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General-purpose Yes/No prompt function ("ask") [Bash]

This is a general-purpose function to ask Yes/No questions in Bash, either with or without a default answer. It keeps repeating the question until it gets a valid answer.

function ask {
while true; do
if [ "${2:-}" = "Y" ]; then
prompt="Y/n"
default=Y
elif [ "${2:-}" = "N" ]; then
prompt="y/N"
default=N
else
prompt="y/n"
default=
fi
# Ask the question
read -p "$1 [$prompt] " REPLY
# Default?
if [ -z "$REPLY" ]; then
REPLY=$default
fi
# Check if the reply is valid
case "$REPLY" in
Y*|y*) return 0 ;;
N*|n*) return 1 ;;
esac
done
}
if ask "Do you want to do such-and-such?"; then
echo "Yes"
else
echo "No"
fi
# Default to Yes if the user presses enter without giving an answer:
if ask "Do you want to do such-and-such?" Y; then
echo "Yes"
else
echo "No"
fi
# Default to No if the user presses enter without giving an answer:
if ask "Do you want to do such-and-such?" N; then
echo "Yes"
else
echo "No"
fi
# Only do something if you say Yes
if ask "Do you want to do such-and-such?"; then
said_yes
fi
# Only do something if you say No
if ! ask "Do you want to do such-and-such?"; then
said_no
fi
# Or if you prefer the shorter version:
ask "Do you want to do such-and-such?" && said_yes
ask "Do you want to do such-and-such?" || said_no
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