Shell: Variables

Variable

Naming rules

• Variable names can be made up of letters 、 Numbers or underscores
• And can only start with letters or underscores
• You can use strings of any length as variable names

Variable type

• Shell It's a kind of Dynamic type language and Weak type language

• The data type of the variable does not need to be declared , The data type of variable will vary according to different operations

• Shell Variables in are uniformly stored as strings . Depending on the context of the variable , Allow programs to perform some different operations

Defining variables

• Variable name = A variable's value （ No space around equal sign ）
• Scope of action ： At present only shell Effective in , At present shell The son of shell None of them

#!/bin/bash 

# Defining variables 
a=1 
b="hello" 
c="hello world" 

# Define backup path  
bak_dir=/data/backup

Variable assignment

Direct assignment

• a=111

Use position parameters

• Positional arguments

​

Use the command to output

• `` command
• $( command )

user=`wc -w /`
user=$(wc -w /)

read

read	Options	Parameters	effect
		Variable	Change the value of a variable
	-p	Variable	“ Prompt statement ”
	-n Numbers	Variable	Set character limits , Automatically exit when the number of characters reaches the limit The input data is assigned to the variable
	-t	Variable	Timing input , You need to enter... Within the specified time , return 0 state When the clock is full , Return to non-zero exit status , Returns NULL to the variable
	-s	Variable	Close back display -s Options enable read The data entered in the command is not displayed on the monitor actually , The data is displayed , It's just read The command sets the text color to the same color as the background

read v1

read -n1 -s10 -p "please input" v1

Read from file

• Out of the loop, the value of the variable is empty

while read user
do
	echo $user
done < user.lst

Reference variables

$ Variable name 
${ Variable name }

# Reference variables 
echo $a			#1
echo ${a}		#1
echo $a1		# Undefined 
echo ${a}1		#11
echo ${a}$b		#1hello

y=123
y=${
    y/1/aaa}

echo $y
aaa23

Check the variable ：

• echo $ Variable name
• echo ${ Variable name }
• View all variables
  • set ------ Custom variables and environment variables
• env example env | gurp back
• printenv

Cancel variables ：

• Cancel variables ：unset
• At present only shell Effective in , At present shell The son of shell None of them

command	Options	Parameters
unset		name	Cancel variables , There is no such variable Cancel function
	-v	Variable name	Cancel variables
	-f	Function name	Cancel function

A read-only variable

• Configure the variable to readonly type
• Variables cannot be changed , Also can not unset

command	Options	Parameters	effect
readonly		Variable name = value	Define read-only variables
	-r	Variable name = value	Define read-only variables
	-f		Define read-only functions
	-a		Define read-only array variables
	-p		Displays a list of all variables in the system

Predefined variables

• $0 Script name
• $# Number of parameters
• $* All parameters ： As a whole
• $@ All parameters ： As a collection , Similar list
• $$ Of the current process PID
• $! Of the last background process PID
• $? The return value of the previous command 0 It means success

Common error codes

Error code	explain
127	Command not found Wrong variable definition
2	No files or directories

Positional arguments

$1 $2 $3 $4 $5 $6 $7$8 $9 ${10}

vim test.sh
echo " The first 1 The first positional parameter is $1" 
echo " The first 2 The first positional parameter is $2" 
echo " The first 3 The first positional parameter is $3"     

sh test.sh 1 2 3
 The first 1 The first positional parameter is 1
 The first 2 The first positional parameter is 2
 The first 3 The first positional parameter is 3

sh test.sh {
    1..3}
 The first 1 The first positional parameter is 1
 The first 2 The first positional parameter is 2
 The first 3 The first positional parameter is 3

shift command

• Position parameters can be used shift Command move left

echo " Number of parameters $#“ echo " All the parameters $*" shift echo " Number of parameters $#“
echo " All the parameters $*"
shift 3 
echo " Number of parameters $#“ echo " All the parameters $*"
    
bash a.sh 1 2 3 4 5 6 7
7
1 2 3 4 5 6 7
6
2 3 4 5 6 7
3
5 6 7

Scope of variable

Global variables

• You can define... In a script
• You can define... In a function
• All variables defined in the script are global variables , Its scope is to start from the defined place , Until Shell The script ends or is explicitly deleted

local variable

• The parameters of the function

• Or use inside the function local Definition

• Local variables are used in a small range , Access is usually limited to a program segment

   local v2=200
  

  #!/bin/bash func() {
            echo "$v1"  local v1=2  echo "$v1"  }v1=1 func echo "$v1"121
  

environment variable

• Variable action range ： At present shell Hezi shell It works

env

• View global variables

export

• Set the environment variable
• export Output , At present shell And the current shell The son of shell It works

command	[ Options ]	[ Variable name ]=[ Variable settings ]	effect
export		A="$(date +"%F %T")"	Create environment variables
	-f		representative [ Variable name ] Function name in
	-n		Delete the specified variable Variables are not actually deleted , It will not be output to the execution environment of subsequent instructions
	-p		List all shell The environment variable given to the program

vim /etc/profileexport A="$(date +"%F %T")"a=111export asource /etc/profile      

Global environment variable

• All users active

• /etc/profile The variables set in the file are global variables

  • Now the environment variable will not take effect directly , Use after modification source file or . file The command to update

