Types Annotate
strong variables Variables must be declared before use, and even
weak variables need to be initialized Variables are declared when they are used, even without distinguishing between types

The role of variables: used to save changed data

The variable name is fixed, set by the system or defined by the user
Variable values vary according to user settings and system environment changes

Setting method

variable name = variable value

Types of Shell Variables

Type description
Environment variables are maintained by the system and are used to set the working environment, only individual environment variables can be directly changed by users
Positional variables pass arguments to script programs via the command line
Predefined variables A type of special-purpose variable built into bash that can be called directly, but cannot be directly assigned or modified
Custom variables are set, modified and used by users themselves

environment variable

At system startup, load the variables defined by the system configuration file
Variable names and variable values are system settings
Variable names are usually defined with capital letters
The value stored in the variable is specified. Usually, it is not recommended to modify the value of the system environment variable.
The scope is the current shell process and its subprocesses

configuration file

Profile comments
 /etc/ profile    
 ~/.bashrc_profile

Related operations

env lists all environment variables
 set     lists all variables

Common environment variables

   
PWD    
PATH    
USER    
LOGNAME    
UID    
SHELL    
HOME    
PS1    
PS2

How to view variable values

echo $ variablename

position variable: read-only variable

position variable read-only variable
    　　　　　　When executing a script or shell, pass a value to a script or function
$ 1 .....$n is represented as $n, where n is a positive integer
${10}......${N}

Predefined variables (special variables)

predefined variables

The predefined variables in the shell used by the current script
Variable names are fixed, and variable values are usually not modified

variable name

variable name meaning
$$ The PID number of the currently running process
$ ?     The return status value after the last command is executed, 0 means normal, 1 or other values are abnormal
$ 0     currently running process or script name
$# number of position variables
$ * treats all command line parameters as a single string, equivalent to " $1$2$3 " 
$ !     The PID number of the last process in the background
$@ treats each command line argument as a separate string, equivalent to " $1 " , " $2 " , " $3 "

Exercise: Write a script

Pass two parameters ( integers ) to the script ;

Display the sum and product of the two

#!/bin/bash
#
if [ $# -lt 2 ]; then
  echo "Usage: cacl.sh ARG1 ARG2"
  exit 8
be

echo "The sum is: $[$1+$2]."
echo "The prod is: $[$1*$2]."

Exit status and what it means:

Exit status and its meaning Comment
 0    　　　　　　 means the operation was successful, the program execution did not encounter any problems
 1 ~ 125     　　　means the operation failed, script command, system command error or parameter passing error
 126     　　　　The command was found but could not be executed
 127 The command     　　　　to run was not found
 > 128     　　　command is forcibly ended by the system
 1 , 2 , 127      reserved by the system

Difficulty Theory and Practical Analysis

Difference between $* and $@
$ *: Treat all command line parameters as a single string, equivalent to " $1$2$3 " 
$@: Treat each command line parameter as a separate string, equivalent to " $1 " , " $2 " , " $3 " . This is the best way to pass parameters to other programs because it preserves any whitespace embedded in each parameter.

2. Shell variables