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linux In the system shell It's a special application , It lies between the operating system kernel and the user , Acting as a “ command interpreter ” Role . Be responsible for accepting the operation instructions input by the user and explaining , Pass the required operations to the kernel for execution , And output the result .

common shell There are many interpreter programs , Use different shell when , Its internal instructions , There are some differences in command line prompt and so on . adopt /etc/shells File understand what the current system supports shell Types of scripts .

2,shell The concept of script

Save the commands to be executed in order to a text file
Give the file executable permissions
It can combine all kinds of shell Control statements to perform more complex operations

3,shell Script application scenarios

Repetitive operations
Interactive tasks
Batch transactions
Service running status monitoring
Scheduled task execution

Two ,,shell Script execution

1, Method 1 ： Command to specify the path , It is required that the document must have x jurisdiction

chmod +x /root/first.sh
 Specify the absolute path ：/root/first.sh
 Specify the relative path ：./first.sh

[[email protected] opt]# ./nxx.sh 
-bash: ./nxx.sh:  Not enough permissions 
[[email protected] opt]# chmod +x nxx.sh 
[[email protected] opt]# ./nxx.sh 
 Please enter who is a stupid dog ：^C

3, Method 2 ： Appoint shell To explain the script , It is not required that the document must have x jurisdiction

sh  Script path ：sh first..sh
source  Script path ： .first.sh  perhaps  source first.sh: perhaps bash first.sh

[[email protected] opt]# chmod -x nxx.sh 
[[email protected] opt]# ./nxx.sh 
-bash: ./nxx.sh:  Not enough permissions 
[[email protected] opt]# . nxx.sh 
 Please enter who is a stupid dog ：

[[email protected] opt]# source nxx.sh 
 Please enter who is a stupid dog ：

[[email protected] opt]# 
[[email protected] opt]# bash nxx.sh 
 Please enter who is a stupid dog ：

[[email protected] opt]#

3、 ... and , Pipe operator operation

The pipe symbol "|" The command output on the left , As input to the command on the right （ Deal with people ）, Multiple pipes can be used in the same command line .

stay shell Script , Pipeline operation is usually used to filter the required customs information .

Example

[[email protected] opt]# netstat -natp |wc
     15     110    1468
[[email protected] opt]# netstat -natp |wc -l
15
[[email protected] opt]#

Example 2, combination awk Print using spaces for segmentation , Intercept the status of the running socket

[[email protected] opt]# netstat -natp |awk '{print $6}'
established)
Foreign
LISTEN
LISTEN
LISTEN
LISTEN
LISTEN
LISTEN
ESTABLISHED
LISTEN
LISTEN
LISTEN
LISTEN
LISTEN
LISTEN
[[email protected] opt]#

Four , Redirect

1, Interactive hardware devices

type	Device file	Document description number	Default device
The standard input	/dev/stdin	0	keyboard
standard output	/dev/stdout	1	Monitor
Standard error output	/dev/stderr	2	Monitor ,2,

2, Redirection operation

type	The operator	purpose
Redirect input	<	Read data from the specified file
Redirect output	>	Put the standard output result , preservation , To the specified file
Redirect output	>>	Append the output result to the end of the specified file
Standard error output	2>	Put the error message preservation To the specified file , And cover the original content
Standard error output	2>>	Put the error message preservation To the specified file , Don't cover the original content
Mixed output	&>	Standard output , Standard errors are saved to the same file
Mixed output	2>&1	Redirect standard error output to standard output

example 1 ">" ">>"

[[email protected] opt]# echo "zzzzz" > z.txt
[[email protected] opt]# cat z.txt 
zzzzz
[[email protected] opt]# echo "pppppp" >> z.txt
[[email protected] opt]# cat z.txt 
zzzzz
pppppp
[[email protected] opt]# echo "dfwfwf" > z.txt
[[email protected] opt]# cat z.txt 
dfwfwf
[[email protected] opt]# 

> Indicates coverage 
>> Indicates append

example 2 "<"

