[linux] 结构化命令-for

1 for命令

# for：迭代循环；默认空格为分隔符

for var in list
do
    commands
done

　　1.1 读取列表中的值

#!usr/bin/bash
for test in Hello Python Student School
do
    echo The next wprd is $test
done
echo The last state is $test
#一直保持最后迭代的值，除非删除（或更改）它

　　1.2 读取列表中复杂的值

# 使用转义字符（反斜线），转义特殊字符
# 使用双引号定义用到的单引号（或反之）
# 默认空格为分隔符
#!usr/bin/bash
for test in I don't know if "this'll" work
do
    echo "word: $test"
done

# 双引号创建字符串变量，并作为一个整体出现在列表中

$cat fortest01.sh
#!usr/bin/bash
for var in "the test bash shell"
do
        echo word: $var
done

$sh fortest01.sh
word: the test bash shell

$cat fortest01.sh
#!usr/bin/bash
for var in "the test bash shell" "the last test"
do
        echo word: $var
done

$sh fortest01.sh
word: the test bash shell
word: the last test

1.3 从变量中读取列表

# 注意与直接读取列表的区别：
    # 列表：
        * 列表中特殊字符的转义字符
        * 双引号-特殊字符和字符串变量（整体）
    # 变量
        * 定义字符串变量-双引号/单引号
        * 字符串添加元素
        * for迭代遍历变量元素
$cat varfortest02.sh
#!usr/bin/bash
list01='the first test example'
list02=" the second test example"
list03=$list02" the thrid test example"
list04=$list01$list02$list03
echo $list04
n=0
for var in $list04
do
        (( n++ ))
        echo cycle $n: $var
done

$sh varfortest02.sh
the first test example the second test example the second test example the thrid test example
cycle 1: the
cycle 2: first
cycle 3: test
cycle 4: example
cycle 5: the
cycle 6: second
cycle 7: test
cycle 8: example
cycle 9: the
cycle 10: second
cycle 11: test
cycle 12: example
cycle 13: the
cycle 14: thrid
cycle 15: test
cycle 16: example

 1.4 从命令读取值

# 反引号
        * 使用文件的绝对路径，除非位于同一个目录
        * 默认空格为分隔符
$cat commandfor.sh
#!usr/bin/bash
n=0
for var in `cat varfortest02.sh`
do
        n=$[ $n+1 ]
        echo line $n: $var
done

$sh commandfor.sh
line 1: #!usr/bin/bash
line 2: list01='the
line 3: first
... ...
line 24: do
line 25: ((
line 26: n++
line 27: ))
... ...
line 31: $var
line 32: done

1.5 更改字段分隔符

# 环境变量IFS-内部字段分隔符
        * 默认分隔符：空格；制表符；换行符
        * 更改：单个：IFS=$'
'-(换行)
               多个：IFS=$'
:;"'-(换行/冒号/分号/双引号)
        * 保存与恢复：
                IFS.OLD=$IFS
                IFS=$'
'
                ...
                IFS=$IFS.OLD    

$cat commandfor.sh
#!usr/bin/bash
n=0
IFS.OLD=$IFS
IFS=$'
'
for var in `cat varfortest02.sh`
do
        n=$[ $n+1 ]
        echo line $n: $var
done
IFS=$IFS.OLD

$sh commandfor.sh
commandfor.sh: line 3: IFS.OLD=: command not found
line 1: #!usr/bin/bash
line 2: list01='the first test example'
line 3: list02=" the second test example"
line 4: list03=$list02" the thrid test example"
line 5: list04=$list01$list02$list03
line 6: echo $list04
line 7: n=0
line 8: for var in $list04
line 9: do
line 10:        (( n++ ))
line 11:        echo cycle $n: $var
line 12: done

1.6 用通配符读取文件/目录

# 文件/目录变量尽量用双引号括起来
# 文件/目录查找方法和列表方法合并进同一个for语句
　　　　* 可以添加任意多个通配符，for语句先匹配文件或目录形成列表，然后遍历列表
$cat filefor.sh
#!usr/bin/bash
for file in ./* ./tsttst
do
        if [ -e "$file" ]
        then
                echo The file is $file
        else
                echo The fiel $file do not exist!
        fi
done

$sh filefor.sh
The file is ./commandfor.sh
The file is ./filefor.sh
The file is ./fortest01.sh
The file is ./varfortest02.sh
The fiel ./tsttst do not exist!