While and Until loops check an exit status of a command. In both cases this check is made before deciding on wether or not to run an itteration of the code inside the loop. In the case of the Whille loop, the code inside the control flow is run while the check is true. In the case of the Until loop, the code is run until the check becomes true.
The basic form of the While loop is:
while Check something do # Runs code here while the check returns true Some Code done
The basic form of the Until loop is:
until Check something do # Runs code here until the check returns true Some Code done
Here’s an example of an until loop, used to wait until a file exists.
The script starts tomcat, then waits until a tomcat pid file has been created before it returns a success message.
bin/catalina.sh start # Command to start tomcat app server until test -f tomcat.pid do sleep 300 done echo "Tomcat has started"
Shell scripters can be games writers too! Here’s an example of a script that makes use of a While loop:
echo "Are you bored? y/n" read ANSWER while test $ANSWER = "n" do echo "How about now?" read ANSWER done echo "OK. Goodbye"
Adding the above code to a file called "dullgame" means that we can play the game like this:
bash$ sh dullgame Are you bored? y/n n How about now? n How about now? y OK. Goodbye
Of course, there are many good uses for While and Until loops too. Have fun looking out for them!