Command-Line Flag Parsing in Go

Introduction

The purpose of this lab is to implement a command-line program that supports basic command-line flag parsing using the flag package in Golang.

Skills Graph

Command-line flags

Implement a Golang program that parses command-line flags and outputs the parsed options and any trailing positional arguments. The program should support the following flags:

• word: a string flag with a default value of "foo".

• numb: an integer flag with a default value of 42.

• fork: a boolean flag with a default value of false.

• svar: a string flag that uses an existing variable declared elsewhere in the program.

• The program should use the flag package to parse command-line flags.

• The program should output the parsed options and any trailing positional arguments.

• The program should support the word, numb, fork, and svar flags as described above.

## 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:
...

There is the full code below:

// [_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())
}

Summary

In this lab, we learned how to use the flag package in Golang to parse command-line flags. We implemented a program that supports basic flag parsing and outputs the parsed options and any trailing positional arguments.