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Script statement （ Interpreter ）： If the first act “#!/bin/bash” , Indicates that the code statement below this line is passed /bin/bash Program to execute ,#!/bin/bash Is the default interpreter , There are other types of interpreters , such as #!/usr/bin/python、#!/usr/bin/expect.

Annotation information ： With “#” The opening statement is expressed as a comment message , The annotated statement will not be executed when the script is run .

Executable statement ： such as echo command , For output “ ” String between .

Four 、Shell Script execution

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

chmod -x /root/liji.sh

Specify the absolute path ：/root/liji.sh

Specify the relative path ：./first.sh

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

sh Script path ：sh liji.sh

source Script path ：.liji.sh or soure liji.sh: or bash liji.sh

5、 ... 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 .

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

6、 ... and 、 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：> Indicates coverage ;>> Indicates append

Example ②

Example ③： Standard error output

7、 ... 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

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

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

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 .

4、 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

8、 ... and 、read Command to get input

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

grammar read [ Options ] Variable name

1、 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 ）

2、 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

3、 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

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

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

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

Nine 、 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 .