[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=
 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 : the
 cycle : first
 cycle : test
 cycle : example
 cycle : the
 cycle : second
 cycle : test
 cycle : example
 cycle : the
 cycle : second
 cycle : test
 cycle : example
 cycle : the
 cycle : thrid
 cycle : test
 cycle : 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
 $cat commandfor.sh
 #!usr/bin/bash
 n=
 for var in `cat varfortest02.sh`
 do
         n=$[ $n+ ]
         echo line $n: $var
 done

 $sh commandfor.sh
 line : #!usr/bin/bash
 line : list01='the
 line : first
 ... ...
 line : do
 line : ((
 line : n++
 line : ))
 ... ...
 line : $var
 line : 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=$'\n'-(换行)
                多个：IFS=$'\n:;"'-(换行/冒号/分号/双引号)
         * 保存与恢复：
                 IFS.OLD=$IFS
                 IFS=$'\n'
                 ...
                 IFS=$IFS.OLD    

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

 $sh commandfor.sh
 commandfor.sh: line : IFS.OLD=: command not found
 line : #!usr/bin/bash
 line : list01='the first test example'
 line : list02=" the second test example"
 line : list03=$list02" the thrid test example"
 line : list04=$list01$list02$list03
 line : echo $list04
 line : n=
 line : for var in $list04
 line : do
 line :        (( n++ ))
 line :        echo cycle $n: $var
 line : 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!