Parsing command-line flags in Go

Golang has a built-in package called flag that enables one to parse command-line flags. The flag package allows you to define String, Bool, or Int flags.

To be able to access the parameters passed as arguments to the command line execution of your program, three steps need to be completed.

Define the flag.
Bind the flag.
Parse the flag.

Syntax

An example of how flag is used with command-line arguments is shown below.

flag.Int("n", def_val, description)

view raw goflag_syntax.go hosted with ❤ by GitHub

Parameters

"n": the variable to be parsed.
def_val: the default value of the variable.
description: the description of the flag.

Define the flag

	var help = flag.Bool("help", false, "Show help")
	var boolFlag = false
	var stringFlag = "Hello There!"
	var intFlag int

view raw goflag_define.go hosted with ❤ by GitHub

Bind the flag

	flag.BoolVar(&boolFlag, "boolFlag", false, "A boolean flag")
	flag.StringVar(&stringFlag, "stringFlag", "Hello There!", "A string flag")
	flag.IntVar(&intFlag, "intFlag", 4, "An integer flag")

view raw goflag_bind.go hosted with ❤ by GitHub

Parse the flag

flag.Parse()

view raw goflag_parse.go hosted with ❤ by GitHub

Bringing it all together

	package main

	import (
	"flag"
	"fmt"
	"os"
	)


	// Define the flag
	var help = flag.Bool("help", false, "Show help")
	var boolFlag = false
	var stringFlag = "Hello There!"
	var intFlag int

	func main() {
	// Bind the flag
	flag.BoolVar(&boolFlag, "boolFlag", false, "A boolean flag")
	flag.StringVar(&stringFlag, "stringFlag", "Hello There!", "A string flag")
	flag.IntVar(&intFlag, "intFlag", 4, "An integer flag")

	// Parse the flag
	flag.Parse()

	// Usage Demo
	if *help {
	flag.Usage()
	os.Exit(0)
	}

	fmt.Println("Boolean Flag is ", boolFlag)
	fmt.Println("String Flag is ", stringFlag)
	fmt.Println("Int Flag is ", intFlag)
	}

view raw goflags.go hosted with ❤ by GitHub

To run the code in the terminal, the following command is added.

./main -boolFlag=true -stringFlag hi

You can use -h or --help flags to get automatically generated help text for the command-line program.

Checking if the flag is set explicitly.

If you would like to check explicitly if a flag has been set in Go, you can use the flag.Visit() function in Go.

	func isFlagPassed(name string) bool {
	found := false
	flag.Visit(func(f *flag.Flag) {
	if f.Name == name {
	found = true
	}
	})
	return found
	}

view raw goflags_visit.go hosted with ❤ by GitHub

This function needs to be called after the flag.Parse()

	package main

	import (
	"flag"
	"fmt"
	"os"
	)

	// Define the flag
	var help = flag.Bool("help", false, "Show help")
	var boolFlag = false
	var stringFlag = "Hello There!"
	var intFlag int

	func main() {
	// Bind the flag
	flag.BoolVar(&boolFlag, "boolFlag", false, "A boolean flag")
	flag.StringVar(&stringFlag, "stringFlag", "Hello There!", "A string flag")
	flag.IntVar(&intFlag, "intFlag", 4, "An integer flag")

	// Parse the flag
	flag.Parse()

	if !isFlagPassed("boolFlag") {
	fmt.Println("BoolFlag is not passed !!!")
	}
	// Usage Demo
	if *help {
	flag.Usage()
	os.Exit(0)
	}

	fmt.Println("Boolean Flag is ", boolFlag)
	fmt.Println("String Flag is ", stringFlag)
	fmt.Println("Int Flag is ", intFlag)
	}

	func isFlagPassed(name string) bool {
	found := false
	flag.Visit(func(f *flag.Flag) {
	if f.Name == name {
	found = true
	}
	})
	return found
	}

view raw goflags_visited_full.go hosted with ❤ by GitHub

If the boolFlag flag is not passed, the output of the above code would be

	BoolFlag is not passed !!!
	Boolean Flag is false
	String Flag is Hello There!
	Int Flag is 4

view raw goflags_output.sh hosted with ❤ by GitHub

That’s all that I had to cover on Flags in Go. Stay tuned for more posts on Golang.