Case insensitive comparison in Bash

Question

I'm trying to write a comparison in a while statement that's case insensitive. Basically, I'm simply trying to shorten the following to act on a yes or no question prompt to the user ...

while[ $yn == "y" | $yn == "Y" | $yn == "Yes" | $yn == "yes" ] ; do

What would be the best way to go about this?

Answer 1

shopt -s nocasematch
while [[ $yn == y || $yn == "yes" ]] ; do

or :

shopt -s nocasematch
while [[ $yn =~ (y|yes) ]] ; do

Note

• [[ is a bash keyword similar to (but more powerful than) the [ command. See http://mywiki.wooledge.org/BashFAQ/031 and http://mywiki.wooledge.org/BashGuide/TestsAndConditionals
  Unless you're writing for POSIX sh, we recommend [[.
• The =~ operator of [[ evaluates the left hand string against the right hand extended regular expression (ERE). After a successful match, BASH_REMATCH can be used to expand matched groups from the pattern. Quoted parts of the regex become literal. To be safe & compatible, put the regex in a parameter and do [[ $string =~ $regex ]]

Answer 2

No need to use shopt or regex. The easiest and quickest way to do this (as long as you have Bash 4):

if [ "${var1,,}" = "${var2,,}" ]; then
  echo "matched"
fi

All you're doing there is converting both strings to lowercase and comparing the results.