shell中while的使用方法

2016-10-17 16:26:02 2185

0 在编写脚本时，一定要注意空格基本语法: while [ condition ]do command1 command2 command3donecondition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 line echo $linedoneexec 0<&3 You can easily evaluate the options passed on the command line for a script using while loop: ........while getopts ae:f:hd:s:qx: optiondo case "${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 :do echo "infinite loops [ hit CTRL+C to stop]"doneConditional 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) then break #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 :do read -p "Enter two numnbers ( - 1 to quit ) : " a b if [ $a -eq -1 ] then break fi ans=$(( a + b )) echo $ansdoneEarly 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) then continue #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) then break #Abandon the while lopp. fi statements3 #While good and, no disaster-condition.done 0
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