getopt

Overview ¶

Package getopt (v1) provides traditional getopt processing for implementing commands that use traditional command lines. The standard Go flag package cannot be used to write a program that parses flags the way ls or ssh does, for example.

A new version of this package (v2) (whose package name is also getopt) is available as:

"github.com/pborman/getopt/v2"

Getopt supports functionality found in both the standard BSD getopt as well as (one of the many versions of) the GNU getopt_long. Being a Go package, this package makes common usage easy, but still enables more controlled usage if needed.

Typical usage:

// Declare the flags to be used
helpFlag := getopt.Bool('?', "display help")
cmdFlag := getopt.StringLong("command", 'c', "", "the command")

func main() {
	// Parse the program arguments
	getopt.Parse()
	// Get the remaining positional parameters
	args := getopt.Args()

If you don't want the program to exit on error, use getopt.Getopt:

err := getopt.Getopt(nil)
if err != nil {
	// code to handle error
	fmt.Fprintln(os.Stderr, err)
}

Support is provided for both short (-f) and long (--flag) options. A single option may have both a short and a long name. Each option may be a flag or a value. A value takes an argument.

Declaring no long names causes this package to process arguments like the traditional BSD getopt.

Short flags may be combined into a single parameter. For example, "-a -b -c" may also be expressed "-abc". Long flags must stand on their own "--alpha --beta"

Values require an argument. For short options the argument may either be immediately following the short name or as the next argument. Only one short value may be combined with short flags in a single argument; the short value must be after all short flags. For example, if f is a flag and v is a value, then:

-vvalue    (sets v to "value")
-v value   (sets v to "value")
-fvvalue   (sets f, and sets v to "value")
-fv value  (sets f, and sets v to "value")
-vf value  (set v to "f" and value is the first parameter)

For the long value option val:

--val value (sets val to "value")
--val=value (sets val to "value")
--valvalue  (invalid option "valvalue")

Values with an optional value only set the value if the value is part of the same argument. In any event, the option count is increased and the option is marked as seen.

-v -f          (sets v and f as being seen)
-vvalue -f     (sets v to "value" and sets f)
--val -f       (sets v and f as being seen)
--val=value -f (sets v to "value" and sets f)

There is no convience function defined for making the value optional. The SetOptional method must be called on the actual Option.

v := String("val", 'v', "", "the optional v")
Lookup("v").SetOptional()

var s string
StringVar(&s, "val", 'v', "the optional v).SetOptional()

Parsing continues until the first non-option or "--" is encountered.

The short name "-" can be used, but it either is specified as "-" or as part of a group of options, for example "-f-". If there are no long options specified then "--f" could also be used. If "-" is not declared as an option then the single "-" will also terminate the option processing but unlike "--", the "-" will be part of the remaining arguments.

Normally the parsing is performed by calling the Parse function. If it is important to see the order of the options then the Getopt function should be used. The standard Parse function does the equivalent of:

func Parse() {
	if err := getopt.Getopt(os.Args, nil); err != nil {
		fmt.Fprintln(os.Stderr, err)
		s.usage()
		os.Exit(1)
	}

When calling Getopt it is the responsibility of the caller to print any errors.

Normally the default option set, CommandLine, is used. Other option sets may be created with New.

After parsing, the sets Args will contain the non-option arguments. If an error is encountered then Args will begin with argument that caused the error.

It is valid to call a set's Parse a second time to amend the current set of flags or values. As an example:

var a = getopt.Bool('a', "", "The a flag")
var b = getopt.Bool('b', "", "The a flag")
var cmd = ""

var opts = getopt.CommandLine

opts.Parse(os.Args)
if opts.NArgs() > 0 {
    cmd = opts.Arg(0)
    opts.Parse(opts.Args())
}

If called with set to { "prog", "-a", "cmd", "-b", "arg" } then both and and b would be set, cmd would be set to "cmd", and opts.Args() would return { "arg" }.

Unless an option type explicitly prohibits it, an option may appear more than once in the arguments. The last value provided to the option is the value.

SYNTAX ¶

For each option type there are an unfortunately large number of ways, 8, to initialize the option. This number is derived from three attributes:

1. Short or Long name
2. Normal vs Var
3. Command Line vs Option Set

The first two variations provide 4 signature:

Option(name rune, [value type,]  helpvalue... string)
OptionLong(name string, short rune, [value type,]  helpvalue... string)
OptionVar(p *type, name rune, helpvalue... string)
OptionVarLong(p *type, name string, short rune, helpvalue... string)

Foo can actually be expressed in terms of FooLong:

func Foo(name rune, value type, helpvalue... string) *type {
    return FooLong("", name, value, helpvalue...)
}

Normally Foo is used, unless long options are needed. Setting short to 0 creates only a long option.

The difference bentween Foo and FooVar is that you pass a pointer, p, to the location of the value to FooVar. The default value is simply *p. The initial value of *p is the defaut value of the option.

Foo is actually a wrapper around FooVar:

func Foo(name rune, value type, helpvalue... string) *type {
    p := value
    FooVar(&p, name, helpvalue... string)
    return &p
}

The third variation provides a top-level function and a method on a Set:

func Option(...)
func (s *Set) Option(...)

The top-level function is simply:

func Option(...) *type {
    return CommandLine.Option(...) {
}

To simplfy documentation, typically only the main top-level function is fully documented. The others will have documentation when there is something special about them.

VALUEHELP ¶

All non-flag options are created with a "valuehelp" as the last parameter. Valuehelp should be 0, 1, or 2 strings. The first string, if provided, is the usage message for the option. If the second string, if provided, is the name to use for the value when displaying the usage. If not provided the term "value" is assumed.

The usage message for the option created with

StringLong("option", 'o', "defval", "a string of letters")

is

-o, -option=value

StringLong("option", 'o', "defval", "a string of letters", "string")

is

-o, -option=string