[tap]
1. For loop
Repeating a series of commands is common in programming. Often you need to repeat a set of commands until a certain condition is reached, such as processing all files in a directory, all users on the system, or all lines in a text file.
Two common loops are commonly used in scripts.
for loop
while loop
Syntax: for variable name in condition; do …; done
for var in list
do
commands
done
在list参数中，提供了迭代中要用的一系列值Example 1: Use a for loop to write a sum of 1-100.
Ideas:
- [ ] First need to loop 1-100
- [ ] Loop adds one at a time and assigns it to sum
- [ ] output the value of sum
- seq This is to traverse the numbers 1-100.
[root@xavi ~]# !vim
vim sum01.sh
#!/bin/bash
sum=0
for i in `seq 1 100`
do
sum=$[ $sum+$i ]
echo $i
done
echo $sumExample 2: Traverse a directory's directories or files
#!/bin/bash
cd /etc/ //脚本中如果查询的目录在多级目录下，首先要进入到目录，或者，直接跟绝对路径
for a in `ls /etc/` //遍历/etc目录
do
if [ -d $a ] //一个一个的去判断是否为目录
then
ls -d $a //如果是目录，就查看下目录内的文件+子目录
fi
done[root@xavi ~]# !vim
vim for02.sh
#!/bin/bash
cd /etc/
for a in `ls /etc/`
do
[ -d $a ] && ls $a
# 判断是否是目录，并列出其下文件和子目录
if [ -d $a ]
then
echo $a
ls $a
fi
done
~ Special for loop example: when the list loops, spaces or carriage returns are used as separators
[root@xavi ~]# mkdir xavi
[root@xavi ~]# cd xavi/
[root@xavi xavi]# ls
[root@xavi xavi]# touch 1 2
[root@xavi xavi]# touch 3\ 4.txt
[root@xavi xavi]# ls
1 2 3 4.txt
[root@xavi xavi]# for i in `ls ./`; do echo $i ; done
1
2
3
4.txt //把3 4.txt一个文件拆分成两个了2. while loop
grammar:
while test command
do
other commands
doneCase 1: Write a script to monitor system load
#!/bin/bash
while : //：为死循环的意思,也相当于ture,
do
load=`w|head -1|awk -F 'load average: ' '{print $2}'|cut -d. -f1`
if [ $load -gt 10 ]
then
top|mail -s "load is high: $load" [email protected]
fi
sleep 30
// 负载有时候不可能一秒钟就起来了，所以每隔30秒判断一次即可。我们使用sleep 去暂停30秒钟。然后再次执行。。。
doneCase 2: Get a value through interaction with the user, and judge.
Output the number entered by the user.
- [ ] First judge, what if the user does not input?
- [ ] What if the user input is not a pure number?
- [ ] According to the number entered by the user, and output.
#!/bin/bash
while :
do
read -p "Please input a number: " n
if [ -z "$n" ]
then
echo "you need input sth."
continue
fi
n1=`echo $n|sed 's/[0-9]//g'`
if [ -n "$n1" ] // -n = ! -z ,记得加双引号
then
echo "you just only input numbers."
continue
fi
break
done
echo $n
-
[ ] continue: When the user has no input, first prompt, and then continue to let the user input. Until the input is a real number, break out of the if statement.
- [ ] break: If it is a real number, break out of the entire while statement.
Third, the usage of break
Example:
#! /bin/bash
for i in `seq 1 5`
do
echo $i
if [ $i == 3 ]
then
break
fi
echo $i
done
echo aaaThe execution steps are as follows:
[root@xavi xavi]# sh -x break.sh
++ seq 1 5
+ for i in '`seq 1 5`'
+ echo 1
1
+ '[' 1 == 3 ']'
+ echo 1
1
+ for i in '`seq 1 5`'
+ echo 2
2
+ '[' 2 == 3 ']'
+ echo 2
2
+ for i in '`seq 1 5`'
+ echo 3
3
+ '[' 3 == 3 ']'
+ break
+ echo aaaa
aaaaThat is to say, once a break is encountered, it will jump out of the current loop and proceed to the next step.
Fourth, the usage of continue:
Ignore the code under continue and proceed directly to the next loop.
#! /bin/bash
for i in `seq 1 5`
do
echo $i
if [ $i == 3 ]
then
continue
fi
echo $i
done
echo aaaThe execution result is as follows:
1
1
2
2
3
4
4
5
5
aaaThat is to say, when the execution reaches continue, any of the following statements are ignored directly. . Go directly to the next for loop.
Five, exit to exit the entire script
#! /bin/bash
for i in `seq 1 5`
do
echo $i
if [ $i == 3 ]
then
exit
fi
echo $i
done
echo aaaThe execution steps are as follows:
++ seq 1 5
+ for i in '`seq 1 5`'
+ echo 1
1
+ '[' 1 == 3 ']'
+ echo 1
1
+ for i in '`seq 1 5`'
+ echo 2
2
+ '[' 2 == 3 ']'
+ echo 2
2
+ for i in '`seq 1 5`'
+ echo 3
3
+ '[' 3 == 3 ']'
+ exit- [ ] When going straight to 3, exit the script directly.