第八章 shell学习之循环和结构化命令

4r221

for循环
1.
列表for循环
for variable in {list}  #有些像C++/CLR中的for each
do
...
done
如：
1.
[iyunv@localhost tmp]# cat b.sh
#! /bin/bash
for i in 1 2 3 4 5      #1 2 3 4 5等价于{1..5}
do
echo $i
done
[iyunv@localhost tmp]# ./b.sh
1
2
3
4
5
2.
[iyunv@localhost tmp]# cat b.sh
#! /bin/bash
for i in $(seq 1 2 6)   #从1增加到6，步幅为2，相当于`seq 1 2 6`
do
echo $i
done
[iyunv@localhost tmp]# ./b.sh
1
3
5
3.
[iyunv@localhost tmp]# cat b.sh
#! /bin/bash
for i in $( ls )     #用命令ls的内容替换$( ls )
do
echo $i
done
[iyunv@localhost tmp]# ./b.sh
1c
a
a1
a1~
...
4.
[iyunv@localhost tmp]# cat b.sh
#! /bin/bash
for i in "$*"   #把输入的所有参数看做一个字符串整体
do
echo $i
done
[iyunv@localhost tmp]# vim b.sh
[iyunv@localhost tmp]# ./b.sh 1 2 3
1 2 3
[iyunv@localhost tmp]# cat b.sh
#! /bin/bash
for i in "$@"  #把输入的参数分开看待
do
echo $i
done
[iyunv@localhost tmp]# ./b.sh 1 2 3
1
2
3
2.
不带列表的for循环
for variable
do
...
done
相当于
for variable in "$@"
do
...
done
3.
类C风格的for循环
for((expr1;expr2;expr3))
do
...
done
如：
for((i=1;i<=100;i+=2))
do
...
done
while循环
1.
[iyunv@localhost tmp]# cat b1.sh
#! /bin/bash
echo "please enter start num:"
read i
echo ""$i"~10:"
while [ "$i" -ne 10 ]
do
echo $i
let "i++"
done
[iyunv@localhost tmp]# ./b1.sh
please enter start num:
6
6~10:
6
7
8
9
2.类C语言模式
[iyunv@localhost tmp]# cat b1.sh
#! /bin/bash
echo "please enter start num:"
read i
echo ""$i"~10:"
while ((i!=10))   #注意(())，和[ "$i" -ne 10 ]等价
do
echo $i
let "i++"
done
[iyunv@localhost tmp]# ./b1.sh
please enter start num:
6
6~10:
6
7
8
9
3.
[iyunv@localhost tmp]# cat b2.sh
#! /bin/bash
echo "you enter "$#" arguments"  #$#为参数的个数
while [ "$*" != "" ]  #注意空格，等价于[ "$#" != 0 ](把"$#"当做字符串)和[ "$#" -ne 0 ]（吧$#当做数字）
do
echo "$1"
shift                        #shift为丢弃第一个参数（$1），然后$1变为$2，$2变为$3以此类推
done
[iyunv@localhost tmp]# ./b2.sh hello world !
you enter 3 arguments
hello
world
!
until循环
和while循环类似，只是while当条件为真时继续循环，而until当条件为假时继续循环
[iyunv@localhost tmp]# cat b2.sh
#! /bin/bash
echo "you enter "$#" arguments"
until [ "$#" = 0 ]
do
echo "$1"
shift
done
[iyunv@localhost tmp]# ./b2.sh hello world !
you enter 3 arguments
hello
world
!
控制字符
break 跳出最内层的循环
continue 跳出当前循环执行下一次的循环
select结构
select结构为bash的扩展结构，类似于case，但交互性更好
例：
1.
[iyunv@localhost tmp]# cat b3.sh
#! /bin/bash
echo "what's your favorite color?"
select color in "red" "blue" "green"
do
break
done
echo "you have selected $color"
[iyunv@localhost tmp]# ./b3.sh
what's your favorite color?
1) red
2) blue
3) green
#? 2
you have selected blue
2.
[iyunv@localhost tmp]# cat b3.sh
#! /bin/bash
echo "what's your favorite color?"
select color          #相当于select color in "$@"，类似于for color的情况
do
break
done
echo "you have selected $color"
[iyunv@localhost tmp]# ./b3.sh red blue green
what's your favorite color?
1) red
2) blue
3) green
#? 2
you have selected blue