shell脚本6--循环，比较

for循环

for var in list;

do

　　commands;#使用变量$var

done

 

example:

　　for i in {a..z}; do actions; done;

后者

for((i=0;i<10;i++))

{

　　commands;

}

 

 

while condition

do

　　commands;

done

 

 

until循环

example:

x=0;

until [ $x -eq 9 ];

do

　　let x++; echo $x;

done

 

if比较语句

if contions;

then 

　　commands;

if

else if和else语句

if condition;

then

　　commands;

else if conditon;then 

　　commands;

else

　　commands;

fi

简洁用法：

[ condition ] && action;

[ condition ] || action;

 

算数比较：
[ $var -eq 0 ]

[ $var -ne 0 ]

逻辑与和逻辑或

[ $var1 -ne 0 -a  $var2 -gt 2 ]

[ $var1 -ne 0 -o $var2 -gt 2 ]

 

 

example:

　　

#!/bin/bash
#filename:compare.sh
fpath="/etc/passwd"
if [ -e $fpath ]; then
    echo file exits;
else
    echo does not exists;
fi