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
command3
done

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/bash
x=1
while [ $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/bash
counter=$1
factorial=1
while [ $counter -gt 0 ]
do
factorial=$(( $factorial * $counter ))
counter=$(( $counter - 1 ))
done
echo $factorialTo run just type:

$ chmod +x script.sh
$ ./script.sh 5

Output:120While loops are frequently used for reading data line by line from file:#!/bin/bash
FILE=$1
# read $FILE using the file descriptors
exec 3<&0
exec 0<$FILE
while read line
do
# use $line variable to process line
echo $line
done
exec 0<&3You can easily evaluate the options passed on the command line for a script using while loop:......
..
while getopts ae:f:hd:s:qx: option
do
case "${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;;
esac
done
.......
..

How do I use while as infinite loops?

Infinite for while can be created with empty expressions, such as:#!/bin/bash
while :
do
echo "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)
then
break #Abandon the while lopp.
fi
statements3 #While good and, no disaster-condition.
doneIn 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 $ans
done

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)
then
continue #Go to next iteration of I in the loop and skip statements3
fi
statements3
done

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