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

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?

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 [[ $REPLY =~ ^[^Yy]$ ]]

use case/esac.

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

advantage:

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

Try the read shell builtin:

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

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

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

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".

I hope you like it,
Cheers!

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

[[ -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.

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

#!/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.

qnd: use

read VARNAME
echo $VARNAME

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