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// Copyright 2012 Jesse van den Kieboom. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package flags
import (
// A Parser provides command line option parsing. It can contain several
// option groups each with their own set of options.
type Parser struct {
// Embedded, see Command for more information
// A usage string to be displayed in the help message.
Usage string
// Option flags changing the behavior of the parser.
Options Options
// NamespaceDelimiter separates group namespaces and option long names
NamespaceDelimiter string
// UnknownOptionsHandler is a function which gets called when the parser
// encounters an unknown option. The function receives the unknown option
// name, a SplitArgument which specifies its value if set with an argument
// separator, and the remaining command line arguments.
// It should return a new list of remaining arguments to continue parsing,
// or an error to indicate a parse failure.
UnknownOptionHandler func(option string, arg SplitArgument, args []string) ([]string, error)
internalError error
// SplitArgument represents the argument value of an option that was passed using
// an argument separator.
type SplitArgument interface {
// String returns the option's value as a string, and a boolean indicating
// if the option was present.
Value() (string, bool)
type strArgument struct {
value *string
func (s strArgument) Value() (string, bool) {
if s.value == nil {
return "", false
return *s.value, true
// Options provides parser options that change the behavior of the option
// parser.
type Options uint
const (
// None indicates no options.
None Options = 0
// HelpFlag adds a default Help Options group to the parser containing
// -h and --help options. When either -h or --help is specified on the
// command line, the parser will return the special error of type
// ErrHelp. When PrintErrors is also specified, then the help message
// will also be automatically printed to os.Stderr.
HelpFlag = 1 << iota
// PassDoubleDash passes all arguments after a double dash, --, as
// remaining command line arguments (i.e. they will not be parsed for
// flags).
// IgnoreUnknown ignores any unknown options and passes them as
// remaining command line arguments instead of generating an error.
// PrintErrors prints any errors which occurred during parsing to
// os.Stderr.
// PassAfterNonOption passes all arguments after the first non option
// as remaining command line arguments. This is equivalent to strict
// POSIX processing.
// Default is a convenient default set of options which should cover
// most of the uses of the flags package.
Default = HelpFlag | PrintErrors | PassDoubleDash
// Parse is a convenience function to parse command line options with default
// settings. The provided data is a pointer to a struct representing the
// default option group (named "Application Options"). For more control, use
// flags.NewParser.
func Parse(data interface{}) ([]string, error) {
return NewParser(data, Default).Parse()
// ParseArgs is a convenience function to parse command line options with default
// settings. The provided data is a pointer to a struct representing the
// default option group (named "Application Options"). The args argument is
// the list of command line arguments to parse. If you just want to parse the
// default program command line arguments (i.e. os.Args), then use flags.Parse
// instead. For more control, use flags.NewParser.
func ParseArgs(data interface{}, args []string) ([]string, error) {
return NewParser(data, Default).ParseArgs(args)
// NewParser creates a new parser. It uses os.Args[0] as the application
// name and then calls Parser.NewNamedParser (see Parser.NewNamedParser for
// more details). The provided data is a pointer to a struct representing the
// default option group (named "Application Options"), or nil if the default
// group should not be added. The options parameter specifies a set of options
// for the parser.
func NewParser(data interface{}, options Options) *Parser {
p := NewNamedParser(path.Base(os.Args[0]), options)
if data != nil {
g, err := p.AddGroup("Application Options", "", data)
if err == nil {
g.parent = p
p.internalError = err
return p
// NewNamedParser creates a new parser. The appname is used to display the
// executable name in the built-in help message. Option groups and commands can
// be added to this parser by using AddGroup and AddCommand.
func NewNamedParser(appname string, options Options) *Parser {
p := &Parser{
Command: newCommand(appname, "", "", nil),
Options: options,
NamespaceDelimiter: ".",
p.Command.parent = p
return p
// Parse parses the command line arguments from os.Args using Parser.ParseArgs.
// For more detailed information see ParseArgs.
func (p *Parser) Parse() ([]string, error) {
return p.ParseArgs(os.Args[1:])
// ParseArgs parses the command line arguments according to the option groups that
// were added to the parser. On successful parsing of the arguments, the
// remaining, non-option, arguments (if any) are returned. The returned error
// indicates a parsing error and can be used with PrintError to display
// contextual information on where the error occurred exactly.
// When the common help group has been added (AddHelp) and either -h or --help
// was specified in the command line arguments, a help message will be
// automatically printed. Furthermore, the special error type ErrHelp is returned.
// It is up to the caller to exit the program if so desired.
func (p *Parser) ParseArgs(args []string) ([]string, error) {
if p.internalError != nil {
return nil, p.internalError
// Add built-in help group to all commands if necessary
if (p.Options & HelpFlag) != None {
compval := os.Getenv("GO_FLAGS_COMPLETION")
if len(compval) != 0 {
comp := &completion{parser: p}
if compval == "verbose" {
comp.ShowDescriptions = true
return nil, nil
s := &parseState{
args: args,
retargs: make([]string, 0, len(args)),
for !s.eof() {
arg := s.pop()
// When PassDoubleDash is set and we encounter a --, then
// simply append all the rest as arguments and break out
if (p.Options&PassDoubleDash) != None && arg == "--" {
if !argumentIsOption(arg) {
// Note: this also sets s.err, so we can just check for
// nil here and use s.err later
if p.parseNonOption(s) != nil {
var err error
prefix, optname, islong := stripOptionPrefix(arg)
optname, _, argument := splitOption(prefix, optname, islong)
if islong {
err = p.parseLong(s, optname, argument)
} else {
err = p.parseShort(s, optname, argument)
if err != nil {
ignoreUnknown := (p.Options & IgnoreUnknown) != None
parseErr := wrapError(err)
if parseErr.Type != ErrUnknownFlag || (!ignoreUnknown && p.UnknownOptionHandler == nil) {
s.err = parseErr
if ignoreUnknown {
} else if p.UnknownOptionHandler != nil {
modifiedArgs, err := p.UnknownOptionHandler(optname, strArgument{argument}, s.args)
if err != nil {
s.err = err
s.args = modifiedArgs
if s.err == nil {
p.eachCommand(func(c *Command) {
c.eachGroup(func(g *Group) {
for _, option := range g.options {
if option.isSet {
}, true)
var reterr error
if s.err != nil {
reterr = s.err
} else if len(s.command.commands) != 0 && !s.command.SubcommandsOptional {
reterr = s.estimateCommand()
} else if cmd, ok :=; ok {
reterr = cmd.Execute(s.retargs)
if reterr != nil {
return append([]string{s.arg}, s.args...), p.printError(reterr)
return s.retargs, nil