一、for命令
在shell编程中,有时我们需要重复执行一直命令直至达到某个特定的条件,bash shell中,提供了for命令,允许你创建一个遍历一系列值的循环,每次迭代都通过一个该系列中的值执行一组预定义的命令。
for的基本格式:
for var in list
do
commands
done
在list中,你提供了迭代中要用的一系列值。在每个迭代中,变量var包含列表中的当前值,第一个迭代会适用列表中的第一个值,第二个迭代使用第二个值,以此类推,直至列表中的所有值都过一遍。
1.1读取列表中的值
1
2
3
4
5
6
7
8
9
10
11
12
|
[root@sh shell]
# cat for1.sh
#!/bin/bash
for
test
in
aaa bbb ccc ddd
do
echo
the next state is $
test
done
[root@sh shell]
# sh for1.sh
the next state is aaa
the next state is bbb
the next state is ccc
the next state is ddd
|
$test变量的值会在shell脚本的最后一次迭代中一直保持有效,除非你修改了它
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
|
[root@sh shell]
# cat for1.sh
#!/bin/bash
for
test
in
aaa bbb ccc ddd
do
echo
the next state is $
test
done
echo
"the last state we visited was $test"
test
=fff
echo
"wait. now we're visiting $test"
[root@sh shell]
# sh for1.sh
the next state is aaa
the next state is bbb
the next state is ccc
the next state is ddd
the last state we visited was ddd
wait. now we're visiting fff
|
1.2读取列表中的复杂值
在shell脚本中,优势你会遇到难处理的数。下面是个给shell脚本程序员带来麻烦的经典例子:
1
2
3
4
5
6
7
8
9
10
11
|
[root@sh shell]
# cat for2.sh
#!/bin/bash
for
test
in
I don
't know if this'
ll work
do
echo
"word:$test"
done
[root@sh shell]
# sh for2.sh
word:I
word:dont know
if
thisll
word:work
|
解决办法:使用转义符或者双引号
1
2
3
4
5
6
7
8
9
10
11
12
13
14
|
[root@sh shell]
# cat for2.sh
#!/bin/bash
for
test
in
I don\
't know if "this'
ll" work
do
echo
"word:$test"
done
[root@sh shell]
# sh for2.sh
word:I
word:don't
word:know
word:
if
word:this'll
word:work
|
记住:for循环假定每一个值都是用空格分割的,如果在单独的数字值中有空格,那么你必须使用双引号来将这些值圈起来。
1.3从变量读取列表
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
|
[root@sh shell]
# cat for3.sh
############################
#!/bin/bash
list=
"aaa bbb ccc ddd eee"
list=$list
" Connecticut"
for
state
in
$list
do
echo
"Have you ever visited $state"
done
[root@sh shell]
# sh for3.sh
Have you ever visited aaa
Have you ever visited bbb
Have you ever visited ccc
Have you ever visited ddd
Have you ever visited eee
Have you ever visited Connecticut
|
1.4从命令读取值
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
|
[root@sh shell]
# cat for4.sh
#!/bin/bash
file
=
"/root/shell/states"
#如果是在当前不用绝对路径,file=“states”即可
for
state
in
`
cat
$
file
`
do
echo
"Visit beautiful $state"
done
[root@sh shell]
# sh for4.sh
Visit beautiful shanghai
Visit beautiful beijing
Visit beautiful hangzhou
Visit beautiful nanjing
Visit beautiful guangzhou
[root@sh shell]
# cat states
shanghai
beijing
hangzhou
nanjing
guangzhou
|
1.5更改字段分隔符
-
空格;
-
制表符:
-
换行符
如果bash shell在数据中看到了这些字符中任意一个,它就会假定你在列表中开始了一个新的数据段。
要解决这个问题,你可以在你shell脚本中临时更改IFS环境变量的值来限制一下被bash shell当作字段分隔符的字符。但这种方式有点奇怪,比如,如果你行IFS的值使其只能识别换行符,你必须这么做:
IFS=$'\n'
将这个语句加入到脚本中,告诉bash shell在数据值中忽略空格和制表符。
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
|
[root@sh shell]
# cat for5.sh
#!/bin/bash
file
=
"states"
IFS=$
'\n'
for
state
in
`
cat
$
file
`
do
echo
"Visit beautiful $state"
done
[root@sh shell]
# sh for5.sh
Visit beautiful shanghai
Visit beautiful beijing
Visit beautiful hangzhou
Visit beautiful nanjing
Visit beautiful guang zhou
Visit beautiful nan ning
Visit beautiful jiang nan
|
在处理长脚本中,可能在一个地方需要修改IFS的值,然后忘掉它在脚本中其他地方还原默认值。
例如:
IFS.OLD=$IFS
IFS=$'\n'
<use the new IFS value in code>
IFS=$IFS.OLD
其他的IFS值,如:在/etc/passwd中可能用到
IFS=:
也可以赋值多个IFS:
IFS=$'\n:;"'
1.6用通配符读取目录
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
|
[root@sh shell]
# cat for6.sh
#!/bin/bash
for
file
in
/home/
*
do
if
[ -d
"$file"
];
then
echo
"$file is a directory"
elif
[ -f
"$file"
];
then
echo
"$file is a file"
fi
done
[root@sh shell]
# sh for6.sh
/home/apache-tomcat-8
.0.28.
tar
.gz is a
file
/home/dir1
is a directory
/home/dir2
is a directory
/home/fie1
is a
file
/home/fie2
is a
file
/home/fie22
is a
file
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
|
[root@sh shell]
# cat for7.sh
#!/bin/bash
for
file
in
/home/
*
/home/badtest
do
if
[ -d
"$file"
];
then
echo
"$file is a directory"
elif
[ -f
"$file"
];
then
echo
"$file is a file"
else
echo
"$file doesn't exist"
fi
done
[root@sh shell]
# sh for7.sh
/home/apache-tomcat-8
.0.28.
tar
.gz is a
file
/home/dir1
is a directory
/home/dir2
is a directory
/home/fie1
is a
file
/home/fie2
is a
file
/home/fie22
is a
file
/home/badtest
doesn't exist
|
二、C语言风格的for命令
2.1 C语言风格的for命令
C语言的for命令有一个用来指明变量的特殊方法、一个必须保持成立才能继续迭代的条件,以及另一个为每个迭代改变变量的方法。当指定的条件不成立时,for循环就会停止。条件等式通过标准的数字符号定义。
for (i=0; i<10; i++)
{
printf("The next number is %d\n",i):
}
第一部分将一个默认值赋给该变量,中间的部分定义了循环重复的条件,当定义的条件不成立时,for循环就停止迭代,最后一部分定义了迭代的过程。
bash中C语言风格的for循环的基本格式:
for (( variable assignment;condition;iteration process))
for (( a = 1; a < 10; a++ ))
本文转自 HMLinux 51CTO博客,原文链接:http://blog.51cto.com/7424593/1738811