Write the program using an if command and then rewrite it using a case command. This program can be useful when reading yes/no responses from the terminal.

Write a program called isyes that returns an exit status of 0 if its argument is yes, and 1 otherwise. For purposes of this exercise, consider y, yes, Yes, YES, and Y all to be valid yes arguments:
$ isyes yes
$ echo $?
0
$ isyes no
$ echo $?
1

The first version with the if conditional is:
if [ -n "$( echo $1 | grep -E '(^[Yy][Ee]{0,1}[Ss]{0,1}$)')" ] ; then
exit 0
else
exit 1
fi
And the second version with the case statement is:
case "$1" in
[Yy][Ee]{0,1}[Ss]{0,1} ) exit 0 ;;
* ) exit 1 ;;
esac