Shell -- custom variables and assignments

One 、shell The variable of

Variables are used to temporarily store data , And the data can change from time to time , No language can do without variables , If something needs to be used more than once and will reappear , So you can use variables , If you need to modify, just modify the variable directly
common Shell The types of variables include custom variables 、 environment variable 、 A read-only variable 、 Positional variable 、 Predefined variables

1、 Custom variable

1、 Definition of variables
Bash The variable operation in is relatively simple , Unlike other high-level programming languages （ Such as C/C++、Java etc. ） So complicated . When defining a new variable , Generally, there is no need to make a statement in advance , Instead, specify the variable name directly and assign it to the initial value （ Content ） that will do

 Format ： Variable name = A variable's value 

• Variable name ： A place where data is temporarily stored

• A variable's value ： Temporary changeable data

• There is no space on either side of the equal sign . Variable names should start with letters or underscores , Do not include special characters in the name （ Such as +、-、*、/、.、?、%、&、# etc. ）

2、 environment variable

• Environment variable refers to the environment variable generated by Linux A type of variable created by the system in advance , It is mainly used to set the user's working environment , Include user host Directory 、 Command to find the path 、 User current directory 、 Login terminal, etc .

• The value of the environment variable is determined by Linux Automatic system maintenance , It will change with the change of user state .

• Use env The command can view the environment variables in the current working environment , For some common environment variables, we should understand their respective purposes .

  • for example
  • Variable USER Represents the user name
  • HOME Represents the user's Host Directory
  • LANG Represents the language and character set
  • PWD Indicates the current working directory
  • PATH Indicates command search path, etc
  • RANDOM Represents a random number , Returns the 0-32767 The integer of
  • USER Indicates the account name of the current account , It is generally defined in all uppercase , Note the distinction between and custom variables

  [[email protected] ~]# echo $RANDOM    # It returns a random number 
  20315
  [[email protected] ~]# echo $RANDOM
  25684
  [[email protected] ~]# echo $HOME
  /root
  [[email protected] ~]# echo $USER
  root
  [[email protected] ~]# echo $UID
  0
  [[email protected] ~]# echo $PWD
  /root
  [[email protected] ~]# echo $LANG
  zh_CN.UTF-8
  [[email protected] ~]# echo $SHELL
  /bin/bash
  

3、PATH Variable

PATH Variable is used to set the default search path for the executable program , When only the file name is specified to execute the command program ,Linux The system will be in PATH Variable to find the corresponding executable file , If it cannot be found, it will prompt “command not found”.

[[email protected] ~]# test.sh
bash: test.sh:  Command not found ...
[[email protected] ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/root/bin
[[email protected] ~]# pwd
/root

This is because test.sh be not in $PATH In the directory of , So the system can't recognize it and can't use it directly , You need to keep up with the absolute path and use the script

Method 1 ： Add the directory of the script to $PATH

[[email protected] ~]# PATH="$PATH:/root"   # This is temporary , If it takes effect permanently, you need to edit /etc/profile file 
[[email protected] ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/root/bin:/root
[[email protected] ~]# test.sh 
hello world

Method 2 ： Put your own script into $PATH A directory in

stay Linux In the system , The global configuration file of the environment variable is /etc/profile, The variables defined in this document act For all users . besides , Each user also has its own independent configuration file （～/.bash_profile）.
After the modification, you have to log in again to take effect , If you want to take effect immediately , have access to source

4、 A read-only variable

Shell There is a special case in variables , Once set , Its value is unchangeable , This variable is called a read-only variable The amount . When you create a variable, you can set it as a read-only property , You can also set an existing variable as a read-only property , only Reading variables is mainly used when the value of variables cannot be modified
Read only variables cannot be changed or deleted

[[email protected] ~]# test=123
[[email protected] ~]# readonly test        #readonly Used to define read-only variables , Once the use readonly Defined variables cannot be changed in a script 
[[email protected] ~]# echo $test
123
[[email protected] ~]# test=456
-bash: test:  A read-only variable 
[[email protected] ~]# unset test
-bash: unset: test:  Can't reset :  read-only  variable

• Methods for deleting read-only variables ： Exit the account to refresh the memory , Eliminate variables ！！！

  [[email protected] ~]# exit
   Log out 
  
  Connection closed.
  
  Disconnected from remote host(centos7-2) at 22:38:32.
  