• /etc/profile Will execute first /etc/profile.d/ All under directory *.sh file

  • Create a global environment variable , It is suggested that /etc/profile.d/ Next, create a new .sh file

User local variables

• User's home directory .bashrc、.bash_profile file , Only useful for the current user
  • Use after modification source file or . file The command to update
• .bash_profile Only read when logging in ,.bashrc Will log in and open a new shell When reading

Variable file reading order

To configure java Environment variables of

• . Represents the current path
• ： For segmentation

tar -xzf jdk.8u251-linux.tar.gz -C /usr/javavim /etc/profile.d/java.shexport JAVAHOME=/usr/java/jdk1.8.0_60export CLASSPATH=.:$JAVA_HOME/lib/dt.jar:$JAVA_HOME/lib/tools.jarexport PATH=$JAVAHOME/bin:$PATH

export

• [ Variable name ]=[ Variable settings ] Defining variables , want export Output , Otherwise it won't work

command	[ Options ]	[ Variable name ]=[ Variable settings ]	effect
export		A="$(date +"%F %T")"	Create environment variables
	-f		representative [ Variable name ] Function name in
	-n		Delete the specified variable Variables are not actually deleted , It will not be output to the execution environment of subsequent instructions
	-p		List all shell The environment variable given to the program

vim /etc/profileexport A="$(date +"%F %T")"source /etc/profile    

history

command	Options	Parameters	effect
history			View history command
		n（ Numbers ）	Print the latest n Historical orders
		!n（ Numbers ）	Execute the command on this line again
		!$	The last parameter of the previous command
	-c		Clear current history command
	-w		Writes the current history command buffer command to the history command file Cover
	-a		Writes commands in the history command buffer to the history command file Additional
	-r		Read the commands in the historical command file into the current historical command buffer Additional

• The files used by each user to store historical commands are generally ~.bash_history
• The system will when the user logs off , Load the commands in the cache into the file

history The command shows the executing user 、 execution time 、 Perform user IP

• Use HISTTIMEFORMAT Environmental variables change

[root@system1 day1]# vim /etc/profile# USER_IP  take ipUSER_IP=$(who -u -m | cut -d'(' -f2 | cut -d')' -f1) =$(who -u -m | awk '{print $NF}'|sed 's/[()]//g')#  Definition history Command display format export HISTTIMEFORMAT="%F %T $(whoami) $USER_IP "export HISTTIMEFORMAT="[$USER] [$user_ip] [%F] [%T] "export HISTTIMEFORMAT="[%F %T] [$(whoami)] [$USER_IP] "[root@system1 day1]# source /etc/profile

System variables

Variable	explain
LANG	The code currently used
SHELL	Look at the system variables
USER	The current user
UID	Current user's UID

Variable substring

expression	say bright
${A}	Return variable A The content of
${#A}	Return variable A Length of content （ By character ）, It also applies to special variables
${A:0}	In variables A in , From the position 0 Then start to extract the substring to the end
S{A:0:5}	In variables A From the middle 0 And then we start to extract the length of 5 The string of
${A:0-5}	take A From the beginning to the end of the penultimate
${A:0-5:2}	A Middle reciprocal 5 Start with two substrings
${A#0}	Dependent variable A Start at the beginning , Delete the shortest match 0 Substring
${A##o}	Dependent variable A Start deleting the longest match at the beginning 0 Substring
${A%o}	Dependent variable A At the end, delete the shortest match 0 Substring
${A%%0}	Dependent variable A At the end, delete the longest matching 0 Substring
${A/O/S}	Use S Instead of the first matching O Does not change the value of the variable
${A//O/S}	Use S Instead of all matching O Does not change the value of the variable

dirname

• Take the pathname

[root@bogon ~]# dirname /root/text/filefile

basename

• Take file name

[root@bogon ~]# basename /root/text/file/root/text

Shell Description of special extended variables

expression	say bright
${A:-Q}	If A The variable value of is empty or not assigned , Will return Q String and replace the value of the variable
	purpose ： If the variable is undefined , Then the alternate value is returned , Prevent variables from being null or causing exceptions due to undefined values
${A:=Q}	If A The variable value of is empty or not assigned , Set the value of this variable to Q, And return its value . Position variables and special variables are not applicable
	purpose ： Basically the same as the previous one $(parameter:-word), But this variable gives an extra parameter The variable is assigned a value
${A:?Q}	If A The variable value is empty or not assigned , that Q The string will be output as standard error , Otherwise, output the value of the variable .
	purpose ： Used to catch errors caused by undefined variables , And quit the program
${A:+Q}	If A The variable value is empty or not assigned , Then do nothing , otherwise Q String will replace variable ; Does not change the value of the variable

[root@bogon ~]# echo ${a:-2}2[root@bogon ~]# b=${a:-2}[root@bogon ~]# echo $b2

Built-in variables

Variable	explain
$RANDOM	Take random values