Exec: func(ctx context.Context, args []string) error {
		if nargs := len(args); nargs != 1 {
			return fmt.Errorf("repeat requires exactly 1 argument, but you provided %d", nargs)
		for i := 0; i < *n; i++ {
			fmt.Fprintln(os.Stdout, args[0])
		return nil

if err := root.ParseAndRun(context.Background(), os.Args[1:]); err != nil {

Each command may have subcommands, allowing you to build a command tree.

var (
	rootFlagSet   = flag.NewFlagSet("textctl", flag.ExitOnError)
	verbose       = rootFlagSet.Bool("v", false, "increase log verbosity")
	repeatFlagSet = flag.NewFlagSet("textctl repeat", flag.ExitOnError)
	n             = repeatFlagSet.Int("n", 3, "how many times to repeat")

repeat := &ffcli.Command{
	Name:       "repeat",
	ShortUsage: "textctl repeat [-n times] <arg>",
	ShortHelp:  "Repeatedly print the argument to stdout.",
	FlagSet:    repeatFlagSet,
	Exec:       func(_ context.Context, args []string) error { ... },

count := &ffcli.Command{
	Name:       "count",
	ShortUsage: "textctl count [<arg> ...]",
	ShortHelp:  "Count the number of bytes in the arguments.",
	Exec:       func(_ context.Context, args []string) error { ... },

root := &ffcli.Command{
	ShortUsage:  "textctl [flags] <subcommand>",
	FlagSet:     rootFlagSet,
	Subcommands: []*ffcli.Command{repeat, count},

if err := root.ParseAndRun(context.Background(), os.Args[1:]); err != nil {

ParseAndRun can also be split into distinct Parse and Run phases, allowing you to perform two-phase setup or initialization of e.g. API clients that require user-supplied configuration.


See the examples directory. If you'd like an example of a specific type of program structure, or a CLI that satisfies a specific requirement, please file an issue.



Package ffcli is for building declarative commandline applications.

See the README at https://github.com/peterbourgon/ff/tree/master/ffcli for more information.




This section is empty.


View Source
var ErrUnparsed = errors.New("command tree is unparsed, can't run")

ErrUnparsed is returned by Run if Parse hasn't been called first.


func DefaultUsageFunc

func DefaultUsageFunc(c *Command) string

DefaultUsageFunc is the default UsageFunc used for all commands if no custom UsageFunc is provided.


type Command

type Command struct {
	// Name of the command. Used for sub-command matching, and as a replacement
	// for Usage, if no Usage string is provided. Required for sub-commands,
	// optional for the root command.
	Name string

	// ShortUsage string for this command. Consumed by the DefaultUsageFunc and
	// printed at the top of the help output. Recommended but not required.
	// Should be one line of the form
	//     cmd [flags] subcmd [flags] <required> [<optional> ...]
	// If it's not provided, the DefaultUsageFunc will use Name instead.
	// Optional, but recommended.
	ShortUsage string

	// ShortHelp is printed next to the command name when it appears as a
	// sub-command, in the help output of its parent command. Optional, but
	// recommended.
	ShortHelp string

	// LongHelp is consumed by the DefaultUsageFunc and printed in the help
	// output, after ShortUsage and before flags. Typically a paragraph or more
	// of prose-like text, providing more explicit context and guidance than
	// what is implied by flags and arguments. Optional.
	LongHelp string

	// UsageFunc generates a complete usage output, written to the io.Writer
	// returned by FlagSet.Output() when the -h flag is passed. The function is
	// invoked with its corresponding command, and its output should reflect the
	// command's short usage, short help, and long help strings, subcommands,
	// and available flags. Optional; if not provided, a suitable, compact
	// default is used.
	UsageFunc func(c *Command) string

	// FlagSet associated with this command. Optional, but if none is provided,
	// an empty FlagSet will be defined and attached during the parse phase, so
	// that the -h flag works as expected.
	FlagSet *flag.FlagSet

	// Options provided to ff.Parse when parsing arguments for this command.
	// Optional.
	Options []ff.Option

	// Subcommands accessible underneath (i.e. after) this command. Optional.
	Subcommands []*Command

	// Exec is invoked if this command has been determined to be the terminal
	// command selected by the arguments provided to Parse or ParseAndRun. The
	// args passed to Exec are the args left over after flags parsing. Optional.
	// If Exec returns flag.ErrHelp, then Run (or ParseAndRun) will behave as if
	// -h were passed and emit the complete usage output.
	// If Exec is nil, and this command is identified as the terminal command,
	// then Parse, Run, and ParseAndRun will all return NoExecError. Callers may
	// check for this error and print e.g. help or usage text to the user, in
	// effect treating some commands as just collections of subcommands, rather
	// than being invocable themselves.
	Exec func(ctx context.Context, args []string) error
	// contains filtered or unexported fields

Command combines a main function with a flag.FlagSet, and zero or more sub-commands. A commandline program can be represented as a declarative tree of commands.

func (*Command) Parse

func (c *Command) Parse(args []string) error

Parse the commandline arguments for this command and all sub-commands recursively, defining flags along the way. If Parse returns without an error, the terminal command has been successfully identified, and may be invoked by calling Run.

If the terminal command identified by Parse doesn't define an Exec function, then Parse will return NoExecError.

Example (Then_Run)
package main

import (


func main() {
	// Assume our CLI will use some client that requires a token.
	type FooClient struct {
		token string

	// That client would have a constructor.
	NewFooClient := func(token string) (*FooClient, error) {
		if token == "" {
			return nil, fmt.Errorf("token required")
		return &FooClient{token: token}, nil

	// We define the token in the root command's FlagSet.
	var (
		rootFlagSet = flag.NewFlagSet("mycommand", flag.ExitOnError)
		token       = rootFlagSet.String("token", "", "API token")

	// Create a placeholder client, initially nil.
	var client *FooClient

	// Commands can reference and use it, because by the time their Exec
	// function is invoked, the client will be constructed.
	foo := &ffcli.Command{
		Name: "foo",
		Exec: func(context.Context, []string) error {
			fmt.Printf("subcommand foo can use the client: %v", client)
			return nil

	root := &ffcli.Command{
		FlagSet:     rootFlagSet,
		Subcommands: []*ffcli.Command{foo},

	// Call Parse first, to populate flags and select a terminal command.
	if err := root.Parse([]string{"-token", "SECRETKEY", "foo"}); err != nil {
		log.Fatalf("Parse failure: %v", err)

	// After a successful Parse, we can construct a FooClient with the token.
	var err error
	client, err = NewFooClient(*token)
	if err != nil {
		log.Fatalf("error constructing FooClient: %v", err)

	// Then call Run, which will select the foo subcommand and invoke it.
	if err := root.Run(context.Background()); err != nil {
		log.Fatalf("Run failure: %v", err)


subcommand foo can use the client: &{SECRETKEY}

func (*Command) ParseAndRun

func (c *Command) ParseAndRun(ctx context.Context, args []string) error

ParseAndRun is a helper function that calls Parse and then Run in a single invocation. It's useful for simple command trees that don't need two-phase setup.

func (*Command) Run

func (c *Command) Run(ctx context.Context) (err error)

Run selects the terminal command in a command tree previously identified by a successful call to Parse, and calls that command's Exec function with the appropriate subset of commandline args.

If the terminal command previously identified by Parse doesn't define an Exec function, then Run will return NoExecError.

type NoExecError

type NoExecError struct {
	Command *Command

NoExecError is returned if the terminal command selected during the parse phase doesn't define an Exec function.

func (NoExecError) Error

func (e NoExecError) Error() string

Error implements the error interface.

Source Files

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