[[email protected] opt]# passwd --stdin jack < z.txt
 Change user  jack  Password  .
passwd： All authentication tokens have been successfully updated .
[[email protected] opt]#

example 3, Standard error output

[[email protected] opt]# dauhfidauhfa 2> z.txt 
[[email protected] opt]# cat z.txt 
bash: dauhfidauhfa:  Command not found ...
[[email protected] opt]# dddddddddfwefc 2>> z.txt 
[[email protected] opt]# cat z.txt 
bash: dauhfidauhfa:  Command not found ...
bash: dddddddddfwefc:  Command not found ...
 One represents coverage 
 One means to append

example 4, Mixed output

[[email protected] opt]# cat /etc/passwd &> /dev/null


 Standard output , Standard errors are guaranteed in the same file 

[[email protected] opt]# cat /saada/daada > z.txt 2>&1
[[email protected] opt]# cat z.txt 
cat: /saada/daada:  There is no file or directory 
[[email protected] opt]# 

 Redirect standard error output to standard output

5、 ... and , Classification of variables

1, Classification of variables

It is used to store specific parameters that the system and users need to use
environment variable ： System maintenance , Application settings working environment
Positional variable ： Pass parameters to the script through the command line
Custom variable ： Defined by the user , Modify and use
Predefined variables ：bash A class of variables built into , Cannot be modified directly

2, Define a new variable

Format ： Variable name = A variable's value

Variable naming rule ： Start with a letter or underline , Case sensitive

[[email protected] opt]# zz=shagou
[[email protected] opt]# bb=caiji
[[email protected] opt]# echo $zz
shagou
[[email protected] opt]# echo $bb
caiji
[[email protected] opt]#

3, Custom variable

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 , In general, you don't need

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 +、 One 、*、/、.、 ?、8、&、# etc. )

■ 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
example 1

[[email protected] opt]# unset zz
[[email protected] opt]# unset bb
[[email protected] opt]# zz=aaa
[[email protected] opt]# bb=cxk
[[email protected] opt]# echo $zz$bb
aaacxk
[[email protected] opt]#

When variable names are easily confused with other characters immediately following them , Need to add “{}” Cover up , Otherwise, the correct variable name cannot be determined , For undefined variables , Will display as empty .

example 2,

[[email protected] opt]# unset zz
[[email protected] opt]# unset bb
[[email protected] opt]# zz=aaa
[[email protected] opt]# bb=cxk
[[email protected] opt]# echo $zz$bb
aaacxk
[[email protected] opt]# echo ${zz} ${bb}
aaa cxk
[[email protected] opt]#

Use quotation marks when assigning values
Double quotes ： Allowed to pass through $ The symbol references other variable values
Single quotation marks ： Do not reference other variable values ,$ Treat as normal characters
Apostrophe ： Command substitution , Extract the output of the command after execution .`..` and $(..) The same effect

 Apostrophe 
[[email protected] opt]# vim zz.sh
[[email protected] opt]# cat zz.sh 
#!/bin/bash
let i+=10
echo "$i"
[[email protected] opt]# bash zz.sh 
10
[[email protected] opt]# num =`bash zz.sh`
bash: num:  Command not found ...
[[email protected] opt]# num=`bash zz.sh`
[[email protected] opt]# echo "$num"
10
[[email protected] opt]# 

**
 Extract the output result after the command is executed through the apostrophe 
 Backhand cannot be nested

6、 ... and ,read Command to get input

Purpose ： Let users themselves , Assign a value to a variable , flexible

grammar read [ Options ] Variable name

Common options

Options	paraphrase
-p	Define information to prompt users
-n	Define the number of characters （ Limit the length of variable values ）
-s	No display （ Don't show user input ）
-t	Define timeout , The default unit is seconds （ Limit the timeout of user input variable value ）

example 1,/ Waiting for user input , Assign the entered value to test Variable

[[email protected] opt]# read cxk
123321
[[email protected] opt]# echo $cxk
123321
[[email protected] opt]# 
 Assign value to cxk 
$cxk You can get the assignment

example 2, 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

#!/bin/bash
read -p " Who is a stupid dog ： " num
echo "${num}" It's a stupid dog 
~                                                                                                
~                            
**
[[email protected] opt]# bash cxx.sh 
 Who is a stupid dog ： cxk
cxk It's a stupid dog 
[[email protected] o

Variable action range

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

It can be done by internal command export Exports the specified variable as a global variable , Make user-defined variables in all sub shell Can continue to be used in the environment .

