3.5 循环(loop)

3.5 循环(loop)

一看见循环，就想起了while和for…

3.5.1 while do done ,until do done（不定循环）

对于不定循环，最常见的便是下面两种使用方式：
当condition成立时才循环

while [ condition ]
do			# 开始循环
		程序
done		# 结束循环

当condition成立时就停止循环

until [ condition ]
do 
		程序
done

下面两段程序，都是当用户输入yse或YES时，才会停止，否则会一直提示输入：

# 使用while do
[[e-mail] ~]$ cd bin
[[e-mail] bin]$ vim yes_to_stop.sh
#!/bin/bash
# Peogram:
#       Repcat question until user input correct ansewe.
#History:
#2019/10/13     haoqiqi         First release
PATH=/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin:~/bin
export PATH

while [ "${yn}" != "yse" -a "${yn}" != "YES" ]
do
        read -p "Please input yes/YES to stop this program: " yn
done
echo "You have quit this program."                                    

[[e-mail] bin]$ cp yes_to_stop.sh yes_to_shop_2.sh
[[e-mail] bin]$ vim yes_to_shop_2.sh
#!/bin/bash
# Peogram:
#       Repcat question until user input correct ansewe.
#History:
#2019/10/13     haoqiqi         First release
PATH=/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin:~/bin
export PATH

until [ "${yn}" == "yse" -a "${yn}" == "YES" ]
do
        read -p "Please input yes/YES to stop this program: " yn
done
echo "You have quit this program."

3.5.2 for…do…done（固定循环）

因为对于for循环，一般我们都知道它会循环几次，因此叫固定循环。

for var in con1 con2 con3 ...
do 
		程序
done
# 第i次循环时，var的值为coni

举例如下：
由于系统上的各种账号都是写在/etc/passwd内的第一个字段，因此想要使用cat捕捉账号，然后用id分别检查使用者的标识符与特殊参数。

[[e-mail] bin]$ vim userid.sh

#!/bin/bash
# Peogram:
#       Use id finger commend to cleck system account's information.
#History:
#2019/10/13     haoqiqi         First release
PATH=/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin:~/bin
export PATH

users=$(cut -d ':' -fl/etc/passwd)
for username in {users}
do 
        id ${username}
done

3.5.3 for…do…done的数值处理

除了上述的方法之外，for循环还有另一种用法，即常用的用法：

for (( 初始值;限制值;执行步长))
do
	程序
done

3.5.4 搭配随机数与数组的实验

学到现在的话，其实已经将loop学的差不多了，然后正好书上有这样一个小例子，拿来和大家娱乐一样。

[[e-mail] bin]$ vim what_to_eat.sh
#!/bin/bash
# Peogram:
#       Try do tell you what you may eat.
#History:
#2019/10/13     haoqiqi         First release
PATH=/bin:/sbin:/usr//bin:/usr/sbin:/usr/local/bin:/usr/local/sbin:~/bin
export PATH
eat[1]="肯德基"
eat[2]="麦当劳"
eat[3]="德客士"
eat[4]="星巴克"
eat[5]="红烧牛肉面"
eat[6]="黄焖鸡米饭"
eat[7]="盖浇饭"
eatnum=7

eated=0
while [ "${eated}" -lt 3 ]; do
        check=$(( ${RANDOM}*${eatnum}/32767+1))
        mycheck=0
        if [ "${eated}" -ge 1 ]; then
                for i in $(seq 1 ${eated} )
                do
                        if [ ${eatedcon[$i]} == $check ]; then
                                mycheck=1
                        fi
                done
        fi
        if [ ${mycheck} == 0 ]; then
                echo "You may eat ${eat[${check}]}"
                eated=$(( ${eated} + 1 ))
                eatedcon[${eated}]=${check}
        fi
done

[[e-mail] bin]$ sh what_to_eat.sh
You may eat 德客士
You may eat 星巴克
You may eat 肯德基

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