Shell writing specifications and variables

Catalog

One 、shell Concept

        1.1、shell What can a script do

        1.2、 Statement shell The interpreter for the script

  Two 、shell Variables and assignments

        2.1、 Custom variable

        echo The use of command

        2.2、 Special operations

         2.3、 Predefined variables

3、 ... and 、 Variable operation

        expr and let

Four 、 Redirect

5、 ... and 、 Pipe operator operation

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One 、shell Concept

        shell Is a command interpreter , He is on the outermost layer of the operating system , Responsible for direct dialogue with users , Explain user input to the operating system , And handle the output of various operating systems , Output to screen feedback to user , This kind of dialogue can be interactive or non interactive , The command we input is not recognized by the computer , At this time, the interpreter is needed to compile , Programming a language that a computer can recognize .

        1.1、shell What can a script do

        Automate the installation and deployment of software , Such as installation and deployment LAMP Architecture Services

        Complete the management of the system automatically , Such as adding users in batch

        Automate backup , Such as regular database backup

        Automated analytical processing , Such as website visits

        1.2、 Statement shell The interpreter for the script

        If marked at the beginning of the file     #!/bin/bash

                                        #!/bin/env/bash

                                        #!bin/sh

                                        

  Two 、shell Variables and assignments

        Variables are used to temporarily store data , And the data value can change , 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, you can directly modify the variable value

        2.1、 Custom variable

Format ：

Variable name = A variable's value

# Variable name ： A place where data is temporarily stored

# A variable's value ： Temporary changeable data

                                         

        echo The use of command

use echo View and reference the values of variables

By adding a leading symbol before the 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 , 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 .

  

echo Options 
echo -n Indicates no line feed output 

 Use echo -e Output transfer 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 

 

        2.2、 Special operations

        There are also some special assignment operations , You can assign values to variables more flexibly , So as to be suitable for your complex management tasks .

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 .
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 .
Apostrophe （  `  ）	The apostrophe is mainly used to replace , Allows you to assign the screen output result of executing a command to a variable . The command line that must be executable within the range enclosed by the apostrophe , Otherwise, there will be mistakes .

         2.3、 Predefined variables

 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 
$#： 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 

3、 ... and 、 Variable operation

Operation content	Add (+)、 reduce (-)、 ride (*)、 except (/)、 Remainder (%)
Operation symbol	$(()) and $[ ]
Operation command	expr and let
Computing tools	bc

      

        expr and let

Four 、 Redirect

type	The operator	purpose
Redirect input	<	Read data from the specified file
Redirect output	>	Put the standard output result , Overwrite and save to the specified file
	>>	Optimize the output result to the end of the specified file
Standard error output	2>	Put the error message , Overwrite and save to the specified file
	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
	2>&1	Redirect standard error output to standard output

< Example

1. [[email protected] opt]# passwd --stdin yy < a.txt

2. Change user yy Password .

3. passwd： All authentication tokens have been successfully updated .

4. [[email protected] opt]#

>,>> Example  

[[email protected] yy]# cat test.sh 
123456
[[email protected] yy]# echo "123456" >> test.sh 
[[email protected] yy]# cat test.sh 
123456
123456
[[email protected] yy]# echo "aaaa" > test.sh 
[[email protected] yy]# cat test.sh 
aaaa
[[email protected] yy]# 

 2>,2>> Example

[[email protected] yy]# cat test.sh 
aaaa
[[email protected] yy]# asdadf 2> test.sh 
[[email protected] yy]# cat test.sh 
bash: asdadf: Command not found ...
[[email protected] yy]# sssss  2>> test.sh 
[[email protected] yy]# cat test.sh 
bash: asdadf: Command not found ...
bash: sssss: Command not found ...
[[email protected] yy]# 

example 4, Mixed output

&> You can save the error and correct prompt information to the same file

[[email protected] yy]# cat test.sh 
123456
[[email protected] yy]# echo "111111" &> test.sh 
[[email protected] yy]# cat test.sh 
111111
[[email protected] yy]# aaaaa  &> test.sh 
[[email protected] yy]# cat test.sh 
bash: aaaaa: Command not found ...
[[email protected] yy]# 

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 .

Example

[[email protected] yy]# netstat -natp |wc -l
15