Handling Input Confirmations in Linux Shell Scripting

Handling input confirmations in Linux shell scripting

yes | ./script will answer y for everything.

Otherwise, write a script that prints the answers you want, eg:

 echo N
 echo Y
 echo Y

How do I prompt for Yes/No/Cancel input in a Linux shell script?

The simplest and most widely available method to get user input at a shell prompt is the read command. The best way to illustrate its use is a simple demonstration:

while true; do
    read -p "Do you wish to install this program? " yn
    case $yn in
        [Yy]* ) make install; break;;
        [Nn]* ) exit;;
        * ) echo "Please answer yes or no.";;
    esac
done

Another method, pointed out by Steven Huwig, is Bash's select command. Here is the same example using select:

echo "Do you wish to install this program?"
select yn in "Yes" "No"; do
    case $yn in
        Yes ) make install; break;;
        No ) exit;;
    esac
done

With select you don't need to sanitize the input – it displays the available choices, and you type a number corresponding to your choice. It also loops automatically, so there's no need for a while true loop to retry if they give invalid input.

Also, Léa Gris demonstrated a way to make the request language agnostic in her answer. Adapting my first example to better serve multiple languages might look like this:

set -- $(locale LC_MESSAGES)
yesexpr="$1"; noexpr="$2"; yesword="$3"; noword="$4"

while true; do
    read -p "Install (${yesword} / ${noword})? " yn
    if [[ "$yn" =~ $yesexpr ]]; then make install; exit; fi
    if [[ "$yn" =~ $noexpr ]]; then exit; fi
    echo "Answer ${yesword} / ${noword}."
done

Obviously other communication strings remain untranslated here (Install, Answer) which would need to be addressed in a more fully completed translation, but even a partial translation would be helpful in many cases.

Finally, please check out the excellent answer by F. Hauri.

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

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

In Bash, how to add Are you sure [Y/n] to any command or alias?

These are more compact and versatile forms of Hamish's answer. They handle any mixture of upper and lower case letters:

read -r -p "Are you sure? [y/N] " response
case "$response" in
    [yY][eE][sS]|[yY]) 
        do_something
        ;;
    *)
        do_something_else
        ;;
esac

Or, for Bash >= version 3.2:

read -r -p "Are you sure? [y/N] " response
if [[ "$response" =~ ^([yY][eE][sS]|[yY])$ ]]
then
    do_something
else
    do_something_else
fi

Note: If $response is an empty string, it will give an error. To fix, simply add quotation marks: "$response". – Always use double quotes in variables containing strings (e.g.: prefer to use "$@" instead $@).

Or, Bash 4.x:

read -r -p "Are you sure? [y/N] " response
response=${response,,}    # tolower
if [[ "$response" =~ ^(yes|y)$ ]]
...

Edit:

In response to your edit, here's how you'd create and use a confirm command based on the first version in my answer (it would work similarly with the other two):

confirm() {
    # call with a prompt string or use a default
    read -r -p "${1:-Are you sure? [y/N]} " response
    case "$response" in
        [yY][eE][sS]|[yY]) 
            true
            ;;
        *)
            false
            ;;
    esac
}

To use this function:

confirm && hg push ssh://..

or

confirm "Would you really like to do a push?" && hg push ssh://..

How do I prompt for Yes/No/Cancel input in a Linux shell script?

The simplest and most widely available method to get user input at a shell prompt is the read command. The best way to illustrate its use is a simple demonstration:

while true; do
    read -p "Do you wish to install this program? " yn
    case $yn in
        [Yy]* ) make install; break;;
        [Nn]* ) exit;;
        * ) echo "Please answer yes or no.";;
    esac
done

Another method, pointed out by Steven Huwig, is Bash's select command. Here is the same example using select:

echo "Do you wish to install this program?"
select yn in "Yes" "No"; do
    case $yn in
        Yes ) make install; break;;
        No ) exit;;
    esac
done

With select you don't need to sanitize the input – it displays the available choices, and you type a number corresponding to your choice. It also loops automatically, so there's no need for a while true loop to retry if they give invalid input.

Also, Léa Gris demonstrated a way to make the request language agnostic in her answer. Adapting my first example to better serve multiple languages might look like this:

set -- $(locale LC_MESSAGES)
yesexpr="$1"; noexpr="$2"; yesword="$3"; noword="$4"

while true; do
    read -p "Install (${yesword} / ${noword})? " yn
    if [[ "$yn" =~ $yesexpr ]]; then make install; exit; fi
    if [[ "$yn" =~ $noexpr ]]; then exit; fi
    echo "Answer ${yesword} / ${noword}."
done

Obviously other communication strings remain untranslated here (Install, Answer) which would need to be addressed in a more fully completed translation, but even a partial translation would be helpful in many cases.

Finally, please check out the excellent answer by F. Hauri.

Is there any way to check if there is any input then contiue?

There are ways, but you want to keep it simple, yes?

Maybe the logic you want is something like

while [[ -z "$bussno" ]]; do read -r -p "Please enter a business number: " bussno; done
while [[ -z "$startdate" ]]; do read -r -p "Please enter a start date: " startdate; done

This still leaves a lot to be desired in terms of data confirmation, though.

You can add a regex for that if you want, and some follow up confirmations.

For example, for the date,

valid_date='^[[:digit:]]{8}$'
msg="A valid start date is today or later as YYYYMMDD, e.g.: 20220823"
echo "$msg"
until [[ -n "$startdate" && $startdate =~ $valid_date  ]]; do
  read -p "Please enter a valid start date: " startdate
  if date -d "$startdate" >/dev/null 2>&1 &&   # fail if invalid date
     (( "$(date +'%Y%m%d')" <= "$startdate" )) # fail if in past
  then break
  else echo "'$startdate' is not a valid date."
       echo "$msg"
       startdate=                            # reset
  fi
done

These still leave much to be desired, but it's a start.

How do I read user input into a variable in Bash?

Use read -p:

# fullname="USER INPUT"
read -p "Enter fullname: " fullname
# user="USER INPUT"
read -p "Enter user: " user

If you like to get the user's confirmation:

read -p "Continue? (Y/N): " confirm && [[ $confirm == [yY] || $confirm == [yY][eE][sS] ]] || exit 1

You should also quote your variables to prevent pathname expansion and word splitting with spaces:

# passwd "$user"
# mkdir "$home"
# chown "$user:$group" "$home"

User input to set cal month and year in linux shell script

Does this example help?

#!/bin/bash
month=$1
year=`date +%Y`

cal $month $year

get the command line parameter to set the month

set the year with the date command

call cal

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