9.17任务

20.10 for循环

案例1：求1到100的整数的和。

#!/bin/bash
sum=0
for i in `seq 1 100`
do
    sum=$[$sum+$i]
done
echo $sum

[email protected]:~/shell_script# bash for1.sh 
5050

[email protected]:~/shell_script# bash -x for1.sh
+ sum=0
++ seq 1 100
+ for i in '`seq 1 100`'
+ sum=1
+ for i in '`seq 1 100`'
+ sum=3
+ for i in '`seq 1 100`'
+ sum=6
...
+ sum=4851
+ for i in '`seq 1 100`'
+ sum=4950
+ for i in '`seq 1 100`'
+ sum=5050
+ echo 5050
5050

案例2：文件列表循环

#!/bin/bash
for a in `ls /etc/`
do
    [ -d /etc/$a ] && ls /etc/$a
done

这个会列出/etc下所有的文件，并且ls所有的一级目录。下面是部分过程

+ for a in '`ls /etc/`'
+ '[' -d /etc/pam.d ']'
+ ls /etc/pam.d
chfn	  chsh		  common-auth	   common-session		  cron	gnome-screensaver  lightdm-autologin  login	other	polkit-1  runuser    sshd  sudo		 unity
chpasswd  common-account  common-password  common-session-noninteractive  cups	lightdm		   lightdm-greeter    newusers	passwd	ppp	  runuser-l  su    systemd-user
+ for a in '`ls /etc/`'
+ '[' -d /etc/papersize ']'

循环目录的时候要注意文件是否有空格。因为for循环中的条件的分割符是空格或者是回车。

[email protected]:~/shell_script# touch 1\ 2\ 3\ 4
[email protected]:~/shell_script# ll 1\ 2\ 3\ 4 
-rw-r--r-- 1 root root 0 Sep 17 23:58 1 2 3 4
[email protected]:~/shell_script# for i in `ls 1\ 2\ 3\ 4 `; do echo $i ; done
1
2
3
4

案例三：常用条件由seq生成

[email protected]:~/shell_script# seq 1 3
1
2
3
[email protected]:~/shell_script# for i in 1 2 3; do echo $i; done
1
2
3
[email protected]:~/shell_script# for i in `seq 1 3`; do echo $i; done
1
2
3

 

20.11/12 while循环

案例1：每隔30s检查一次系统负载，如果负载过高就报警。

[email protected]# uptime | awk -F 'load average: ' '{print $2}' | cut -d . -f1
0
[email protected]# uptime 
 00:08:19 up 1 day,  9:02,  3 users,  load average: 0.00, 0.00, 0.00

#!/bin/bash
#while :
#while 1   
while true
do
    load=`uptime | awk -F 'load average: ' '{print $2}' | cut -d . -f1`
    if [ $load -gt 10 ]
    then
        /usr/local/sbin/mail.py [email protected] "load high" "$load"
    fi
    sleep 30
done

[email protected]:~/shell_script# bash -x while1.sh 
+ true
++ uptime
++ awk -F 'load average: ' '{print $2}'
++ cut -d . -f1
+ load=0
+ '[' 0 -gt 10 ']'
+ sleep 30

while后的条件冒号，非0数字，true都是恒真。

不希望脚本意外中断可以在screen里运行。

案例2：带交互的while循环。让用户做选择，直到满足条件退出。

#!/bin/bash
while :
do
    read -p "Please input a number: " n
    if [ -z "$n" ]
    then
        echo "you need input sth."
        continue
    fi
    n1=`echo $n|sed 's/[0-9]//g'`
    if [ -n "$n1" ]
    then
        echo "you just only input numbers."
        continue
    fi
    break
done
echo $n

[email protected]:~/shell_script# bash -x while2.sh 
+ :
+ read -p 'Please input a number: ' n
Please input a number:  
+ '[' -z '' ']'
+ echo 'you need input sth.'
you need input sth.
+ continue
+ :
+ read -p 'Please input a number: ' n
Please input a number: a
+ '[' -z a ']'
++ echo a
++ sed 's/[0-9]//g'
+ n1=a
+ '[' -n a ']'
+ echo 'you just only input numbers.'
you just only input numbers.
+ continue
+ :
+ read -p 'Please input a number: ' n
Please input a number: 12
+ '[' -z 12 ']'
++ echo 12
++ sed 's/[0-9]//g'
+ n1=
+ '[' -n '' ']'
+ break
+ echo 12
12

