How do I prompt a user for confirmation in bash script? [duplicate]

Asked 11/12, 2009 at 2:52 Answered 17/7, 2013 at 9:3

840

I want to put a quick "are you sure?" prompt for confirmation at the top of a potentially dangerous bash script, what's the easiest/best way to do this?

Enjoin answered 11/12, 2009 at 2:52 Comment(1)

Related – Muscadine 4/8, 2013 at 21:20

1402

read -p "Are you sure? " -n 1 -r
echo    # (optional) move to a new line
if [[ $REPLY =~ ^[Yy]$ ]]
then
    # do dangerous stuff
fi

I incorporated levislevis85's suggestion (thanks!) and added the -n option to read to accept one character without the need to press Enter. You can use one or both of these.

Also, the negated form might look like this:

read -p "Are you sure? " -n 1 -r
echo    # (optional) move to a new line
if [[ ! $REPLY =~ ^[Yy]$ ]]
then
    [[ "$0" = "$BASH_SOURCE" ]] && exit 1 || return 1 # handle exits from shell or function but don't exit interactive shell
fi

However, as pointed out by Erich, under some circumstances such as a syntax error caused by the script being run in the wrong shell, the negated form could allow the script to continue to the "dangerous stuff". The failure mode should favor the safest outcome so only the first, non-negated if should be used.

Explanation:

The read command outputs the prompt (-p "prompt") then accepts one character (-n 1) and accepts backslashes literally (-r) (otherwise read would see the backslash as an escape and wait for a second character). The default variable for read to store the result in is $REPLY if you don't supply a name like this: read -p "my prompt" -n 1 -r my_var

The if statement uses a regular expression to check if the character in $REPLY matches (=~) an upper or lower case "Y". The regular expression used here says "a string starting (^) and consisting solely of one of a list of characters in a bracket expression ([Yy]) and ending ($)". The anchors (^ and $) prevent matching longer strings. In this case they help reinforce the one-character limit set in the read command.

The negated form uses the logical "not" operator (!) to match (=~) any character that is not "Y" or "y". An alternative way to express this is less readable and doesn't as clearly express the intent in my opinion in this instance. However, this is what it would look like: if [[ $REPLY =~ ^[^Yy]$ ]]

Muscadine answered 11/12, 2009 at 2:56 Comment(51)

thanks, what do the double square brackets do? – Enjoin 11/12, 2009 at 3:2

and can that be negated? i.e. if not match, exit? – Enjoin 11/12, 2009 at 3:4

It has more features than [] including the regex match operator =~. See: mywiki.wooledge.org/BashFAQ/031 – Muscadine 11/12, 2009 at 3:9

one small issue. If user enters anything that starts with y|Y, it will go through. [[ $a =~ "^[Yy]$" ]] – Poindexter 11/12, 2009 at 5:53

after the "read" you should do an "echo" to get the cursor back on the next line. (just a raw "echo" with no params will do the trick) – Smilacaceous 21/5, 2011 at 4:0

The read line should have the variable specified at the end: read -p "Are you sure? " -n 1 -r REPLY – Tammeratammi 12/6, 2012 at 9:29

@tuner: $REPLY is automatically set if no variable name is supplied. – Muscadine 12/6, 2012 at 9:42

@Dennis Williamson: with my cygwin-based bash 4.1.10(4) this didn't work somehow. – Tammeratammi 18/6, 2012 at 8:37

@tuner: "didn't work" provides no information. Please describe exactly how it doesn't, because it should in every version of Bash from 3.2 to 4.2 something. – Muscadine 18/6, 2012 at 9:18

@DennisWilliamson At first it wouldn't carry out the inner part of the if-clause. Checking again it works. Maybe something with the line endings in the first try. Sorry for the noise. – Tammeratammi 20/6, 2012 at 7:20

Just to clarify, the -r option tells read not to interpret backslashes as escape characters. I understand it can also be used on the negated form above. – Bullfinch 26/2, 2013 at 10:4

just a warning on the negated form - if your user runs that in the old Bourne sh shell there is a risk that the read and conditional [[ will fail but the script will continue without exiting - perhaps not want you want - the positive version is therefore safer – Asthenic 1/4, 2013 at 7:49

@ErichBSchulz: But sh doesn't support either [[ or =~. – Muscadine 1/4, 2013 at 11:18

