35.15.1. Looping Until a Command Succeeds

The until loop runs a command repeatedly until it succeeds. That is, if the command returns a nonzero status, the shell executes the body of the loop and then runs the loop control command again. The shell keeps running the command until it returns a zero status, as shown in the following example:

% cat sysmgr
#!/bin/sh
until who | grep "^barb "
do sleep 60
done
echo The system manager just logged on.
% sysmgr &
[1] 2345
     ...time passes...
barb     ttyp7   Jul 15 09:30
The system manager just logged on.

The loop runs who (Section 2.8) and pipes that output to grep (Section 13.1), which searches for any line starting with barb and a space. (The space makes sure that usernames like barbara don't match.) If grep returns a nonzero status (no lines matched), the shell waits 60 seconds. Then the loop repeats, and the script tries the who | grep command again. It keeps doing this until grep returns a zero status -- then the loop is broken and control goes past the done line. The echo command prints a message and the script quits. (I ran this script in the background so I could do something else while I waited for Barb.)

This is also a useful way to get someone with whom you share a machine to turn on their cell phone: just set a loop to wait until they login and then send them a write message (in case they don't always check their email, like a few nocturnal system administrators I know).

[A Bourne shell until loop is not identical to the until construction in most programming languages, because the condition is evaluated at the top of the loop. Virtually all languages with an until loop evaluate the condition at the bottom. -- ML]

35.15.2. Looping Until a Command Fails

Go to http://examples.oreilly.com/upt3 for more information on: catsaway

The while loop is the opposite of the until loop. A while loop runs a command and loops until the command fails (returns a nonzero status). The catsaway program below uses a while loop to watch the who output for the system manager to log off. It's the opposite of the sysmgr script.

/dev/null Section 43.12

% cat catsaway
#!/bin/sh
while who | grep "^barb " > /dev/null
do sleep 60
done
echo "The cat's away..."
% catsaway &
[1] 4567
     ...time passes...
The cat's away...

-- JP