Linux结构化命令which
for命令
for命令允许你创建一个遍历一系列值的循环，每次迭代都是用其中一个值来执行已定义好的命令。具体格式如下：
for var in list
do
commands
done
for循环假定每个值都是用空格分隔的，如果有包含空格的数据值，用双引号圈起来
读取列表中的值
$ cat test1
#!/bin/bash
# basic for command
for test in Alabama Alaska Arizona Arkansas California Colorado
do
echo The next state is $test
done
$ ./test1
The next state is Alabama
The next state is Alaska
The next state is Arizona
The next state is Arkansas
The next state is California
The next state is Colorado
读取列表中的复杂值
看下面的例子：
$ cat badtest1
#!/bin/bash
# another example of how not to use the for command
for test in I don't know if this'll work
do
echo "word:$test"
done
$ ./badtest1
word:I
word:dont know if thisll
word:work
结果并不是我们想要的
有两种办法解决该问题：
- 使用转义字符(反斜线)来将单引号转义
- 使用双引号来定义用到单引号的值
$ cat test2
#!/bin/bash
# another example of how not to use the for command
for test in I don\'t know if "this'll" work
do
echo "word:$test"
done
$ ./test2
word:I
word:don't
word:know
word:if
word:this'll
word:work
从变量读取列表
$ cat test4
#!/bin/bash
# using a variable to hold the list
list="Alabama Alaska Arizona Arkansas Colorado"
list=$list" Connecticut"
for state in $list
do
echo "Have you ever visited $state?"
done
$ ./test4
Have you ever visited Alabama?
Have you ever visited Alaska?
Have you ever visited Arizona?
Have you ever visited Arkansas?
Have you ever visited Colorado?
Have you ever visited Connecticut?
从命令读取值
$ cat test5
#!/bin/bash
# reading values from a file
file="states"
for state in $(cat $file)
do
echo "Visit beautiful $state"
done
$ cat states
Alabama
Alaska
Arizona
Arkansas
Colorado
Connecticut
Delaware
Florida
Georgia
$ ./test5
Visit beautiful Alabama
Visit beautiful Alaska
Visit beautiful Arizona
Visit beautiful Arkansas
Visit beautiful Colorado
Visit beautiful Connecticut
Visit beautiful Delaware
Visit beautiful Florida
Visit beautiful Georgia
更改字段分隔符
造成这个问题的原因是特殊的环境变量IFS，叫做内部字段分隔符，IFS环境变量定义了bash shell用作分隔符的一系列字符。默认情况下，bash shell将下列字符当作字段分隔符：
- 空格
- 制表符
- 换行符
要解决上面遇到的处理文件中含有字段分隔符的问题，可以在shell脚本中临时更改IFS环境变量的值来限制被bahs shell当作字段分隔符的字符
$ cat test5b
#!/bin/bash
# reading values from a file
file="states"
IFS=$'\n'
for state in $(cat $file)
do
echo "Visit beautiful $state"
done
$ ./test5b
Visit beautiful Alabama
Visit beautiful Alaska
Visit beautiful Arizona
Visit beautiful Arkansas
Visit beautiful Colorado
Visit beautiful Connecticut
Visit beautiful Delaware
Visit beautiful Florida
Visit beautiful Georgia
Visit beautiful New York
Visit beautiful New Hampshire
Visit beautiful North Carolina
如果要指定多个IFS字符，只要将它们在赋值行串起来就行：
IFS=$'\n':;"
这个赋值会将换行符、冒号、分号和双引号作为字段分隔符
用通配符读取目录
$ cat test6
#!/bin/bash
# iterate through all the files in a directory
for file in /home/rich/test/*
do
if [ -d "$file" ]
then
echo "$file is a directory"
elif [ -f "$file" ]
then
echo "$file is a file"
fi
done
$ ./test6
/home/rich/test/dir1 is a directory
/home/rich/test/myprog.c is a file
/home/rich/test/myprog is a file
/home/rich/test/myscript is a file
/home/rich/test/newdir is a directory
/home/rich/test/newfile is a file
/home/rich/test/newfile2 is a file
/home/rich/test/testdir is a directory
/home/rich/test/testing is a file
/home/rich/test/testprog is a file
/home/rich/test/testprog.c is a file
C语言风格的for命令
bash中C语言风格的for循环的基本格式:
for (( variable assignment ; condition ; iteration process ))
$ cat test8
#!/bin/bash
# testing the C-style for loop
for (( i=1; i <= 10; i++ ))
do
echo "The next number is $i"
done
$ ./test8
The next number is 1
The next number is 2
The next number is 3
The next number is 4
The next number is 5
The next number is 6
The next number is 7
The next number is 8
The next number is 9
The next number is 10
使用多个变量
C语言风格的for命令也允许为迭代使用多个变量。循环会单独处理每个变量，你可以为每个变量定义不同的迭代过程。尽管可以使用多个变量，但你只能在for循环中定义一种条件:
$ cat test9
#!/bin/bash
# multiple variables
for (( a=1, b=10; a <= 10; a++, b-- ))
do
echo "$a - $b"
done
$ ./test9
1 - 10
2 - 9
3 - 8
4 - 7
5 - 6
6 - 5
7 - 4
8 - 3
9 - 2
10 - 1
联系方式：
wechat: yt056410 Email: tyan@zju.edu.cn QQ: 1051927088 GitHub: https://github.com/YTLogos JianShu: http://www.jianshu.com/u/bd001545cf0b Blog: https://ytlogos.github.io/
个人简介：
严涛 浙江大学作物遗传育种在读研究生 R语言爱好者，爱开源
