opts

opts

A low friction command-line interface library for Go (Golang)

Command-line parsing should be easy. Use configuration structs:

package main

import (
	"fmt"

	"github.com/jpillora/opts"
)

func main() {
	config := struct {
		File  string `help:"file to load"`
		Lines int    `help:"number of lines to show"`
	}{}
	opts.Parse(&config)
	fmt.Println(config)
}

$ go run main.go -f foo -l 12
{foo 12}

Features (with examples)

• Easy to use (simple)
• Promotes separation of CLI code and library code (separation)
• Automatically generated --help text via struct tags help:"Foo bar" (help)
• Subcommands by nesting structs (subcmds)
• Default values by modifying the struct prior to Parse() (defaults)
• Default values from a JSON config file, unmarshalled via your config struct (config)
• Default values from environment, defined by your field names (env)
• Infers program name from package name (and optional repository link)
• Extensible via flag.Value (customtypes)
• Customizable help text by modifying the default templates (customhelp)

Overview

Internally, opts creates flag.FlagSets from your configuration structs using pkg/reflect. So, given the following program:

type Config struct {
	Alpha   string        `help:"a string"`
	Bravo   int           `help:"an int"`
	Charlie bool          `help:"a bool"`
	Delta   time.Duration `help:"a duration"`
}

c := Config{
	Bravo: 42,
	Delta: 2 * time.Minute,
}

opts.Parse(&c)

opts would approximately perform:

foo := Config{}
set := flag.NewFlagSet("Config")
set.StringVar(&foo.Alpha, "", "a string")
set.IntVar(&foo.Bravo, 42, "an int")
set.BoolVar(&foo.Charlie, false, "a bool")
set.DurationVar(&foo.Delta, 2 * time.Minute, "a duration")
set.Parse(os.Args)

And, you get pretty --help output:

$ ./foo --help

  Usage: foo [options]

  Options:
  --alpha, -a    a string
  --bravo, -b    an int (default 42)
  --charlie, -c  an bool
  --delta, -d    a duration (default 2m0s)
  --help, -h

All Examples

See all examples here

Struct Tag API

Common tags

These tags are usable across all types:

• name - Name is used to display the field in the help text (defaults to the field name converted to lowercase and dashes)
• help - Help is used to describe the field (defaults to "")
• type - The opts type assigned the field (defaults using the table below)

type defaults

All fields will have a type. By default a struct field will be assigned a type depending on its field type:

This default assignment can be overridden with a type struct tag. For example you could set a string struct field to be an arg field with type:"arg".

type specific properties

• opt

  An option (opt) field will appear in the options list and by definition, be optional.

  • short - An alias or shortcut to this option (defaults to the first letter of the name property)
  • env - An environment variable to use to retrieve the default (when UseEnv() is set this defaults to name property converted to uppercase and underscores)

  Restricted to fields with type int,string,bool,time.Duration and flag.Value

• arg

  An argument (arg) field will appear in the usage and will be required if it does not have a default value set.

  Restricted to fields with type string

• arglist

  An argument list (arglist) field will appear in the usage. Useful for a you allow any number number of arguments. For example file and directory targets.

  • min - An integer representing the minimum number of args specified

  Restricted to fields with type []string

• subcmd

  A subcommand is nested opts.Opt instance, so its fields behave in exactly the same way as the parent struct.

  You can access the options of a subcommand with prog --prog-opt X subcmd --subcmd-opt Y

  Restricted to fields with type struct

• cmdname

  A special type which will assume the name of the selected subcommand

  Restricted to fields with type string

• embedded

  A special type which causes the fields of struct to be used in the current struct. Useful if you want to extend existing structs with extra command-line options.

Other projects

Other CLI libraries which infer flags from struct tags:

• https://github.com/jessevdk/go-flags is similar though it still could be simpler and more customizable.

Todo

• More tests
• Option groups (Separate sets of options in --help)
• Bash completion
• Multiple short options -aux (Requires a non-pkg/flag parser)

MIT License

Copyright © 2015 <dev@jpillora.com>

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.