Repeated-Action Commands

The Bourne shell provides three repeated-action commands, each of which corresponds to constructs you might have seen before in other programming languages:

• for

• while

• until

These commands cause a program to loop or repeat. They are described next.

The for Loop

A useful shell command is the for loop, which is the simplest way to set up repetition in a shell script. The syntax of a for loop is as follows:

for name [ in wordlist . . . ] 
do list 
done 

Each time a for command is executed, name is set to the next word taken from the in word list. If in word . . . is omitted, the for command executes the do list once for each positional parameter that is set. Execution ends when no more words are in the list. The following example illustrates a simple for loop:

for i in eat run jump play 
do 
    echo See spot $i 
done 

When the program is executed, the system responds with this:

See spot eat 
See spot run 
See spot jump 
See spot play 

If you want to enter data interactively, you can add the shell special command read:

echo Hello- What's your name? 
read name 
for i in $name 
do 
    echo $i 
done 

When executing the program, the user is asked to enter the word list. Notice the use of the backslash () so that the ' and the ? are taken literally and are not used as special characters.

The while Loop

A while loop repeats a set of commands continuously until a condition is met. The syntax for a while loop is as follows:

while condition-list 
do commands 
done 

First, the condition-list is executed. If it returns a true exit status, the do list is executed, and the operation restarts from the beginning. If the condition-list returns a false exit status, the conditional structure is complete.

The following illustrates the while loop. The program checks to see if the file /tmp/errlog is present. If the file is not present, the program exits the loop. If the file is present, the program prints a message and runs again every five seconds until the file is removed.

while [ -f /tmp/errlog ] 
do echo The file is still there ; sleep 5 
done 

The until Loop

The until loop is a variant of the while statement. Just as the while statement repeats as long as the condition-list returns a true value, the until statement repeats as long as the condition-list returns a false value. The following example continues to display a message every five seconds until the file is created:

until [ -f  /tmp/errlog ] 
    do echo the file is missing; sleep 5 
done 

Conditional structures such as while and until are executed by the shell as if they were a single command. The shell scans the entire structure before any part of it is executed.