  Type `help' to learn how to use Xshell prompt.
  [C:\~]$ 
  
  Connecting to 192.168.226.127:22...
  Connection established.
  To escape to local shell, press 'Ctrl+Alt+]'.
  
  Last login: Fri Jul  1 22:38:22 2022 from 192.168.226.1
  [[email protected] ~]# echo $teat    ## Will display as empty 
  
  [[email protected] ~]# 
  

5、 Positional variable

• When performing command line operations , The first field represents the command name or script program name , The remaining string parameters are assigned to the position variable in order from left to right .

• Location variables are also called location parameters , Use $1、$2、$3、…、$9 Express

• The name of the command or script itself uses “$0” Express

example ： Write a simple script to create users and passwords

[[email protected] ~]# vim 1.sh
#!/bin/bash
useradd $1
echo $2 | passwd --stdin $1
[[email protected] ~]# bash 1.sh ct 123
 Change user  ct  Password  .
passwd： All authentication tokens have been successfully updated .
[[email protected] ~]# 
[[email protected] ~]# id ct
uid=1001(ct) gid=1001(ct)  Group =1001(ct)

6、 Predefined variables

The predefined variables are created by Bash A class of special variables pre-defined by a program , Users can only use predefined variables , and Cannot create a new predefined variable , You can't assign values to predefined variables directly . Predefined variables use “$” A combination of a symbol and another symbol indicates

Variable	meaning
$#	Indicates the number of positional parameters in the command line
$*	Represents the contents of all positional parameters , These contents as a whole
[email protected]	Indicates that all position parameters are listed , But it is listed in a single form
$?	Indicates the return status after the execution of the previous command , The return value is 0 The execution is correct , Return any non 0 Values indicate an exception in execution
$0	Indicates the name of the currently executing script or program
$$	Indicates that the process number of the current process is returned
$!	Returns the process number of the last background process

example 1： Understand the meaning of each predefined and location variable according to a simple script

#!/bin/bash
echo $1  
echo “$0  Indicates the name of the currently executing script or program ”
echo “$#  Indicates the number of positional parameters in the command line ”
echo “$*  The contents of all positional parameters , These contents as a whole ” 
echo “[email protected]  Indicates that all position parameters are listed , But it is listed in a single form ”

 understand $* and [email protected] The difference between 
$*、[email protected]: Represents the parameters to be processed by a command or script .
$*: Think of all parameters as a whole string separated by spaces ( Single string ) return , representative "$1 $2 $3 $4".
[email protected]: Separate each parameter with double quotation marks into n A list of the parameters of the , Each parameter is returned as a string , representative "$1""$2""$3""$94".

7、 Define variables interactively

• read command

• Options

  Options	meaning
  -p	Information to prompt users
  -n	Define the number of characters
  -s	Don't show user input , Commonly used to enter passwords read -s -p “input your password:” pass
  -t	Define timeout , If you haven't lost for more than a long time, you will automatically quit

In addition to the above assignment operations , You can also use Bash Built-in commands read To assign a value to a variable .
Used to prompt the user to enter information , So as to realize a simple interactive process . It will be executed from the standard input device （ keyboard ） Read in One line , And use a space as a separator , Assign the read fields to the specified variable in turn （ Assign extra content to The last variable ）. If only one variable is specified , Assign the entire line to this variable .

[[email protected] ~]# read date
4      # Waiting for user input , Assign the entered value to test Variable 
[[email protected] ~]# echo $date
4

Generally speaking, in order to make the interface of interactive operation more friendly , Improve ease of use ,read Commands can be combined with “-p” Option to set the prompt message , In order to inform the user what content to enter and other related matters

[[email protected] ~]# read -p " Today, this week " date
 Today, this week 4
[[email protected] ~]# echo " Today, this week $date"
4

• contrast

[[email protected] ~]# read " week " date
4
-bash: read: ` week ':  Not a valid identifier 
[[email protected] ~]# read -p " week " date
 week 4
[[email protected] ~]# echo "$date"
4

It can be seen that it is used alone read The command cannot use Chinese characters

use read -p Chinese characters can be quoted

Action range of variable

By default , The newly defined variable is only in the current Shell Effective in the environment , So it's called a local variable , When entering a subroutine or a new subroutine Shell Environmental time , Local variables will no longer be used

[[email protected] ~]# bash  # Intron shell Environmental Science 
[[email protected] ~]# 
[[email protected] ~]# echo $name