@DennisWilliamson that's exactly the problem! so if a "bash" script gets run under the old shell (which is possible) the negated form lurches past the test skips the exit... and proceeds straight onto the "dangerous stuff". – Asthenic 1/4, 2013 at 23:59

@ErichBSchulz: Odd, I get [[: not found for both the negated and non-negated forms when run under sh. Which sh are you running? Oh, now I see what you mean. You're not talking about the syntax, but the logic. Yes, your point is very much valid! – Muscadine 2/4, 2013 at 0:17

What about such a construct: [[ $REPLY =~ ^[Yy]$ ]] || exit -1 How would this be evaluated by an older shell? – Jaredjarek 4/6, 2013 at 19:14

@Scolytus: That will work fine in any shell that supports all the features in use by that statement: double square bracket conditionals, $REPLY is the default variable for read and regex matching. Taken together, they are supported by Bash >= 3.2, ksh 93 (at least) and zsh (I don't know the version history). The || (logical or) construct is supported by most versions of most shells. – Muscadine 5/6, 2013 at 0:44

Just to point out - a rookie error in forgetting the hashbang at the top while testing this will throw errors. make sure you add #!/usr/bin/env bash (or similar valid hashbang) to the top of your script – Joyless 18/7, 2013 at 17:2

Can I suggest to insert a echo "" just after fi? This will send to a new line the cursor, it's just for a better user experience :) Before this, the result output was: Are you sure? nroot@server: – Landahl 21/8, 2013 at 10:43

@DJHell: That's a good suggestion. I've added it to my answer. – Muscadine 21/8, 2013 at 10:51

Can someone breakdown what =~ ^[Yy]$ is doing/means in the if statement? if [[ $REPLY =~ ^[Yy]$ ]] – Gorgonian 18/7, 2014 at 14:18

@Tony: =~ is Bash's regex match operator. ^[Yy]$ is a regular expression which means a string which consists only of an upper or lower case y. – Muscadine 18/7, 2014 at 15:3

what if I want it to be either y or press enter? – Mooneye 8/11, 2014 at 10:20

@PineappleUndertheSea: With Bash 4, you can create a default: read -p "Are you sure? " -r -i y -e but the user would need to backspace to be able to change the "y" to an "n", for example. For Bash 3.2 and later, change the pattern in the match in the first example in my answer to ^([Yy]|)$ - this means to match upper or lower case "Y" or nothing (a blank line). – Muscadine 8/11, 2014 at 14:16

@DennisWilliamson how would you add coloring for the message ? – Pieria 15/6, 2016 at 5:8

@angry_kiwi: Example: yellow=$(tput setaf 3); reset=$(tput sgr0); read -p "${yellow}Name: ${reset}" See man 5 terminfo in the Color Handling section for more information. Note that if you use read -e for readline support, you should use \001 and \002 to wrap the control sequences like this:

yellow=$(tput setaf 3); reset=$(tput sgr0); printf -v prompt "\001${yellow}\002Name: \001${reset}\002"; read -e -p "$prompt"

– Muscadine 15/6, 2016 at 16:49

I'm getting conditional binary operator expected , syntax error near =~' , if [[ ! $REPLY =~ ^[Yy]$ ]]' ..... Oh looks like your example is Bash specific stackoverflow.com/questions/11206233/… – Scala 29/3, 2017 at 19:55

@PHPDave: Yes, it's Bash specific. The question is tagged bash and "Bash" is in the question title. – Muscadine 29/3, 2017 at 22:28

@SkippyleGrandGourou: I added some explanation. Please let me know if I missed something. – Muscadine 22/2, 2019 at 19:19

In case anyone else has an issue using read with a file that has a /bin/bash shebang (while using a zsh terminal), this is what worked for me choice=$(bash -c "read -p 'Are you sure? (y/n) ' -n 1 c; echo \$c") – Kermie 14/8, 2019 at 4:37

@theOneWhoKnocks: You shouldn't need to do that. – Muscadine 14/8, 2019 at 12:15

@DennisWilliamson I agree with you, but I had to none the less. – Kermie 15/8, 2019 at 3:54

From where does $REPLY variable come from can someone explain I am new to bash ? – Pauiie 24/9, 2019 at 6:6

@SuhailAbdulRehmanChougule: $REPLY is the builtin variable that read stores the string it reads in if you don't supply a variable name as an argument. I describe it in my answer in the first paragraph under "Explanation". – Muscadine 24/9, 2019 at 12:30

