Shell programming specifications and variables

Catalog

One .shell

1.shell Script Overview

(1)Shell The concept of script

(2)Shell Script application scenarios

Two ,,shell Script execution

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

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

3、 ... and , Pipe operator operation

Four . Redirect

(1) Interactive hardware devices

(2) Redirection operation

5、 ... and , Classification of variables

1, Classification of variables

2, Define a new variable

3, Custom variable

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One .shell

  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 receiving the operation instructions input by the user （ command ） And explain , Pass the operation to be performed to the kernel for execution , And output the execution result .
  Bash（/bin/bash） It's most at the moment 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 .Linux The default version is Shell.

1.shell Script Overview

(1)Shell The concept of script

1 Save the commands to be executed in order to a text file

2 Give the file executable permissions
3 It can combine all kinds of Shell Control statements to perform more complex operations

(2)Shell Script application scenarios

1 Repetitive operations

2 Interactive tasks

3 Batch transactions

4 Service running status monitoring

5 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

2 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]# ./hhd.sh 
-bash: ./nxx.sh:  Not enough permissions 
[[email protected] opt]# . hhd.sh 
 Who comes in? Who's brother ：
 
[[email protected] opt]# source hhd.sh 
 Who comes in? Who's brother ：
 
[[email protected] opt]# 
[[email protected] opt]# bash hhd.sh 
 Who comes in? Who's brother 
 [[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

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) Redirection operation

type	The operator	purpose
Redirect input	<	Read data from the specified file
Redirect output	>	Save the standard output results to the specified file , And cover the original content
	>>	Appends the standard output result to the end of the specified file , Don't cover the original content
Standard error output	2>	Save the error message to the specified file , And cover the original content
	2>>	Append the error message to the end of 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]# 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 

echo " Custom content " > Custom filename
passwd --stdin user name < Custom filename

ls -lh > log.txt 2>&1      Equate to         ls -lh &> log.txt
Originally 1→ The screen （1 Point to the screen ）
perform >log after , 1→log.txt (1 Point to log.txt)
perform 2>&1 after , 2→1 (2 Point to 1, and 1 Point to log.txt, therefore 2 It also points to log.txt)

 

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
xiaoyang
[[email protected] opt]# echo $bb
shini
[[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 "$i"
[[email protected] opt]# bash zz.sh 
10
[r[email protected] opt]# num =`bash zz.sh`
bash: num:  Command not found ...
[[email protected] opt]# num=`bash zz.sh`
[[email protected] opt]# echo "$num"
10
[[email protected] opt]# 
 
**
 Extract the output result after the command is executed through the apostrophe 
 Backhand cannot be nested