flag

Overview ¶

Package flag implements command-line flag parsing.

Usage ¶

Define flags using StringVar, BoolVar, IntVar, etc.

var flagvar int
func init() {
	flag.IntVar(&flagvar, "flagname", 1234, "help message for flagname")
}

You can create custom flags that satisfy the Value interface (with pointer receivers) and couple them to flag parsing by

flag.Var(&flagVal, "name", "help message for flagname")

For such flags, the default value is just the initial value of the variable.

After all flags are defined, call Parse to parse the command line into the defined flags.

flag.Parse()

Flags may then be used directly.

fmt.Println("flagvar has value ", flagvar)

After parsing, the arguments following the flags are available as the slice flag.Args or individually as flag.Arg(i). The arguments are indexed from 0 through flag.NArg-1.

Command line flag syntax ¶

The following forms are permitted:

-flag
--flag   // double dashes are also permitted
-flag=x
-flag x  // non-boolean flags only

One or two dashes may be used; they are equivalent. The last form is not permitted for boolean flags because the meaning of the command

cmd -x *

where * is a Unix shell wildcard, will change if there is a file called 0, false, etc. You must use the -flag=false form to turn off a boolean flag.

Flag parsing stops just before the first non-flag argument ("-" is a non-flag argument) or after the terminator "--".

Integer flags accept 1234, 0664, 0x1234 and may be negative. Boolean flags may be:

1, 0, t, f, T, F, true, false, TRUE, FALSE, True, False

The default set of command-line flags is controlled by top-level functions. The FlagSet type allows one to define independent sets of flags, such as to implement subcommands in a command-line interface. The methods of FlagSet are analogous to the top-level functions for the command-line flag set.

Based on the flag package.