Thanks for your reply @DennisWilliamson but i am getting an usual error "read: illegal option -n" – Pauiie 24/9, 2019 at 18:4

Thanks for your reply @DennisWilliamson but i am getting an usual error "read: illegal option -n" THECODE IN NEXT COMMENT – Pauiie 24/9, 2019 at 18:30

#!/bin/sh BRANCH=`git rev-parse --abbrev-ref HEAD` PROTECTED_BRANCH="master" MSG="$1" bug="bug-fix" if [ "$BRANCH" = "$PROTECTED_BRANCH" ]; then     echo "inside branch if branch=$BRANCH and protected branch=$PROTECTED_BRANCH"     if ! grep -qE "bug-fix" "$MSG";     then 	cat "$MSG" 	echo "Your commit message must contain the word 'bug-fix'" 	exit 1     else 	exit 0     fi else     read -p "Are you sure you want to commit test (y/n) ?" -n 1 -r     echo     if [ "$REPLY" =~ ^[Yy]$ ]     then 	exit 0     else 	echo "commit aborted" 	exit 1     fi fi

– Pauiie 24/9, 2019 at 18:31

@DennisWilliamson the issue got sorted out when instead of #! /bin/sh I used #! /usr/bin/env bash :) Any ways thanks I learned a lot :) – Pauiie 24/9, 2019 at 18:40

@SuhailAbdulRehmanChougule: You're welcome. Yes, the shell invoked by /bin/sh (Often Dash) usually doesn't support Bash features. In this case, its simpler version of read doesn't support the -n option which, in Bash, limits the number of characters that read accepts. – Muscadine 24/9, 2019 at 19:53

syntax error near unexpected token fi'` – Phenacaine 8/11, 2019 at 10:39

To allow a default of y, you can make the match group optional with ?: if [[ $REPLY =~ ^[yY]?$ ]] – Sandhurst 6/8, 2020 at 11:18

Vote down because using unusual -r and -n' – Williawilliam 14/11, 2020 at 7:4

I like this better than what I found on the 'duplicate' Q/A – Keneth 17/5, 2021 at 4:51

I think [[ ${REPLY^} = Y ]] is slightly easier to understand than a regex. Good answer though. – Sodomy 18/8, 2021 at 5:7

Dangerous! If you accidentally hit the Y key twice this will propagate to the next part of the script that requests input, if there is one, which could be quite catastrophic. Probably wise to require the enter key be pressed to end any input. – Fulgurate 17/9, 2021 at 14:32

One-liner: read -p "Continue? [Enter] → Yes, [Ctrl]+[C] → No.". – Ligulate 22/10, 2021 at 7:15

if [[ $REPLY =~ ^[Yy]$ ]] is needed to proceed not if [[ ! $REPLY =~ ^[Yy]$ ]] – Eleen 26/10, 2021 at 3:21

Is this standard ? for me the REPLY variable gets the value e from the following echo then it complains about the unknown cho >_> – Pimental 21/12, 2021 at 19:52

@Enissay: You must be entering the commands (or pasting them) at the prompt instead of in a script. The read command would execute immediately, waiting for the next input which is the echo command. Since the -n option is set to accept one character then exit, the "e" goes into the REPLY variable and the rest of the line goes to the shell for execution. Since the "e" was read, the rest is "cho ..." which isn't a recognized command. If you want to try this from the command line, enter the read command, press a key, then enter the echo command. – Muscadine 28/6, 2022 at 15:55

@mikoop: The negated tests proceeds to exiting/returning if something other that Y/y are pressed. If one of those keys is pressed the negated test will fail and execution proceeds past the if on to the rest of the script. One of the things this does is prevent one from having to wrap a whole script inside the if. – Muscadine 28/6, 2022 at 16:3

Suggestion: Show the expected responses and which response is default. read -p "Are you sure? [y/N] " -n 1 -r And then, echo a newline unless they hit "Enter". [[ -z $REPLY ]] || echo – Strafe 21/9, 2022 at 11:46

250

use case/esac.

read -p "Continue (y/n)?" choice
case "$choice" in 
  y|Y ) echo "yes";;
  n|N ) echo "no";;
  * ) echo "invalid";;
esac

advantage:

neater
can use "OR" condition easier
can use character range, eg [yY][eE][sS] to accept word "yes", where any of its characters may be in lowercase or in uppercase.

