09 Shell Scriptfor循环结构语句

Shell Scriptfor循环结构语句

一、Shell FOR循环语句概述

​ 属于shell的符合语句

​ 可以看出帮助信息给出了两种语法

[root@localhost ~]# help for
for: for NAME [in WORDS ... ] ; do COMMANDS; done
    Execute commands for each member in a list.
    
    The `for' loop executes a sequence of commands for each member in a
    list of items.  If `in WORDS ...;' is not present, then `in "$@"' is
    assumed.  For each element in WORDS, NAME is set to that element, and
    the COMMANDS are executed.
    
    Exit Status:
    Returns the status of the last command executed.
for ((: for (( exp1; exp2; exp3 )); do COMMANDS; done
    Arithmetic for loop.
    
    Equivalent to
    	(( EXP1 ))
    	while (( EXP2 )); do
    		COMMANDS
    		(( EXP3 ))
    	done
    EXP1, EXP2, and EXP3 are arithmetic expressions.  If any expression is
    omitted, it behaves as if it evaluates to 1.
    
    Exit Status:
    Returns the status of the last command executed.

​ 语法：

​ for NAME [in WORDS … ] ; do COMMANDS; done

​ 循环遍历WORDS中的元素，每次将元素赋值给NAME–

​ NAME：变量

​ WORDS：被扩展后得到的一个集合/列表

​ for (( exp1; exp2; exp3 )); do COMMANDS; done

​ 跟据(())种给定的三个算术表达计算循环

​ exp1：用于循环判断的变量初始值

​ exp2：循环判断的结束值

​ exp3：循环控制的步进值

二、Shell FOR循环语句简单实现

​ 对语法：for NAME [in WORDS … ] ; do COMMANDS; done的简单实现：

​ i：变量

​ “a” “b” “c”：一个集合或序列

[root@localhost ~]# for i in "a" "b" "c"; do echo "$i"; done
a
b
c

​ 使用扩展的方式得到一个集合/序列：

​ 可以看到在例子中，集合的位置使用了命令替换

​ 该例子的执行顺序是：

​ 先做命令替换，将命令：ls / 的执行结果扩展到命令的位置

​ 再执行for语句迭代命令替换的结果

[root@localhost ~]# for i in `ls /`; do echo $i;done
bin
boot
data
dev
dump.rdb
etc
home
lib
lib64
media
mnt
opt
proc
root
run
sbin
srv
sys
tmp
usr
var

​ Seq命令获取一个列表用于for循环的迭代方式：

​ seq命令的语法：seq 起始值 步进值 结束值

[root@localhost ~]# for i in `seq 1 3 9`; do echo "$i"; done
1
4
7

​ 对语法：for (( exp1; exp2; exp3 )); do COMMANDS; done的简单实现：

[root@localhost ~]# for ((i=0;i<5;i++)); do echo $i; done
0
1
2
3
4