# The display is empty 

Two 、 View and reference the values of variables

• use echo View and reference the values of variables

• By adding a leading symbol before the variable name “$”, You can reference the value of a variable , Use echo Command can view variables , Can be in one echo View multiple variable values at the same time in the command
  for example

[[email protected] ~]#Product=Python
[[email protected] ~]#Version=2.7.13
[[email protected] ~]#echo $Product$Version
Python2.7.13

• When variable names are easily confused with other characters immediately following them , You need to add braces “{}” Enclose it , Otherwise, the correct variable name cannot be determined . For undefined variables , Will be displayed as a null value
  give an example

  #{} Reference variables 
  
  echo ${Product}2.5
  2.5
  echo ${test}RMB
  RMB
  

echo Options

echo -n Indicates no line feed output

• Use echo -e Output escape characters , Output the escaped content to the screen

The commonly used escape characters are as follows ：

• \c Don't wrap output , stay ”\c” If there are no characters after , The effect is equivalent to echo -n

• \n Line break

• \t After escape, it means to insert tab, That's the tab

notes ：\ Escape character , With the \ The special symbol after that will lose its special meaning , Change to normal characters . Such as "\ $" Will output “ $” Symbol , Improper doing is variable reference

example

[[email protected] ~]# echo -n hello
hello[[email protected] ~]#

[[email protected] ~]# echo -e "hello\t"
hello	

Undefine

unset Variable name

3、 ... and 、 Special operations

There are also some special assignment operations , You can assign values to variables more flexibly , So as to be applicable to various complex management tasks

1、 Double quotes : " "

Double quotation marks are mainly used to define strings , Especially when the content to be assigned contains spaces , Must be enclosed in double quotation marks ; In other cases, double quotation marks can usually be omitted

1、 When there are spaces in the content

echo “hello world”
echo nihao

2、 When the value of a variable is assigned

[[email protected] ~]# version=2
[[email protected] ~]# pyver="python $version"
[[email protected] ~]# echo $pyver
python 2

2、 Single quotation marks : ‘’

When the content to be assigned contains "$、“、”" And other characters with special meaning , Should be enclosed in single quotation marks .
Within single quotation marks , You will not be able to reference the values of other variables , Any character is treated as a normal character . Display what you enter
But the assignment content contains single quotation marks （‘） when , Need to use \’ The symbols are escaped , To avoid conflict .

[[email protected] ~]# test=123
[[email protected] ~]# echo "$test"
123
[[email protected] ~]# echo '$test'
$test

3、 Apostrophe : ``

The apostrophe is mainly used for command substitution , Allows you to assign the screen output result of executing a command to a variable .
The range enclosed by the apostrophe must be an executable command line , Otherwise, there will be mistakes

ls -lh `which useradd`

Through the first which useradd Command to find out useradd The program location of the command , Then list the file attributes according to the search results

[[email protected] ~]# time=`date +%T`
[[email protected] ~]# echo $time
04:23:22

It is difficult to implement nested command substitution in one line of command by using apostrophe , At this time, you can use “$()” Instead of the apostrophe operation , To solve the problem of nesting

for example

[[email protected] ~]# rpm -qc `(rpm -qf `(which useradd)`)`
-bash:  Unexpected symbols  `('  There are grammar errors nearby 
[[email protected] ~]# rpm -qc `(rpm -qf $(which useradd))`   
/etc/default/useradd    
/etc/login.defs
[[email protected] ~]# rpm -qc $(rpm -qf $(which useradd))
/etc/default/useradd
/etc/login.defs

• From that `` Symbols cannot nest instructions ,$() Can be nested

Four 、export command

In order to make user-defined variables in all sub variables Shell Can continue to be used in the environment , Reduce duplicate setup work , It can be done by internal command export Exports the specified variable as a global variable . You can specify multiple variable names as parameters at the same time （ No need to use “$” Symbol ）, Variable names are separated by spaces

[[email protected] ~]# exit
exit
[[email protected] ~]# export name test
[[email protected] ~]# bash
[[email protected] ~]# echo "$name $test"
 Zhang San  123

Use export While exporting global variables , You can also assign values to variables , In this way, when the global variable is newly defined, there is no need to assign a value in advance

• env View the user's current environment variables

• export ABC=123

• Again env You can see it

• export -n ABC Undefined global variables become local variables