What is a script? effect?
The script in Linux is actually similar to the batch processing we write in the windows system, so how do we learn it?
Execute from top to next, if there is an error, it will not be executed, skip to the next command

Here we mainly deal with the requirements of various scenarios and write the corresponding scripts to achieve the process, we go step by step to understand the various syntax of script writing. The meaning of these grammars has actually been understood in the programming languages we have learned before. Here, we mainly need to understand and study its grammatical structure. You will find that it is very easy and interesting to learn...

A simple script

The scene settings are as follows:

Request to set the eth0 network card ip at 9 a.m.

IP：192.168.1.100/24

Gateway: 192.168.1.254

Request to set eth0 network card ip at 2pm

IP：172.16.1.100/24

Gateway: 172.16.1.254

Implementation process:

Stupid way:

①先把ifcfg-eth0文件清空：echo "" > /etc/sysconfig/network-scripts/ifcfg-eth0
②echo "DEVICE=eth0" >> /etc/sysconfig/network-scripts/ifcfg-eth0
   echo "TYPE=Ethernet" >> /etc/sysconfig/network-scripts/ifcfg-eth0
   echo "ONBOOT=yes" >> /etc/sysconfig/network-scripts/ifcfg-eth0
   echo "BOOYPROTO=static" >> /etc/sysconfig/network-scripts/ifcfg-eth0
   echo "IPADDR=192.168.1.100" >> /etc/sysconfig/network-scripts/ifcfg-eth0
   echo "NETMASK=255.255.255.0" >> /etc/sysconfig/network-scripts/ifcfg-eth0
   echo "GATEWAY=192.168.1.254" >> /etc/sysconfig/network-scripts/ifcfg-eth0
③先关了：ifdown eth0，再开启：ifup eth0

I have to lose again in the afternoon and change to 172

Use script:

①Create a file vim fixip.sh first (because you want to modify the ip function, see the name zhiyi , the suffix is sh, because it is written in a shell script)
②The first line of the script is to declare what command interpreter the script is used for To run, such as sh command interpreter, write the location of sh command interpreter (/bin/sh), if it is bash, write (/bin/bash), the
second line and the third line at the beginning of the pound sign are also pound signs At the beginning, write down the role of the script, the creation time, and contact information (you don’t need to write it)

③How does it work? First give it an execution permission

If it turns green, it means that you have the permission to
execute the script. The method of running the script: if it is in the same directory, use ./ to run—— "./fixup.sh

The company will randomly assign different ips to modify

The optimized script is an interactive script, which leads to the variable assignment
100.1.1.100/24
10.1.1.254 that we will talk about below.
Please enter the ip address:
please enter the subnet mask:
please enter the gateway:

Variable assignment

A, as the name implies, is the variable we defined. We assign a value to it, and then call out the value (string) it represents through $A

[root@Waffle Desktop]# read -p "please input ipaddr:" IP
（大写IP说明这个东西是变量，please input ipaddr是显示内容，显示的内容输入完后就存在变量里面了）
please input ipaddr:10.1.1.1
[root@Waffle Desktop]# echo $IP
10.1.1.1

Optimize the script:
write read -p "please input ipaddr:" IP on the second line of the script just written

IF judgment statement

if single branch structure

if condition
then holds sub-statement
fi (to write fi at the end)

Case: Determine whether 3 is less than 5

Note [ Please write a space after it, otherwise an error

if double branch structure

if condition

then sub-statement

else does not hold sub-statement

Case: Determine if 6 is less than 5

There is no requirement for indentation in linux

if multi-branch structure

if condition

then sub-statement

elif condition

then sub-statement

else does not hold sub-statement

Case: Determine the relationship between the number entered on the keyboard and 10

Note: There must be a space before and after [

In Linux, greater than -gt is less than -lt equal to -eq

Practical case: Detect the survival status of hosts on the intranet

Our usual ping operation, we can modify it and change its specific output display:

-c specifies how many packets it sends

-i sending interval time (s)

-W wait for response time (s)

[root@Waffle Desktop]# ping -c2 -i0.2 -W2 12.34.56.79 (to write together, send two packets, the interval time is 0.2s, the waiting time is 2s)

[root@Waffle Desktop]# ping -c2 -i0.2 -W2 12.34.56.79 &> /dev/null (It means to export all the output whether correct or wrong to /dev/null, this file is equivalent to a hole )

Add two dots before and after (on the left side of numeric keypad 1, different from single quotation marks)

optimization:

If you want to test the survival of hosts on a network segment, it is not very useful

Shell loop statement (for, while)

for loops according to the value list
while loops according to the conditions

for loop

Format:
for variable in value list
do
sub-statement
done

Case: How to use for loop to realize 1~10 output on the screen?

while loop

while conditional
do
sub-statement
done

Case: Output 1~3 on the screen

Detect the live state of the host in the intranet (optimized version)

Continuing, we return to the previous detection of host survival. This time we need to detect the host survival status of the LAN in a real environment.

Configure our network properties to bridge mode

1. Set the eth0 network card of centos6-1 to bridge mode

2. Modify the network card configuration

[root@Waffle Desktop]# vim /etc/sysconfig/network-scripts/ifcfg-eth0

3. Write a script

#!/bin/bash
NET=10.0.110.
for i in {1..254}
do
    if `ping -c2 -i0.2 -w2 $NET$IP &> /dev/null`
    then echo -e "$ NET$IP is \033[31mup\033[0m" is to change UP to red and DOWN to green
    else echo -e "$NET$IP is \033[32mdown\033[0m"
    fi
done

Look at the while loop

#!/bin/bash
NET=10.1.1.
IP=200
while [ $IP -lt 254 ]

do
    let IP=IP+1
    if `ping -c2 -i0.2 -w2 $NET$IP &> /dev/null`
    then echo -e "$NET$IP is \033[31mup\033[0m"
    else echo -e "$NET$IP is \033[32mdown\033[0m"
    fi
done

case statement

case

Case study

./case.sh centos
redhat

./case.sh redhat
centos

./cash.sh xxx
usage case.sh {redhat|centos}

#!/bin/bash
case $1 in
redhat)
   echo "centos"
   ;;
centos)
   echo "redhat"
   ;;
*)--》*表示所有
   echo "useage $0 {redhat|centos}"
esac

Execution after empowerment

case format

case variable in
mode 1)
   sub-statement
   ;;
*)
   sub-statement
   ;;
esac

function

Store part of the code in a variable and
then execute this function when you call it

Case: Design a function named A, and the screen will output OK when running A

The return value of the function can be obtained by echo $? (return)

redhat(){ echo cetos return 0 } centos(){ echo redhat return 0 } case $1 in redhat) redhat (equivalent to calling the above function) ;; centos) centos ;; *) echo "Usage $0 {redhat|centos }" esac