Command-Line Flags #

// [_Command-line flags_](https://en.wikipedia.org/wiki/Command-line_interface#Command-line_option)
// are a common way to specify options for command-line
// programs. For example, in `wc -l` the `-l` is a
// command-line flag.

package main

// Go provides a `flag` package supporting basic
// command-line flag parsing. We'll use this package to
// implement our example command-line program.
import (
	"flag"
	"fmt"
)

func main() {

	// Basic flag declarations are available for string,
	// integer, and boolean options. Here we declare a
	// string flag `word` with a default value `"foo"`
	// and a short description. This `flag.String` function
	// returns a string pointer (not a string value);
	// we'll see how to use this pointer below.
	wordPtr := flag.String("word", "foo", "a string")

	// This declares `numb` and `fork` flags, using a
	// similar approach to the `word` flag.
	numbPtr := flag.Int("numb", 42, "an int")
	forkPtr := flag.Bool("fork", false, "a bool")

	// It's also possible to declare an option that uses an
	// existing var declared elsewhere in the program.
	// Note that we need to pass in a pointer to the flag
	// declaration function.
	var svar string
	flag.StringVar(&svar, "svar", "bar", "a string var")

	// Once all flags are declared, call `flag.Parse()`
	// to execute the command-line parsing.
	flag.Parse()

	// Here we'll just dump out the parsed options and
	// any trailing positional arguments. Note that we
	// need to dereference the pointers with e.g. `*wordPtr`
	// to get the actual option values.
	fmt.Println("word:", *wordPtr)
	fmt.Println("numb:", *numbPtr)
	fmt.Println("fork:", *forkPtr)
	fmt.Println("svar:", svar)
	fmt.Println("tail:", flag.Args())
}

# To experiment with the command-line flags program it's
# best to first compile it and then run the resulting
# binary directly.
$ go build command-line-flags.go

# Try out the built program by first giving it values for
# all flags.
$ ./command-line-flags -word=opt -numb=7 -fork -svar=flag
word: opt
numb: 7
fork: true
svar: flag
tail: []

# Note that if you omit flags they automatically take
# their default values.
$ ./command-line-flags -word=opt
word: opt
numb: 42
fork: false
svar: bar
tail: []

# Trailing positional arguments can be provided after
# any flags.
$ ./command-line-flags -word=opt a1 a2 a3
word: opt
...
tail: [a1 a2 a3]

# Note that the `flag` package requires all flags to
# appear before positional arguments (otherwise the flags
# will be interpreted as positional arguments).
$ ./command-line-flags -word=opt a1 a2 a3 -numb=7
word: opt
numb: 42
fork: false
svar: bar
tail: [a1 a2 a3 -numb=7]

# Use `-h` or `--help` flags to get automatically
# generated help text for the command-line program.
$ ./command-line-flags -h
Usage of ./command-line-flags:
  -fork=false: a bool
  -numb=42: an int
  -svar="bar": a string var
  -word="foo": a string

# If you provide a flag that wasn't specified to the
# `flag` package, the program will print an error message
# and show the help text again.
$ ./command-line-flags -wat
flag provided but not defined: -wat
Usage of ./command-line-flags:
...