本身while的条件是恒真的，是不会退出的，为什么能退出就是因为break关键字，下节再讲。

 

20.13 break跳出循环

break的作用就是结束循环，跳出到循环体外。

#!/bin/bash
for i in `seq 1 5`
do
    echo $i first
    if [ $i -eq 3 ]
    then
        break
    fi
    echo $i second
done
echo aaa

[email protected]:~/shell_script# bash -x break.sh 
++ seq 1 5
+ for i in '`seq 1 5`'
+ echo 1 first
1 first
+ '[' 1 -eq 3 ']'
+ echo 1 second
1 second
+ for i in '`seq 1 5`'
+ echo 2 first
2 first
+ '[' 2 -eq 3 ']'
+ echo 2 second
2 second
+ for i in '`seq 1 5`'
+ echo 3 first
3 first
+ '[' 3 -eq 3 ']'
+ break
+ echo aaa
aaa

也就是说4和5就没有被执行，直接跳出循环，而且当前循环的后续部分不会被执行。

[email protected]:~/shell_script# bash break.sh 
1 first
1 second
2 first
2 second
3 first
aaa

 

20.14 continue结束本次循环

忽略continue之下的代码，直接进行下一次循环

#!/bin/bash
for i in `seq 1 5`
do
    echo $i first
    if [ $i -eq 3 ]
    then
        continue
    fi
    echo $i second
done
echo aaa

[email protected]:~/shell_script# bash -x continue.sh 
++ seq 1 5
+ for i in '`seq 1 5`'
+ echo 1 first
1 first
+ '[' 1 -eq 3 ']'
+ echo 1 second
1 second
+ for i in '`seq 1 5`'
+ echo 2 first
2 first
+ '[' 2 -eq 3 ']'
+ echo 2 second
2 second
+ for i in '`seq 1 5`'
+ echo 3 first
3 first
+ '[' 3 -eq 3 ']'
+ continue
+ for i in '`seq 1 5`'
+ echo 4 first
4 first
+ '[' 4 -eq 3 ']'
+ echo 4 second
4 second
+ for i in '`seq 1 5`'
+ echo 5 first
5 first
+ '[' 5 -eq 3 ']'
+ echo 5 second
5 second
+ echo aaa
aaa

最明显的就是continue只结束了本次循环，可以看到3 second没有被输出

[email protected]:~/shell_script# bash continue.sh 
1 first
1 second
2 first
2 second
3 first
4 first
4 second
5 first
5 second
aaa

 

20.15 exit退出整个脚本

#!/bin/bash
for i in `seq 1 5`
do
    echo $i first
    if [ $i -eq 3 ]
    then
        exit
    fi
    echo $i second
done
echo aaa

[email protected]:~/shell_script# bash -x exit.sh 
++ seq 1 5
+ for i in '`seq 1 5`'
+ echo 1 first
1 first
+ '[' 1 -eq 3 ']'
+ echo 1 second
1 second
+ for i in '`seq 1 5`'
+ echo 2 first
2 first
+ '[' 2 -eq 3 ']'
+ echo 2 second
2 second
+ for i in '`seq 1 5`'
+ echo 3 first
3 first
+ '[' 3 -eq 3 ']'
+ exit

碰到exit，这个脚本就直接结束了，最后的aaa输出都没有了。

[email protected]:~/shell_script# bash exit.sh 
1 first
1 second
2 first
2 second
3 first

exit后面可以跟一个数字，就是$?的返回值。

[email protected]:~/shell_script# bash
[email protected]:~/shell_script# exit
exit
[email protected]:~/shell_script# echo $?
0
[email protected]:~/shell_script# bash
[email protected]:~/shell_script# exit 1
exit
[email protected]:~/shell_script# echo $?
1

 

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select用法

转载于:https://my.oschina.net/u/3866688/blog/2053323