Shell Script Basics (iii)

One, for loop

The For loop structure is a very frequent loop structure used in daily operation and maintenance work.

1. For loop specific format:

for 变量名 in 循环条件； do       commanddone

The "loop condition" here can be a set of strings with numbers (separated by spaces) or a command's execution result.

2. For loop instance 1: Calculates the sum of 1 to 5

[[email protected] shell]# vim for01.sh#! /bin/bashsum=0for i in `seq 1 5`do    echo $i    sum=$[$sum+$i]doneecho $sum[[email protected] shell]# sh for01.sh 1234515

Example 2: Listing /etc/Include words in the directory yumThe directory.

[[email protected] shell]# vim for2.sh#! /bin/bashcd /etc/for i in `ls /etc/`; do   if [ -d $i ]; then     ls -ld $i | grep -w ‘yum‘   fidone[[email protected] shell]# sh for2.sh drwxr-xr-x. 6 root root 100 3月  16 04:04 yumdrwxr-xr-x. 2 root root 248 4月  12 14:53 yum.repos.d

Second, while loop 1, while loop format 1:

while 条件; do        commanddone

Format 2: Dead loop

# 冒号代替条件while : ; do        command                sleep 30done

2, while loop instance 1: When the system load is greater than 10, send mail, every 30 seconds to execute

[[email protected] shell]# vim while01.sh#!/bin/bashwhile :do    load=`w|head -1 |awk -F ‘load average: ‘ ‘{print $2}‘| cut -d . -f1`    if [ $load -eq 0 ]    then       python /usr/lib/zabbix/alertscripts/mail.py [email protected] "load is high:$load" "$load"    fi    sleep 30done##python行表示使用邮件脚本发送负载状况，这里为了实验，把load设为等于0，mail.py脚本在之前zabbix实验中设置#‘while :‘表示死循环，也可以写成while true，意思是“真”#Attention：awk -F ‘load average: ‘此处指定‘load average: ‘为分隔符，注意冒号后面的空格#如果不加该空格，过滤出来的结果会带空格，需要在此将空格过滤掉

Experimental results:

Note: If you do not manually stop the script, it will continue to loop (at the end of CTRL + C), with screen in the actual environment.

Example 2: In interactive mode, the user enters a character to detect whether the character meets the criteria, such as: empty, non-numeric, numeric. The characters are judged separately, and then different responses are made.

[[email protected] shell]# vim while02.sh #!/bin/bashwhile truedo  read -p "Please input a number:" n  if [ -z "$n" ]  then       echo "You need input some characters!"      continue  fi  n1=`echo $n|sed ‘s/[-0-9]//g‘`  if [ -n "$n1" ]  then      echo "The character must be a number!"      continue  fi  breakdoneecho $n#continue:中断本次while循环后重新开始；#break：表示跳出本层循环，即该while循环结束

Iii. break, continue and exit usage

Break jumps out of the loop
Continue end of this cycle
Exit exits the entire script

[[email protected] shell]# vim break.sh #!/bin/bashfor i in `seq 1 5`do  echo $i  if [ $i -eq 3 ]  then      break           #此处课替换continue和exit，执行脚本查看区别  fi  echo $idoneecho $i

Shell Script Basics (iii)