Linux shell编程学习笔记17：for循环语句

Linux Shell 脚本编程和其他编程语言一样，支持算数、关系、布尔、字符串、文件测试等多种运算，同样也需要进行根据条件进行流程控制，提供了if、for、while、until等语句。
之前我们探讨了if语句，现在我们来探讨for循环语句。

Linux Shell中的for语句十分灵活，格式多样，我们通过实例看看一些常用的格式。

一、数字条件类循环

（一）格式1：C语言风格

# csdn @ edu in ~ [22:14:49]
$ for (( i=1; i <=10; i++)); do
for> echo "$i * 2 = $(expr $i \* 2)";
for> done
1 * 2 = 2
2 * 2 = 4
3 * 2 = 6
4 * 2 = 8
5 * 2 = 10
6 * 2 = 12
7 * 2 = 14
8 * 2 = 16
9 * 2 = 18
10 * 2 = 20

# csdn @ edu in ~ [22:15:25]
$

（二）格式2：in + 完整数列

1.在zsh中

# csdn @ edu in ~ [23:22:07] C:127
$ for i in 1 2 3 4 5 6 7 8 9 10; do
for> echo "$i * 2 = $(expr $i \* 2)";
for> done
1 * 2 = 2
2 * 2 = 4
3 * 2 = 6
4 * 2 = 8
5 * 2 = 10
6 * 2 = 12
7 * 2 = 14
8 * 2 = 16
9 * 2 = 18
10 * 2 = 20

# csdn @ edu in ~ [23:22:44]
$

2.在bash中

[csdn ~]$ for i in 1 2 3 4 5 6 7 8 9 10; do echo "$i * 2 = $(expr $i \* 2)"; done
1 * 2 = 2
2 * 2 = 4
3 * 2 = 6
4 * 2 = 8
5 * 2 = 10
6 * 2 = 12
7 * 2 = 14
8 * 2 = 16
9 * 2 = 18
10 * 2 = 20
[csdn ~]$

（三）格式3：使用 in + seq集合

# csdn @ edu in ~ [23:27:05]
$ for i in $(seq 1 10) ; do
for> echo "$i * 2 = $(expr $i \* 2)";
for> done
1 * 2 = 2
2 * 2 = 4
3 * 2 = 6
4 * 2 = 8
5 * 2 = 10
6 * 2 = 12
7 * 2 = 14
8 * 2 = 16
9 * 2 = 18
10 * 2 = 20

# csdn @ edu in ~ [23:28:42]
$

（四）格式4：用 in + {..}集合

1.多余空格导致错误

# csdn @ edu in ~ [23:36:02]
$ for i in {1..10}; do
zsh: command not found: do
zsh: command not found: do
zsh: command not found: do
zsh: command not found: do
zsh: command not found: do
zsh: command not found: do
zsh: command not found: do
zsh: command not found: do
zsh: command not found: do
zsh: command not found: do

导致出错的原因是命令

for i in {1..10}; do

中的 }; 和行末的 do 之间多放了几个空格，只需要一个空格就行。

2.正确的格式

# csdn @ edu in ~ [23:36:09] C:127
$ for i in {1..10}; do
for> echo "$i * 2 = $(expr $i \* 2)";
for> done
1 * 2 = 2
2 * 2 = 4
3 * 2 = 6
4 * 2 = 8
5 * 2 = 10
6 * 2 = 12
7 * 2 = 14
8 * 2 = 16
9 * 2 = 18
10 * 2 = 20

# csdn @ edu in ~ [23:36:47]
$

（五）格式5：在awk中应用

# csdn @ edu in ~ [22:41:42]
$ awk 'BEGIN {for( i=1; i<=10; i++ ) print i "*2=" i*2}'
1*2=2
2*2=4
3*2=6
4*2=8
5*2=10
6*2=12
7*2=14
8*2=16
9*2=18
10*2=20

# csdn @ edu in ~ [22:41:57]
$

二、字符条件类循环

（一）格式1：in 字符串

1.在bash中有效

[csdn ~]$ s="b d 3";for i in $s; do if [[ $i > 'a' && $i < 'z' ]]; then echo $i is a lowercase letter; else echo $i is not a letter; fi; done
b is a lowercase letter
d is a lowercase letter
3 is not a letter
[csdn ~]$

2.在zsh中无效

[csdn ~]$ exec zsh

# csdn @ edu in ~ [14:00:44]
$ s="b d 3";for i in $s; do if [[ $i > 'a' && $i < 'z' ]]; then echo $i is a lowercase letter; else echo $i is not a letter; fi; done
b d 3 is a lowercase letter

