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MIT License

mcli is a minimal but powerful cli library for Go.
m stands for minimal and magic.

It is extremely easy to use, it makes you like writing cli programs in Go.
The idea is borrowed from


  1. Extremely easy to use, dead simple buf powerful API to define commands, flags and arguments.
  2. Add arbitrary level sub-command with single line code.
  3. Define your command flags and arguments inside the command processor using a simple struct tag.
  4. Set default value for flags and arguments.
  5. Read environment variables for flags and arguments.
  6. Set default value for flags and arguments.
  7. Work with slice, map out of box (of course the bool, (u)int, (u)int16, (u)int32,
    (u)int64, float, duration, string types are also supported).
  8. Automatic help for commands, flags and arguments.
  9. Mark commands, flags as hidden, hidden commands and flags won’t be showed in help,
    except that when a special flag “–mcli-show-hidden” is provided.
  10. Mark flags, arguments as required, it reports error when not given.
  11. Mark flags as deprecated.
  12. Automatic suggestions like git.
  13. Compatible with the standard library’s flag.FlagSet.


Work in main function:

func main() {
    var args struct {
        Name string `cli:"-n, --name, Who do you want to say to" default:"tom"`

        // This argument is required.
        Text string `cli:"#R, text, The 'message' you want to send"`
    fmt.Printf("Say to %s: %s\n", args.Name, args.Text)

$ go run say.go
argument is required but not given: text
  tt [flags] <text>

  -n, --name string    Who do you want to say to (default "tom")

  text message (REQUIRED)    The message you want to send

exit status 2

$ go run say.go hello
Say to tom: hello

Use with sub-commands:

func main() {
    mcli.Add("cmd1", runCmd1, "An awesome command cmd1")

    mcli.AddGroup("cmd2", "This is a command group called cmd2")
    mcli.Add("cmd2 sub1", runCmd2Sub1, "Do something with cmd2 sub1")
    mcli.Add("cmd2 sub2", runCmd2Sub2, "Brief description about cmd2 sub2")

    // A sub-command can also be registered without registering the group.
    mcli.Add("group3 sub1 subsub1", runGroup3Sub1Subsub1, "Blah blah Blah")

    // This is a hidden command, it won't be showed in help,
    // except that when flag "--mcli-show-hidden" is given.
    mcli.AddHiden("secret-cmd", secretCmd, "An secret command won't be showed in help")


func runCmd1() {
    var args struct {
        Branch    string `cli:"-b, --branch, Select another branch by passing in the branch name"`
        Commit    bool   `cli:"-c, --commit, Open the last commit"`
        NoBrowser bool   `cli:"-n, --no-browser, Print destination URL instead of opening the browser"`
        Projects  bool   `cli:"-p, --projects, Open repository projects"`
        Repo      string `cli:"-R, --repo, Select another repository using the '[HOST/]OWNER/REPO' format"`
        Settings  bool   `cli:"-s, --settings, Open repository settings"`
        Wiki      bool   `cli:"-w, --wiki, Open repository wiki"`

        Location  string `cli:"location, A browser location can be specified using arguments in the following format:\n- by number for issue or pull request, e.g. \"123\"; or\n- by path for opening folders and files, e.g. \"cmd/gh/main.go\""`

    // Do something

type Cmd2CommonArgs struct {
    Repo string `cli:"-R, --repo, Select another repository using the '[HOST/]OWNER/REPO' format"`

func runCmd2Sub1() {
    // Note that the flag/argument description can be seperated either
    // by a comma or spaces, and can be mixed.
    var args struct {
        Body     string `cli:"-b, --body        Supply a body. Will prompt for one otherwise."`
        BodyFile string `cli:"-F, --body-file   Read body text from 'file' (use \"-\" to read from standard input)"`
        Editor   bool   `cli:"-e, --editor,     Add body using editor"`
        Web      bool   `cli:"-w, --web,        Add body in browser"`

    // Do something

See example_test for a more sophisticated example which mimics Github’s cli command gh.

Tag syntax

Struct tag is a powerful feature in Go, mcli uses struct tag to define flags and argumens.

  • tag cli defines the name and description for flags and arguments
  • tag default optionally provides a default value to a flag or argument
  • tag env tells Parse to lookup environment variables when user doesn’t provide a value

The syntax is

/* cli tag, only Name is required.
 * Short name and long name are both optional, but at least one must be given.
 * See below for details about Modifiers.
 * e.g.
 * - `cli:"-c, Open the last commit"`
 * - `cli:"#R, -b, --branch, Select another branch by passing in the branch name"`
 * - `cli:"--an-obvious-flag-dont-need-description"`
CliTag           <-  ( Modifiers ',' Space? )? Name ( ( ',' | Space ) Description )?
Modifiers        <-  '#' [DHR]+
Name             <-  ( ShortName LongName? ) | LongName
Description      <-  ( ![\r\n] . )*

/* default value tag, optional.
 * e.g.
 * - `default:"1.5s"` // duration
 * - `default:"true"` // bool
DefaultValueTag  <-  ( ![\r\n] . )*

/* env tag, optional.
 * Multiple environment names can be specified.
 * e.g.
 * - `env:"SOME_ENV"`
 * - `env:"ANOTHER_ENV_1, ANOTHER_ENV_2"`
EnvTag           <-  ( EnvName ',' Space? )* EnvName


Modifier represents an option to a flag, it sets the flag to be
deprecated, hidden, or required. In a cli tag, modifiers appears as
the first segment, starting with a # character.

Fow now the following modifiers are available:

  • D – marks a flag or argument as deprecated, “DEPRECATED” will be showed in help
  • R – marks a flag or argument as required, “REQUIRED” will be showed in help
  • H – marks a flag as hidden, see below for more about hidden flags

Hidden flags won’t be showed in help, except that when a special flag
“–mcli-show-hidden” is provided.

Modifier H shall not be used for an argument, else it panics.
An argument must be showed in help to tell user how to use the program

Some modifiers cannot be used together, else it panics, e.g.

  • H & R – a required flag must be showed in help to tell user to set it
  • D & R – a required flag must not be deprecated, it does not make sense
    and make user confusing

Compatibility with package flag

Parse returns a *flag.FlagSet if success, all defined flags are available
with the flag set, including both short and long names.

Note that there is a little difference with flag package, while the flag
package requires command line flags must present before arguments, and
arguments can be accessed using flag.Arg(i), this library doesn’t require
that, the order that user pass flags and arguments doesn’t matter.
Arguments should be defined in the struct given to Parse, command line
arguments will be set to the struct. As a bonus, you can use slice and
map arguments, it just works.

When command line arguments are given before flags, calling FlagSet.Arg(i)
won’t get the expected arguments.


Well, definitely command line parsing won’t be your hot path, performance
is not a main consideration for this library, we always want simpler API
and better usage instruction for end-users.
(This does not mean the library has poor performance.)


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