Shell script series (1) getting started

Shell Script entry

1. Shell Concept

• Shell Is a command interpreter , The function is to explain and execute the commands and programs entered by the user , Every command the user enters ,Shell Just explain and implement one of them . This kind of input commands from the keyboard , You can get an immediate response , It is called the way of interaction .

• Shell Scripting language is a weakly typed language ( You don't need to define the type of the variable to use ).

Check out the shell Support

• Centos7.8

$ cat /etc/shells 
/bin/sh
/bin/bash
/usr/bin/sh
/usr/bin/bash

• ubuntu18.04

$ cat /etc/shells 
/bin/sh
/bin/bash
/bin/rbash
/bin/dash

2 Shell Scripting language advantages

• Shell Scripting language is more suitable for dealing with the business at the bottom of the operating system .
• Linux Regular expressions and three swordsmen grep、awk、sed Wait for the order .
• Use Shell More in line with Linux Simple operation and maintenance 、 Easy to use 、 Three basic principles of efficiency .

3. Common operating systems default Shell

• CentOS Linux By default Shell yes : bash

• see CentOS Linux By default Shell

# echo $SHELL
# grep root /etc/passwd 

1.4 Shell Script format

• Check out the shell edition

# bash --version

• upgrade bash edition

# yum -y update bash

• At the beginning of the script ( first line ) : The first line indicates which program interpreter executes the script content

#! /bin/bash

• Script comment

  • # The following content indicates the comments , Used to annotate scripts
  • Try not to use Chinese for notes , It is better not to have Chinese in the script
  • Shell The script is from top to bottom 、 From left to right, execute each line of commands and statements in turn , That is, after executing one command, execute the next .
  • In execution Shell Script time , Will request the system kernel to start a new process , In order to execute the script's commands and subprograms in the process Shell Script .

• Shell Script execution

1. bash  xxx.sh
2. chmod +x xxx.sh   then ./xxx.sh
3. source xxx.sh   Use source Or point number to read or load the specified shell Script files .

Conclusion ：
adopt source or . Load the executed script , Because in the present Shell Execute script in , So after the script ends , Variables in scripts （ Include function ） Value in current Shell There is still .

sh and bash Executing the script will start a new child Shell perform , After execution, return to the parent Shell. therefore , Variable （ Include function ） Values, etc. cannot be reserved .

It's going on Shell Script development , If there is a need to reference or execute the contents of other scripts or configuration files in the script , It is best to . or source Load the script or configuration file first , After that , Then load them under the script , You can call source The loaded script and variables and functions in the configuration file .

• Use sh Executing the script will cause the current Shell Unable to get variable value .

1.5 Shell Basic specifications and habits of script development

shell The script normalization format is as follows

#!/bin/bash
set +o posix

#: <<'COMMENT'
#'''
#@File : xxxxx.sh
#@Time : 2021/12/20 14:03:14
#@Modify : 2021/12/20 16:13:14
#@Author : Li yuanjie
#@Version : 1.1
#@Contact : [email protected]
#@Desc : Project shell scripts
#'''
#COMMENT
#  Commissioning switch 
# set -x
#set -euxo pipefail
## -------------------------------------------------------------------------
export PATH=/sbin:/usr/sbin:/usr/local/sbin:/usr/local/bin:/usr/bin:/bin
export TERM="xterm-256color"
export WORKDIR="$(cd $(dirname '$0') && pwd )"
cd "${WORKDIR}" || exit 1
## -------------------------------------------------------------------------

info() {
    
    date +"%F %T $( tput bold ; tput setaf 2)Info: [email protected]$( tput sgr0)"
}
warn() {
    
    date +"%F %T $( tput bold ; tput setaf 3)Warning: [email protected]$( tput sgr0)"
}
error() {
    
    date +"%F %T $( tput bold ; tput setaf 1)Error: [email protected]$( tput sgr0)"
}
err_exit() {
    
    date +"%F %T $( tput bold ; tput setaf 1)Error: [email protected]$( tput sgr0)"
    exit 1
}

• Shell The first line of the script specifies the script interpreter
  #!/bin/bash

• Shell The script starts with a version , Copyright and other information

• stay Shell Try not to use Chinese in the script .

• Shell The name of the script should be ．sh Extension name .

• Shell Scripts should be stored in a fixed path .

Code writing habits

• Pairs of symbols should be written at once , Then backspace and add content to the symbol , To prevent omission .
• brackets （[]） At least... At both ends 1 A space .
• For process control statements , The format should be written once , Add content .
• Make the code easier to read by indentation
• For the string definition of a regular variable, the variable value should be enclosed in double quotation marks , And there should be no spaces before and after the equal sign , Strong references are required （ Refers to WYSIWYG character references ）, Use single quotation marks （' '）, If it is a reference to a command , Use back quotation marks .
• Single quotation marks in scripts 、 Double quotation marks and back quotation marks must be symbols in English .