bash shell while语法

在编写脚本时，一定要注意空格

基本语法:

while [ condition ]do   command1   command2   command3done

condition为true时命令1到命令3将会一直执行，知道条件为false ，例如：

#!/bin/bashx=1while [ $x -le 5 ]do  echo "Welcome $x times"  x=$(( $x + 1 ))done

Here is a sample shell code to calculate factorial using while loop:

#!/bin/bashcounter=$1factorial=1while [ $counter -gt 0 ]do   factorial=$(( $factorial * $counter ))   counter=$(( $counter - 1 ))doneecho $factorial

To run just type:
$ chmod +x script.sh
$ ./script.sh 5
Output:

120

While loops are frequently used for reading data line by line from file:

#!/bin/bashFILE=$1# read $FILE using the file descriptorsexec 3<&0exec 0<$FILEwhile read linedo# use $line variable to process lineecho $linedoneexec 0<&3

You can easily evaluate the options passed on the command line for a script using while loop:

........while getopts ae:f:hd:s:qx: optiondo        case "${option}"        in                a) ALARM="TRUE";;                e) ADMIN=${OPTARG};;                d) DOMAIN=${OPTARG};;                f) SERVERFILE=$OPTARG;;                s) WHOIS_SERVER=$OPTARG;;                q) QUIET="TRUE";;                x) WARNDAYS=$OPTARG;;                \?) usage                    exit 1;;        esacdone.........

How do I use while as infinite loops?

Infinite for while can be created with empty expressions, such as:

#!/bin/bashwhile :doecho "infinite loops [ hit CTRL+C to stop]"done

Conditional while loop exit with break statement

You can do early exit with the break statement inside the whil loop. You can exit from within a WHILE using break. General break statement inside the while loop is as follows:

while [ condition ]do   statements1      #Executed as long as condition is true and/or, up to a disaster-condition if any.   statements2  if (disaster-condition)  thenbreak          #Abandon the while lopp.  fi  statements3          #While good and, no disaster-condition.done

In this example, the break statement will skip the while loop when user enters -1, otherwise it will keep adding two numbers:

#!/bin/bash while :doread -p "Enter two numnbers ( - 1 to quit ) : " a bif [ $a -eq -1 ]thenbreakfians=$(( a + b ))echo $ansdone

Early continuation with the continue statement

To resume the next iteration of the enclosing WHILE loop use the continue statement as follows:

while [ condition ]do  statements1      #Executed as long as condition is true and/or, up to a disaster-condition if any.  statements2  if (condition)  thencontinue   #Go to next iteration of I in the loop and skip statements3  fi  statements3done

while [ condition ]do   statements1      #Executed as long as condition is true and/or, up to a disaster-condition if any.   statements2  if (disaster-condition)  thenbreak          #Abandon the while lopp.  fi  statements3          #While good and, no disaster-condition.done