shell for和while用法

写法一：
----------------------------------------------------------------------------
#!/bin/bash


while read line
do
    echo $line
done < filename
----------------------------------------------------------------------------


写法二：（并发脚本慎用，grep不能输出全部匹配的信息）
----------------------------------------------------------------------------
#!/bin/bash


cat filename | while read line
do
    echo $line
done
----------------------------------------------------------------------------


写法三：
----------------------------------------------------------------------------
for line in `cat filename`
do
    echo $line
done
----------------------------------------------------------------------------


说明：
for逐行读和while逐行读是有区别的,如:
$ cat filename
aaaa
bbbb  fff  ggg
cccc dddd


$ cat filename | while read line; do echo $line; done
aaaa
bbbb  fff  ggg
cccc dddd


$ for filename in `cat filename`; do echo $line; done
aaaa
bbbb
fff
ggg
cccc
dddd