Format 1：export Variable name

Format 2：export Variable name = A variable's value

[[email protected] ~]# nxx=cxk                      
[[email protected] ~]# echo $nxx
cxk                                      Define a variable 
[[email protected] ~]# bash]\
> ^C
[[email protected] ~]# ^C
[[email protected] ~]# bash
[[email protected] ~]# bash                      Load new environment 
[[email protected] ~]# echo $nxx                    This environment variable cannot be found 

[[email protected] ~]# exit
exit
[[email protected] ~]# exit
exit
[[email protected] ~]# export nxx                   use export  Add it to the global variable 
[[email protected] ~]# bash
[[email protected] ~]# bash                          Loading environment 
[[email protected] ~]# echo $nxx               Sure echo To this variable 
cxk
[[email protected] ~]#

The operation of integer variables

expression	give an example
$(())	echo $((9+9))
$[]	echo $[15-9]
expr	expr 10/5
let	i=1;let i+=10 be equal to let i=i+10

Example

[[email protected] ~]# echo $((18+25))
43
[[email protected] ~]# echo $[2*4]
8

*
[[email protected] ~]# expr 4 \* 8
32
[[email protected] ~]# i=4
[[email protected] ~]# a=3
[[email protected] ~]# let $i \* $a
bash: let: *:  Grammar mistakes :  Expecting operands  （ The error symbol is  "*"）
[[email protected] ~]# expr  $i \* $a
12
[[email protected] ~]# let i+=10
[[email protected] ~]# echo $i
14
[[email protected] ~]# 
[[email protected] ~]# let a=a+11
[[email protected] ~]# echo $a
14

i++ amount to i=$[$i+1]
i-- amount to i=$[$i-1]
i+=10 amount to i=$[$i+10]

7、 ... and , environment variable

Use env The command can view the environment variables of the current working environment
Variable user Represents the user name ,home Indicates the user's add directory ,lang Represents the language and character set ,pwd Indicates the current working directory , Variable path Represents the default search path for executable programs .

8、 ... and , A read-only variable

Used when the variable value cannot be modified

[[email protected] ~]# cxk=shagou
[[email protected] ~]# readonly cxk                Set to read-only variable 
[[email protected] ~]# cxk=caiji                     Variables cannot be reassigned 
bash: cxk:  A read-only variable 
[[email protected] ~]# unset cxk
bash: unset: cxk:  Can't reset :  read-only  variable         Read only variables cannot be deleted , Restart and disappear .
[[email protected] ~]# 
🦼🦼**********

Nine , Positional variable

When executing command operation , The first character represents the command name or script program name , The rest of the characters indicate that they are assigned to the position variable once in order from left to right .
$n:n Is a number ,$0 For the order itself ,1~9 Represents the first to ninth parameters , More than ten parameters need to be represented by braces , For example, the tenth parameter is ${10}.

Ten , Custom variable

Built in commands	meaning
`==$?==`	The status returned after the last command is executed ; The status value is 0 Indicates normal execution ,== Not 0== Indicates execution exception or error
$0	The name of the program or script currently executing
==$#==	Of the parameters followed by the script == Number ==
==$*==	After the script == All the parameters ==, Parameters are output as a whole , Each variable parameter is separated by a space
[email protected]==	After the script == All the parameters ==, The parameters are independent , It's all output
==$1~$9==	After the script == Positional arguments ==,$1 It means the first one 1 Position parameters , By analogy
${10}~${n}	Extended location parameters , The first 10 Position variables must use {} Enclosed in braces (2 Add more than one digit )
==$$==	The process number of the current process , Such as `echo $$`
$！	The last process number running in the background ( Current terminal
!$	Call... In the last command history == Parameters ==

result

Further connection $* and [email protected] The difference between

Learn more $?

For the status code returned by the previous command .

[[email protected] ~]# bash weizhi.sh  A B C D E
A
B
C
D
E
___________________________________
A B C D E
[[email protected] ~]# echo $?                         The previous command is correct , return 0
 0
[[email protected] ~]# bash weizhi.shssss  
bash: weizhi.shssss:  There is no file or directory 
[[email protected] ~]# echo $?
127                                           The command of the previous day reported an error , Display the error return value 
[[email protected] ~]#

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