Poindexter answered 11/12, 2009 at 3:48 Comment(5)

Good solution. Personally, if the bash script could really be crippling I like to have the person type out 'yes'. – Pyrimidine 11/12, 2009 at 5:33

How can I use this to exit when the input is no (maybe echo "Quitting script."; exit;;), but if the input is yes, the script will just continue with whatever comes after esac? – Tabular 13/2, 2014 at 23:53

If you do n|N ) echo "no"; return;; the script will end there if you say 'n' and continue with the rest otherwise, is that what you mean? – Etam 14/2, 2014 at 13:2

@Pyrimidine if it's really, really crippling you could have the user type out "The quick brown fox jumps over the lazy dogs" :P – Excel 10/12, 2015 at 23:4

When the user input is invalid, how to ask the user to input again. And do the 'return;' if got invalid three times. – Balsam 10/10, 2016 at 16:26

Try the read shell builtin:

read -p "Continue (y/n)?" CONT
if [ "$CONT" = "y" ]; then
  echo "yaaa";
else
  echo "booo";
fi

Spendthrift answered 11/12, 2009 at 2:58 Comment(0)

This way you get 'y' 'yes' or 'Enter'

 read -r -p "Are you sure? [Y/n]" response
 response=${response,,} # tolower
 if [[ $response =~ ^(y| ) ]] || [[ -z $response ]]; then
    your-action-here
 fi

If you are using zsh try this:

read "response?Are you sure ? [Y/n] "
response=${response:l} #tolower
if [[ $response =~ ^(y| ) ]] || [[ -z $response ]]; then
    your-action-here
fi

Heelpiece answered 12/5, 2013 at 14:53 Comment(3)

Not accepting default Y – Domash 20/2, 2015 at 14:43

For default Y if [[ $response =~ ^(yes|y| ) ]] | [ -z $response ]; then – Domash 20/2, 2015 at 14:48

In the regex, there is no need to match both yes and y, just y beginning is enough. Also, if default needs to be no, then the following would be helpful. read "response?Are you sure ? [Y/n] " && if [[ "$response" =~ ^[Yy] ]]; then echo "Yes, you did it."; else echo "No, narrow escape"; fi – Amarillas 28/6, 2018 at 20:42

Here's the function I use:

function ask_yes_or_no() {
    read -p "$1 ([y]es or [N]o): "
    case $(echo $REPLY | tr '[A-Z]' '[a-z]') in
        y|yes) echo "yes" ;;
        *)     echo "no" ;;
    esac
}

And an example using it:

if [[ "no" == $(ask_yes_or_no "Are you sure?") || \
      "no" == $(ask_yes_or_no "Are you *really* sure?") ]]
then
    echo "Skipped."
    exit 0
fi

# Do something really dangerous...

The output is always "yes" or "no"
It's "no" by default
Everything except "y" or "yes" returns "no", so it's pretty safe for a dangerous bash script
And it's case insensitive, "Y", "Yes", or "YES" work as "yes".

Zamir answered 17/7, 2013 at 9:3 Comment(0)

This what I found elsewhere, is there a better possible version?

read -p "Are you sure you wish to continue?"
if [ "$REPLY" != "yes" ]; then
   exit
fi

Enjoin answered 11/12, 2009 at 2:59 Comment(0)

[[ -f ./${sname} ]] && read -p "File exists. Are you sure? " -n 1

[[ ! $REPLY =~ ^[Yy]$ ]] && exit 1

used this in a function to look for an existing file and prompt before overwriting.

Homans answered 6/2, 2012 at 4:21 Comment(0)

echo are you sure?
read x
if [ "$x" = "yes" ]
then
  # do the dangerous stuff
fi

Phenanthrene answered 11/12, 2009 at 2:57 Comment(0)

#!/bin/bash
echo Please, enter your name
read NAME
echo "Hi $NAME!"
if [ "x$NAME" = "xyes" ] ; then
 # do something
fi

I s a short script to read in bash and echo back results.

Ewall answered 11/12, 2009 at 2:59 Comment(0)

qnd: use

read VARNAME
echo $VARNAME

for a one line response without readline support. Then test $VARNAME however you want.

Linis answered 11/12, 2009 at 2:55 Comment(0)

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