Книга: Fedora™ Unleashed, 2008 edition

The while Statement

The while Statement

The while statement can be used to execute a series of commands while a specified condition is true. The loop terminates as soon as the specified condition evaluates to False. It is possible that the loop will not execute at all if the specified condition initially evaluates to False. You should be careful with the while command because the loop will never terminate if the specified condition never evaluates to False.

Endless Loops Have Their Place in Shell Programs

Endless loops can sometimes be useful. For example, you can easily construct a simple command that constantly monitors the 802.11b link quality of a network interface by using a few lines of script:

#!/bin/sh
while :
do
 /sbin/iwconfig eth0 | grep Link | tr 'n' 'r'
done

The script outputs the search, and then the tr command formats the output. The result is a simple animation of a constantly updated single line of information:

Link Quality:92/92 Signal level:-11 dBm Noise level:-102 dBm

This technique can also be used to create a graphical monitoring client for X that outputs traffic information and activity about a network interface:

#!/bin/sh
xterm -geometry 75x2 -e
bash -c
"while :; do
 /sbin/ifconfig eth0 |
 grep 'TX bytes' | tr 'n' 'r' ;
done"

The simple example uses a bash command-line script (enabled by -c) to execute a command line repeatedly. The command line pipes the output of the ifconfig command through grep, which searches ifconfig's output and then pipes a line containing the string "TX bytes" to the tr command. The tr command then removes the carriage return at the end of the line to display the information inside an /xterm X11 terminal window, automatically sized by the -geometry option:

RX bytes:4117594780 (3926.8 Mb) TX bytes:452230967 (431.2 Mb)

Endless loops can be so useful that Linux includes a command that repeatedly executes a given command line. For example, you can get a quick report about a system's hardware health by using the sensors command. But rather than use a shell script to loop the output endlessly, you can use the watch command to repeat the information and provide simple animation:

$ watch "sensors -f | cut -c 1-20"

In bash, the following format is used for the while flow-control construct:

while expression
do
 statements
done

If you want to add the first five even numbers, you can use the following shell program in bash:

#!/bin/bash loopcount=0 result=0
while [ $loopcount -lt 5 ] do
 loopcount=`expr $loopcount + 1`
 increment=`expr $loopcount * 2`
 result=`expr $result + $increment`
done
echo "result is $result"

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