Shell variables and operators

-Local variable-commands that affect Variables
-Environment variables-quotation marks
-Replace variables-Operators
-Location variable-expression replacement
-Standard shell variable-operator priority
-Special Variables

-Variable:
-What is a shell variable?
-Local Variables
-Environment Variables
-Replace variables (display variables)
-Location variable
-Standard Variables
-Special Variables
-Commands that affect Variables

-Local variables:
-Local variables are used in users' Current shell lifecycle scripts.
-Variable-name = value
-Set: displays all local variables.
-Readonly variable-name
# LOCALTEST = "test"
# Echo $ {LOCALTEST}
Test
# Echo $ LOCALTEST
Test
# Set view variables through set
# Exit and check whether the LOCALTEST variable exists.
# Readonly LOCALTEST
# LOCALTEST = "test"
# Readonly view the read-only variables in the Current shell
# Readonly-p
-Environment Variables
-Environment variables are used for all user processes (often referred to as subprocesses ). the logon process is called the parent process. user processes executed in shell are called sub-processes. unlike local variables (only for the current shell), environment variables can be used for all sub-processes, including editors, scripts, and applications.
-$ HOME/. bash_profile (/etc/profile)
-Export
-Env
# Cat/etc/profile
# Export Antiy = "Xi'an"
# Env and search for the variable Antiy
# Export can also be viewed
# Readonly Antiy is set to read-only
# Export Antiy = "xidian" to check whether it can be modified. The system prompts "no" because it is read-only.

-Variable replacement:
-Replace its name with the value of the variable.
-Echo
-Add $ before the variable name and use the echo command to display the values of a single variable.
# Testvar = "this is a test"
# Echo $ testvar
This is a test
# Echo $ {testvar}
Variable replacement-continued
$ {Variable name} shows the actual value to variable name
$ {Variable name: + value} If variable name is set, its value is displayed. Otherwise, it is null!
$ {Variable name :? Value} If variable name is not set, the user-defined error message value is displayed!
$ {Variable name:-value} if no variable name is set, its value is displayed.
$ {Variable name: = value} if no variable name is set, set its value and display
# Echo $ {testvar: + "xidian "}
Xidian
# Echo $ {testvar1: + "xidian "}
Null
# Echo ${testvar1 :? "No defined "}
Bash: testvar1: no defined
# Echo $ {testvar1:-"xidian "}
# Echo $ {testvar:-"xidian "}
Xidian
# Echo $ {testvar: = "xidian "}
This is a test
# Echo $ {testvar2: = "xidain "}
Xidian
# Echo $ testvar2
Xidian

-Clear variables:

# Echo $ testvar
This is a test
# Unset testvar
# Echo $ testvar
Null
# Echo $ testvar2
# Readonly testvar2
# Unset testvar2
-Location variable:
-The position variable indicates $0, $1, $2,... $9
-Use location parameters to scripts
-Pass parameters to system commands
Example01:
#! /Bin/bash
# Parm01
Echo "this is the Script Name: $0"
Echo "This is the script's 1st location parameter: $1"
Echo "This is the script's 1st location parameter: $2"
Echo "This is the script's 1st location parameter: $3"
Echo "This is the script's 1st location parameter: $4"
Echo "This is the script's 1st location parameter: $5"
Echo "This is the script's 1st location parameter: $6"
Echo "This is the script's 1st location parameter: $7"
Echo "This is the script's 1st location parameter: $8"
Echo "This is the script's 1st location parameter: $9"
Example02:
#! /Bin/bash
# Parm02
Find/home/shell/-name $1-print

-Standard variables:
-Bash creates some standard environment variables by default, which can be defined in/etc/profile.
-EXINIT
-HOME
-IFS
Echo $ IFS
IFS = ':'
-LOGNAME
-MAIL
-MAILCHECK
-MAILPATH
-TERM
-PATH
-TZ
-PS1
# Set | grep "PS"
PS1 = '[\ u @ \ h \ w] \ $'
PS2 = '>'
-PS2
-PWD
-SHELL
-MANPATH
-TERMINFO
-Special variables:
-$ #: Number of parameters passed to the script
-$ *: Displays all parameters passed to the script in a single string. Different from the location variable, this option can contain more than 9 parameters.
-$: ID of the current process running the script
-$! : ID of the last process running in the background
-$ @: It is the same as $ *, but it is enclosed by quotation marks and each parameter is returned in quotation marks.
-$-: Displays the current options used by the shell, which have the same functions as the set command.
-$? : Displays the exit status of the last command. 0 indicates no error. Other values indicate errors.
#! /Bin/bash
# Parm01
Echo "this is the Script Name: $0"
Echo "This is the script's 1st location parameter: $1"
Echo "This is the script's 1st location parameter: $2"
Echo "This is the script's 1st location parameter: $3"
Echo "This is the script's 1st location parameter: $4"
Echo "This is the script's 1st location parameter: $5"
Echo "This is the script's 1st location parameter: $6"
Echo "This is the script's 1st location parameter: $7"
Echo "This is the script's 1st location parameter: $8"
Echo "This is the script's 1st location parameter: $9"
Echo "Number of displayed parameters: $ #"
Echo "show all Script Parameters: $ *"
Echo "display process ID: $"
Echo "shows the status after the previous command is run: $? "
Commands that affect variables:
-Declare
-Export
-Readonly
-Set
-Shift
-Typeset
-Unset
Quotation marks, double quotation marks, single quotation marks, reverse quotation marks, and backslash
Operator and operator type

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