Shell script Basics - basic grammar knowledge

SHELL Script Basics

One 、shell Script

• Application scenarios
  • Repeat the work 、 Automatic execution
• It's essentially a text file

Two 、 Output statement

1、echo

• Single quotation marks
  • All characters will be displayed as normal characters
• Double quotes
  • Special characters will be escaped, such as （!! Will call the last historical command ）

[[email protected] ~]# echo -e "SHELL\nPython\nGOlang"
SHELL
Python
GOlang

[[email protected] ~]# echo -e "Linux\tWindows\tUNIX"
Linux	Windows	UNIX

[[email protected] ~]# echo "hello!!"
echo "hellocat /etc/hosts"
hellocat /etc/hosts

[[email protected] ~]# echo 'hello!!'
hello!!

2、here document

• Easy to display multi line content

[[email protected] ~]# cat << eof
> 1.  Create a virtual machine 
> 2.  View virtual machine 
> 3.  Destroy virtual machine 
> 4.  View system resources 
> eof

• Write a script

[[email protected] ~]# vim 1.sh 
cat << eof 1、 Create a virtual machine  2、 View virtual machine  3、 Delete virtual machine  4、 Restore virtual machine  5、 Go back to the previous step  eof
[[email protected] ~]# bash 1.sh 
1、 Create a virtual machine 
2、 View virtual machine 
3、 Delete virtual machine 
4、 Restore virtual machine 
5、 Go back to the previous step 

3、 ... and 、 Redirection symbol

1、 type

• Output redirection

  > >> 2> &>
  

• Input redirection

  <
  

2、 Three devices

• /dev/stdin Standard input devices keyboard 、 mouse 0
• /dev/stdout Standard output equipment Monitor 1
• /dev/stderr Standard error output device Monitor 2

3、 Output redirection

• > Cover
  • Only standard output can be redirected

[[email protected] ~]# ls -ldh /etc/ > /tmp/file01

[[email protected] ~]# cat /tmp/file01 
drwxr-xr-x. 76 root root 8.0K Sep 17 09:02 /etc/

[[email protected] ~]# ls -ldh /dev/ > /tmp/file01 

[[email protected] ~]# cat /tmp/file01
drwxr-xr-x 20 root root 3.2K Sep 17 09:02 /dev/

• >> Additional
  • Only standard redirected output can be

[[email protected] ~]# ls -ldh /dev/ > /tmp/file01 

[[email protected] ~]# cat /tmp/file01
drwxr-xr-x 20 root root 3.2K Sep 17 09:02 /dev/
[[email protected] ~]# 
[[email protected] ~]# ls -ldh /etc/ >> /tmp/file01 
[[email protected] ~]# 
[[email protected] ~]# cat /tmp/file01
drwxr-xr-x 20 root root 3.2K Sep 17 09:02 /dev/
drwxr-xr-x. 76 root root 8.0K Sep 17 09:02 /etc/

• /dev/null Black hole file

[[email protected] ~]# ls -ldh /etc/ > /dev/null 

• 2>
  • Can only redirect error messages

[[email protected] ~]# ls -ldh /dsjlfdsjlfdsjlf 2> /dev/null 

[[email protected] ~]# ls -ldh /etc/ 2> /dev/null
drwxr-xr-x. 76 root root 8.0K Sep 17 09:02 /etc/

• &>
  • Rewrite all information

4、 Input redirection

• Use the contents of the file as keyboard input

[[email protected] ~]# tr 'a-z' 'A-Z' < /tmp/file01
ABCDE

Four 、 Use of variables

1、 Variable type

• Custom variable
• environment variable
• Special variables

2、 Custom variable

• Defining variables

 Variable name = value 

• Canonical variable name
  • Only letters can appear , Numbers , Underline
  • Cannot appear with shell Keyword conflict
  • Know what you know
• Call variables ( Be careful not to write the variable name wrong )
  • $ Variable name
  • ${ Variable name }

[[email protected] ~]# ip_address=192.168.152.10
[[email protected] ~]# echo $ip_address
192.168.152.10
[[email protected] ~]# echo $(ip_address)

[[email protected] ~]# data=server
[[email protected] ~]# echo "There are some $datas"
There are some 
[[email protected] ~]# echo "There are some ${data}s"
There are some servers

1、 Define variables interactively

• ​ # read -p “ Prompt information ” Variable name

[[email protected] ~]# read -p " user name : " name
 user name : userA
[[email protected] ~]# echo $name
userA

[[email protected] ~]# vim 1.sh 
read -p " user name ：" user
echo " Hello! ：" $user
[[email protected] ~]# bash 1.sh 
 user name ：lijianhua
 Hello! ： lijianhua

2、shell All variables stored in are characters

[[email protected] ~]# a=10
[[email protected] ~]# b=20

[[email protected] ~]# c=a+b
[[email protected] ~]# 
[[email protected] ~]# echo $c
a+b
[[email protected] ~]# c=$a+$b
[[email protected] ~]# echo $c
10+20

3、 Mathematical operations

• + - * %( Remainder )
• $(())

[[email protected] ~]# a=10
[[email protected] ~]# b=20
[[email protected] ~]# echo $((a+b))
30

• let keyword

[[email protected] ~]# let c=a+b
[[email protected] ~]# echo $c
30

• declare

[[email protected] ~]# declare -i c=a+b
[[email protected] ~]# echo $c
30

4、 obtain 100 Inner random number

[[email protected] ~]# let rand=$RANDOM%100
[[email protected] ~]# echo $rand
43

3、 environment variable

• Save the current operating environment of the system

1、 View environment variables

env

2、 Definition 、 Modify environment variables

• Environment variable names must be capitalized

 Provisional entry into force ：export  environment variable = value 
 permanent ：vim /etc/profile
		export  environment variable = value 

Example 1 ： Define the time of the historical command

export HISTTIMEFORMAT="%F_%T "

Example 2 ： Define the number of command lines in the current user history , Default 1000

export HISTSIZE=10

Example 3 ： Modify system language

export LANG=en_us.UTF-8

4、 Special variables

• $?
  • The execution status code of the last command
  • Value range 0—255
    • 0 success
    • Not 0 Failure

5、 ... and 、shell Script example

1、 Configure local yum Source

#!/bin/bash
#
mkdir /mnt/cdrom
sed -i '$a \/dev/sr0 /mnt/cdrom iso9660 defaults 0 0' /etc/fstab
mount -a
mkdir /etc/yum.repos.d/backup
mv /etc/yum.repos.d/CentOS* /etc/yum.repos.d/backup/
touch /etc/yum.repos.d/yum.repo
cat << eof > /etc/yum.repos.d/yum.repo [centos] name=centos baseurl=file:///mnt/cdrom enabled=1 gpgcheck=0 eof
yum repolist

2、 Configure network parameters

#!/bin/bash
#

read -p " Manual configuration IP Address , Please enter ：" ip
nmcli connection modify ens160 autoconnect yes ipv4.method manual ipv4.addresses $ip ipv4.gateway 192.168.152.2 ipv4.dns 223.5.5.5
nmcli connection reload
nmcli connection up ens160 >> /dev/null
IP=$(ifconfig ens160 | sed -n '2p' | awk '{print $2}')
ROUTE=$(route -n | sed -n '3p' | awk '{print $2}')
DNS=$(cat /etc/resolv.conf|sed -n '2p' | awk '{print $2}')
echo "=============================="
echo -e " The changed IP：$IP\n The changed gateway is ：$ROUTE\n The changed DNS by ：$DNS"