shell的循环结构及其流程控制

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while(当型)循环结构

标准用法，当 condition 条件满足时执行循环体，否则退出循环

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while [[ condition ]] ; do
    # statements1
    # statements2
    # ...........
done

死循环,当触发某条件时退出循环

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while : ; do
     # statements1
     # statements2 && break
done

将文件内容作为标准输入，遍历每一行进行循环体内的所有操作

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while read col1 col2 ; do
    # statements1
    # statements2
    # ...........
done < /path/to/filename

将进程替换的内容作为标准输入，遍历每一行进行循环体内的所有操作

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while read col1 col2 ; do
    # statements1
    # statements2
    # ...........
done < <(command)

until(直到型)循环结构

直到 condition 条件满足时退出循环，否则继续执行循环体

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until [[ condition ]] ; do
    # statements1
    # statements2
    # ...........
done

for循环结构

foreach型循环结构

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for variable in WordList ; do
    # statements1
    # statements2
    # ...........
done
# in WordList 可省略，省略时相当于 in "$@"

C语言型的循环结构

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for (( exp1 ; exp2 ; exp3 )) ; do
    # statements1
    # statements2
    # ...........
done

示例一

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#!/bin/bash
for ((i=1;i<=100;i++));do
    ((sum=sum+i))
done
echo $sum

示例二

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for i in {1..5} ; do
	# statements1
	# statements2
	# …………..
done

示例三

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for i in $(seq 5) ; do
	# statements1
	# statements2
	# …………..
done

示例四

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for i in 1 2 3 4 5 ; do
	# statements1
	# statements2
	# …………..
done

循环控制

break 用于打断，跳出当前的整个循环

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[user1@study ~]$ cat test.sh
#!/bin/bash

for i in Tom 1 2 3 4 Jerry 5 6 7 8;do
    if [[ "${i}" == "Jerry" ]];then
        break
        echo 'test'
    else
        echo ${i}
    fi
done

[user1@study ~]$ bash test.sh 
Tom
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[user1@study ~]$

continue 会跳过本次循环，忽略剩余代码，进入循环的下一次迭代

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[user1@study ~]$ cat test.sh  
#!/bin/bash

for i in Tom 1 2 3 4 Jerry 5 6 7 8;do
    if [[ "${i}" == "Jerry" ]];then
        continue
        echo 'test'
    else
        echo ${i}
    fi
done

[user1@study ~]$ bash test.sh 
Tom
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[user1@study ~]$

: 后面的同一行的语句不做任何操作，可以理解为将语句做了注释，需要注意 : 属于语句，而 # 只是注释符

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[user1@study ~]$ cat test.sh 
#!/bin/bash

for i in Tom 1 2 3 4 Jerry 5 6 7 8;do
    if [[ "${i}" == "Jerry" ]];then
        : echo "yes"
        echo 'test'
    else
        echo ${i}
    fi
done

[user1@study ~]$ bash test.sh 
Tom
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test
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[user1@study ~]$

在命令行执行 exit 会退出当前的 shell 终端

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[user1@study ~]$ exit
logout

在脚本中不管在什么地方，执行语句遇到 exit 就会结束整个脚本

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[user1@study ~]$ cat test.sh 
#!/bin/bash

for i in Tom 1 2 3 4 Jerry 5 6 7 8;do
    if [[ "${i}" == "6" ]];then
        exit
        echo 'test'
    else
        echo ${i}
    fi
done

[user1@study ~]$ bash test.sh
Tom
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Jerry
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[user1@study ~]$
有钱任性，请我吃包辣条
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