# csdn @ edu in ~ [14:00:55]
$

（二）格式1：in 字符1 字符2 …… 字符n

1.在bash中部分有效

# csdn @ edu in ~ [14:26:43]
$ exec bash
[csdn ~]$ for i in 'b' 'd' '3'; do if [[ $i > 'a' && $i < 'z' ]]; then echo $i is a lowercase letter; else echo $i is not a letter; fi; done
b is a lowercase letter
d is a lowercase letter
3 is not a letter
[csdn ~]$ for i in b d 3; do if [[ $i > 'a' && $i < 'z' ]]; then echo $i is a lowercase letter; else echo $i is not a letter; fi; done
>
> ^C
[csdn ~]$ for i in 'b d 3'; do if [[ $i > 'a' && $i < 'z' ]]; then echo $i is a lowercase letter; else echo $i is not a letter; fi; done ;
> ^C
[csdn ~]$ for i in "b d 3"; do if [[ $i > 'a' && $i < 'z' ]]; then echo $i is a lowercase letter; else echo $i is not a letter; fi; done ;
> ^C
[csdn ~]$ for i in "b d 3"; do if [[ $i > 'a' && $i < 'z' ]]; then echo $i is a lowercase letter; else echo $i is not a letter; fi; done
> ^C
[csdn ~]$

2.在zsh中有效

# csdn @ edu in ~ [14:24:52]
$ for i in 'b' 'd' '3'; do if [[ $i > 'a' && $i < 'z' ]]; then echo $i is a lowercase letter; else echo $i is not a letter; fi; done
b is a lowercase letter
d is a lowercase letter
3 is not a letter

# csdn @ edu in ~ [14:25:11]
$ for i in b d 3; do if [[ $i > 'a' && $i < 'z' ]]; then echo $i is a lowercase letter; else echo $i is not a letter; fi; done
b is a lowercase letter
d is a lowercase letter
3 is not a letter

# csdn @ edu in ~ [14:26:43]
$

（三）格式3：in {..}

1.在bash中有效

[csdn ~]$ for i in {b..d}; do
> if [[ $i > 'a' && $i < 'z' ]]; then
> echo "$i is a letter";
> else
> echo "$i is not a letter";
> fi;
> done
b is a letter
c is a letter
d is a letter
[csdn ~]$

或者

for i in {b..d}; do if [[ $i > 'a' && $i < 'z' ]]; then echo "$i is a letter"; else echo "$i is not a letter"; fi; done

这条命令在csdn的linux学习环境中显示如下：

2.在zsh中无效

[csdn ~]$ exec zsh

# csdn @ edu in ~ [21:58:49]
$ for i in {b..d}; do
for> if [[ $i > 'a' && $i < 'z' ]]; then
for then> echo "$i is a letter";
for then> else
for else> echo "$i is not a letter";
for else> fi;
for> done
{b..d} is not a letter

# csdn @ edu in ~ [21:59:03]
$

（四） in [ 字母1,字母n ]

注意： [后面和] 的前面加了空格

1.在bash中

[csdn ~]$ for i in [ b,d ]; do
> if [[ $i > 'a' && $i < 'z' ]]; then
> echo "$i is a letter";
> else
> echo "$i is not a letter";
> fi;
> done
[ is not a letter
b,d is a letter
] is not a letter
[csdn ~]$

2.在zsh中

# csdn @ edu in ~ [23:08:28]
$ for i in [ b,d ]; do
for> if [[ $i > 'a' && $i < 'z' ]]; then
for then> echo "$i is a letter";
for then> else
for else> echo "$i is not a letter";
for else> fi;
for> done
[ is not a letter
b,d is a letter
] is not a letter

# csdn @ edu in ~ [23:12:14]
$

（五） in [ 字母1 ，字母n ]

注意：[ 字母1 ，字母n ] 之间均有空格间隔。

1.在bash中

[csdn ~]$ for i in [ b , d ]; do
> if [[ $i > 'a' && $i < 'z' ]]; then
> echo "$i is a letter";
> else
> echo "$i is not a letter";
> fi;
> done
[ is not a letter
b is a letter
, is not a letter
d is a letter
] is not a letter
[csdn ~]$

2.在zsh中

[csdn ~]$ exec zsh

# csdn @ edu in ~ [23:08:21]
$ for i in [ b , d ]; do
for> if [[ $i > 'a' && $i < 'z' ]]; then
for then> echo "$i is a letter";
for then> else
for else> echo "$i is not a letter";
for else> fi;
for> done
[ is not a letter
b is a letter
, is not a letter
d is a letter
] is not a letter

（六）一些无效的格式