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
1.in 字母1-字母n
（1）在basth中
# csdn @ edu in ~ [22:36:03]
$ exec bash
[csdn ~]$ for i in b-d; do
> if [[ $i > 'a' && $i < 'z' ]]; then
> echo "$i is a letter";
> else
> echo "$i is not a letter";
> fi;
> done
b-d is a letter
[csdn ~]$
（2）在zsh中
# csdn @ edu in ~ [22:01:51]
$ 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-d is a letter# csdn @ edu in ~ [22:02:09]
$
2. in [字母1-字母n]
（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
[b-d] is a letter
（2）在zsh中
# csdn @ edu in ~ [22:25:27]
$ 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
zsh: no matches found: [b-d]# csdn @ edu in ~ [22:28:11]
$
3. in [字母1,字母n]
（1）在bash中
[csdn ~]$ for i in [b,d]; do
> if [[ $i > 'a' && $i < 'z' ]]; then
> echo "$i is a letter";
> else
> echo "$i is not a letter";
> fi;
> done
[b,d] is a letter
[csdn ~]$
（2）在zsh中
# csdn @ edu in ~ [22:29:14]
$ 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
zsh: no matches found: [b,d]# csdn @ edu in ~ [22:33:29]
$
4. in '字母1-字母n'
（1）在bash中
[csdn ~]$ for i in 'b-d'; do
> if [[ $i > 'a' && $i < 'z' ]]; then
> echo "$i is a letter";
> else
> echo "$i is not a letter";
> fi;
> done
b-d is a letter
[csdn ~]$
（2）在zsh中
# csdn @ edu in ~ [22:33:29]
$ 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 a letter# csdn @ edu in ~ [22:36:03]
$
5. in '字母1'-'字母n'
（1）在bash中
[csdn ~]$ for i in 'b'-'d'; do
> if [[ $i > 'a' && $i < 'z' ]]; then
> echo "$i is a letter";
> else
> echo "$i is not a letter";
> fi;
> done
b-d is a letter
[csdn ~]$
（2）在zsh中
# csdn @ edu in ~ [22:46:27]
$ 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 a letter# csdn @ edu in ~ [22:46:35]
$
6.in ['字母1'-'字母n']
（1）在bash中
# csdn @ edu in ~ [22:51:56]
$ exec bash
[csdn ~]$ for i in ['b'-'d']; do
> if [[ $i > 'a' && $i < 'z' ]]; then
> echo "$i is a letter";
> else
> echo "$i is not a letter";
> fi;
> done
[b-d] is a letter
[csdn ~]$
（2）在zsh中
# csdn @ edu in ~ [22:51:06]
$ 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
zsh: no matches found: [b-d]# csdn @ edu in ~ [22:51:56]
$
三、文件和目录类循环操作
（一）实例1：列出根目录/下的内容，判断其是文件还是目录并输出
# csdn @ edu in ~ [11:39:57] C:1
$ cd /# csdn @ edu in / [11:40:01]
$ ls
anaconda-post.log bin dev etc home lib lib64 media mnt opt proc root run sbin srv sys tmp usr var# csdn @ edu in / [11:40:04]
$ for i in `ls`; do if [ -f $i ]; then echo $i is a file\! ; elif [ -d $i ]; then echo $i is a directory\! ; else echo $i is other file\!; fi; done
anaconda-post.log is a file!
bin is a directory!
dev is a directory!
etc is a directory!
home is a directory!
lib is a directory!
lib64 is a directory!
media is a directory!
mnt is a directory!
opt is a directory!
proc is a directory!
root is a directory!
run is a directory!
sbin is a directory!
srv is a directory!
sys is a directory!
tmp is a directory!
usr is a directory!
var is a directory!# csdn @ edu in / [11:43:15]
$
我们使用 cd / 命令让根目录/变成当前目录，然后用引用符（Esc键下面的`） 将ls命令获取的根目录/内容作为循环变量用if语句进行判断处理：
用 -f判断是否为文件，如果是文件就输出文件名称并注明is a file
否则用 -d判断是否为目录 ，如果是目录就输出目录名称并注明is a directory
否则就输出文件名称并注明is other file
需要注意的是，我们用引用符（Esc键下面的`） 来引有ls命令获取的根目录/内容。
（二）列出当前目录下的.sh文件名并输出提示is a shell script file !
# csdn @ edu in ~ [14:03:58]
$ ls *.sh
zsh: no matches found: *.sh# csdn @ edu in ~ [14:05:37] C:1
$ echo "hello" > hello.sh# csdn @ edu in ~ [14:05:55]
$ cat hello.sh
hello# csdn @ edu in ~ [14:06:02]
$ for f in $(ls *.sh);do echo $f is a shell script file \! ; done
hello.sh is a shell script file !# csdn @ edu in ~ [14:07:16]
$
在上面的实例中，我们先用ls命令检查，发现当前目录下没有.sh文件
然后 我们用输出重定向创建了一个hello.sh
接着我们用for语句列出当前目录下的.sh文件名，并输出提示is a shell script file !
这里我们同样是引入了ls命令执行的结果，但引用的方法跟 （一）实例1：列出根目录/下的内容，判断其是文件还是目录并输出里不同，大家可以对比一下这两种方法 。
