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SHELL中$@和$*的比较

时间:2015-07-29 06:32:56      阅读:149      评论:0      收藏:0      [点我收藏+]

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[root@localhost ceshi]# cat test.sh

#!/bin/bash

# test.sh

echo "-------------ISF is set to \"-seperator\" ------------"

IFS="-seperator";  # 注意 seperator前有一个减号(-)

for i in "$@"; do echo "@ ‘$i‘"; done

for i in "$*"; do echo "* ‘$i‘"; done


echo "-------------ISF is set to null ------------------------"

IFS=

for i in "$@"; do echo "@ ‘$i‘"; done

for i in "$*"; do echo "* ‘$i‘"; done


echo "-------------ISF is unset ------------------------"

unset IFS

for i in "$@"; do echo "@ ‘$i‘"; done

for i in "$*"; do echo "* ‘$i‘"; done



echo ‘---------$* and $@ are not put into double quotes(" ")-------‘

for i in $@; do echo "@ ‘$i‘"; done

for i in $*; do echo "* ‘$i‘"; done


test.sh aa bb cc执行的结果如下:

[root@localhost ceshi]# sh test.sh aa bb cc 

-------------ISF is set to "-seperator" ------------

@ ‘aa‘

@ ‘bb‘

@ ‘cc‘

* ‘aa-bb-cc‘

-------------ISF is set to null ------------------------

@ ‘aa‘

@ ‘bb‘

@ ‘cc‘

* ‘aabbcc‘

-------------ISF is unset ------------------------

@ ‘aa‘

@ ‘bb‘

@ ‘cc‘

* ‘aa bb cc‘

---------$* and $@ are not put into double quotes(" ")-------

@ ‘aa‘

@ ‘bb‘

@ ‘cc‘

* ‘aa‘

* ‘bb‘

* ‘cc‘


由此可见,

1) 当不加双引号("")时, $*,$@被展开时的行为是一样的;

2) 当$*,$@都被放到双引号("")内;

    2.1) 如果设置了变量IFS的值并且该值非空, 则$*被展开时使用${IFS}的第一个字符将所有参数(除了参数$0)连接起来, 即"$1c$2c$3c...",其中c表示${IFS}的第一个字符;

    2.2) 如果变量IFS为空, 则$*被展开时只是将所有参数(除了参数$0)简单连接起来, 即 “$1$2$3..."

    2.3)如果变量IFS没有被定义, 则$*被展开时使用空格字符将所有参数(除了参数$0) 连接起来, 即"$1 $2 $3 ..."

    但$@的展开和没有加双引号的情形是一致的。


更多的Shell脚本例子:

#!/bin/bash

set ‘apple pie‘ pears peaches

for i in $* #单引号被去掉,循环单个字符输出#

do echo $i

done


[root@localhost ceshi]# sh test1.sh

apple

pie

pears

peaches

--------------------------------------------------------------


#!/bin/bash

set ‘apple pie‘ pears peaches

for i in "$*" #单引号被去掉,但当作字符串一次输出#

do echo $i

done


[root@localhost ceshi]# sh test2.sh

apple pie pears peaches

-----------------------------------------------------------------




#!/bin/bash

set ‘apple pie‘ pears peaches

for i in $@ #单引号被去掉,循环单个字符输出#

do echo $i 

done

[root@localhost ceshi]# sh test3.sh

apple

pie

pears

peaches

-----------------------------------------------------------------


#!/bin/bash

set ‘apple pie‘ pears peaches

for i in "$@" #每个位置参数都将当成一个加引号的字符串:"apple pie","pears ","peaches"#

do echo $i

done

[root@localhost ceshi]# sh test4.sh

apple pie

pears

peaches

-----------------------------------------------------------------




test5.sh


sum=0

for var

in "$@"

do

let sum=sum+$var

done

echo "sum=$sum"

 


./test1.sh 1 2 3


结果为sum=6

-----------------------------------------------------------------

 

test6.sh


sum=0

for var

in "$*"

do

let sum=sum+$var

done

echo "sum=$sum"



./test2.sh 1 2 3


结果为sum=1

-----------------------------------------------------------------

 


分析一:


bash -x test1.sh


结果如下:


+ sum=0

+ for var in ‘ "$@" ‘

+ let sum=sum+1

+ for var in ‘ "$@" ‘

+ let sum=sum+2

+ for var in ‘ "$@" ‘

+ let sum=sum+3

+ echo sum=6

sum=6


可以看出for执行了三次,说明for是以""为边界的,因为每个参数是以""被包围的


 


分析二:


bash -x test2.sh


结果如下:


+ sum=0

+ for var in ‘"$*"‘

+ let sum=sum+1 2 3

+ echo sum=1

sum=1


可以看出循环只执行了一次,说明$*整体是以""包围的,不是每个参数被""包围,因此只执行一次


 



一个更直接的例子是:


for var

in "a b c"

do

echo "${var}@"

done


打印:


a b c@


 


for var

in "a" "b" "c"

do

echo "${var}@"

done


打印:


a@


b@


c@


本文出自 “Flylinux” 博客,转载请与作者联系!

SHELL中$@和$*的比较

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原文地址:http://flylinux.blog.51cto.com/9896978/1679435

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