Go (golang): How to ask for user confirmation via command line

confirm.go

/* MIT License
 *
 * Copyright (c) 2017 Roland Singer [roland.singer@desertbit.com]
 *
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 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

package main

import (
	"bufio"
	"fmt"
	"log"
	"os"
	"strings"
)

// askForConfirmation asks the user for confirmation. A user must type in "yes" or "no" and
// then press enter. It has fuzzy matching, so "y", "Y", "yes", "YES", and "Yes" all count as
// confirmations. If the input is not recognized, it will ask again. The function does not return
// until it gets a valid response from the user.
func askForConfirmation(s string) bool {
	reader := bufio.NewReader(os.Stdin)

	for {
		fmt.Printf("%s [y/n]: ", s)

		response, err := reader.ReadString('\n')
		if err != nil {
			log.Fatal(err)
		}

		response = strings.ToLower(strings.TrimSpace(response))

		if response == "y" || response == "yes" {
			return true
		} else if response == "n" || response == "no" {
			return false
		}
	}
}

func main() {
	c := askForConfirmation("Do you really want to reset your system?")
	if c {
		fmt.Println("Sorry dude, I need root rights...")
		return
	}
}

That doesn't work if the user enters garbage, like oh yeah!

@das-vinculum Yes, it should. The user will be asked again.

@das-vinculum:

If the input is not recognized, it will ask again. The function does not return until it gets a valid response from the user.

It should probably be made to fail after several tries.

My approach is a little more strict (anything other than 'y' is false), but could be tweaked to only accept 'y' or 'n'.

// confirm displays a prompt `s` to the user and returns a bool indicating yes / no
// If the lowercased, trimmed input begins with anything other than 'y', it returns false
// It accepts an int `tries` representing the number of attempts before returning false
func confirm(s string, tries int) bool {
	r := bufio.NewReader(os.Stdin)

	for ; tries > 0; tries-- {
		fmt.Printf("%s [y/n]: ", s)

		res, err := r.ReadString('\n')
		if err != nil {
			log.Fatal(err)
		}

		// Empty input (i.e. "\n")
		if len(res) < 2 {
			continue
		}

		return strings.ToLower(strings.TrimSpace(res))[0] == 'y'
	}

	return false
}

Just for learning sake, How would you write a unit test for something like this?

you'd have to mock out the call out ReadString for testing

@davideasaf You can slightly change the function signature:

func askForConfirmation(s string, in io.Reader) bool {
    reader := bufio.NewReader(in)
    ...
}

That will allow you to pass in a mock reader in unit tests and os.Stdin in your production code.

My approach is a little more strict (anything other than 'y' is false), but could be tweaked to only accept 'y' or 'n'.

// confirm displays a prompt `s` to the user and returns a bool indicating yes / no
// If the lowercased, trimmed input begins with anything other than 'y', it returns false
// It accepts an int `tries` representing the number of attempts before returning false
func confirm(s string, tries int) bool {
	r := bufio.NewReader(os.Stdin)

	for ; tries > 0; tries-- {
		fmt.Printf("%s [y/n]: ", s)

		res, err := r.ReadString('\n')
		if err != nil {
			log.Fatal(err)
		}

		// Empty input (i.e. "\n")
		if len(res) < 2 {
			continue
		}

		return strings.ToLower(strings.TrimSpace(res))[0] == 'y'
	}

	return false
}

@ryanbaer Works great, but I'm getting an error when just pressing the enter key. Shouldn't it be if len(res) < 3 { continue } as "\n" already has a length of 2?

if you don't care about retries:

func askForConfirmation(s string) bool {
	reader := bufio.NewReader(os.Stdin)
	fmt.Printf("%s [y/N]: ", s)
	response, err := reader.ReadString('\n')
	if err != nil {
		panic(err)
	}
	response = strings.ToLower(strings.TrimSpace(response))
	if response == "y" || response == "yes" {
		return true
	}
	return false
}

if you don't care about retries:

func askForConfirmation(s string) bool {
	reader := bufio.NewReader(os.Stdin)
	fmt.Printf("%s [y/N]: ", s)
	response, err := reader.ReadString('\n')
	if err != nil {
		panic(err)
	}
	response = strings.ToLower(strings.TrimSpace(response))
	if response == "y" || response == "yes" {
		return true
	}
	return false
}