Part 9: Case

The case statement saves going through a whole set of if .. then .. else statements. Its syntax is really quite simple:

talk.sh#!/bin/sh

echo "Please talk to me ..."
while :
do
  read INPUT_STRING
  case $INPUT_STRING in
	hello)
		echo "Hello yourself!"
		;;
	bye)
		echo "See you again!"
		break
		;;
	*)
		echo "Sorry, I don't understand"
		;;
  esac
done
echo 
echo "That's all folks!"

Okay, so it's not the best conversationalist in the world; it's only an example!

Try running it and check how it works...

$ ./talk.sh
Please talk to me ...
hello
Hello yourself!
What do you think of politics?
Sorry, I don't understand
bye
See you again!

That's all folks!
$

The syntax is quite simple: The case line itself is always of the same format, and it means that we are testing the value of the variable INPUT_STRING.

The options that we understand are then listed and followed by a right bracket, as hello) and bye).
This means that if INPUT_STRING matches hello then that section of code is executed, up to the double semicolon.
If INPUT_STRING matches bye then the goodbye message is printed and the loop exits. Note that if we wanted to exit the script completely then we would use the command exit instead of break. break just breaks out of the while loop and continues the next part of the script.

The third option here, the *), is the default catch-all condition; it is not required, but is often useful for debugging purposes even if we think we know what values the test variable will have.

The whole case statement is ended with esac (case backwards!) then we end the while loop with a done.

That's about as complicated as case conditions get, but they can be a very useful and powerful tool. They are often used to parse the parameters passed to a shell script, amongst other uses.