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 promp

Answer 1

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.

Answer 2

try this one too:

 [[ $yn =~ "^[yY](es)?$" ]] 

Answer 3

Here's an answer that uses extended patterns instead of regular expressions:

shopt -s nocasematch
shopt -s extglob
while [[ $yn = y?(es) ]]; do
  ...
done

Note that starting in version 4.1, bash always uses extended patterns inside the [[ ... ]] conditional expression, so the shopt line is no necessary.

Answer 4

I prefer using grep -E or egrep for short:

while egrep -iq '^(y|yes)$' <<< $yn; do
  your_logic_here
done

Here you have explanations from grep manual:

-i, --ignore-case

Ignore case distinctions in both the PATTERN and the input files. (-i is specified by POSIX.)

-q, --quiet, --silent

Quiet; do not write anything to standard output. Exit immediately with zero status if any match is found, even if an error was detected. Also see the -s or --no-messages option. (-q is specified by POSIX.)

Answer 5

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 ]]