当前位置：网站首页>Shell programming specifications and variables

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 Linux The default version is Shell.

2.shell species

common shell There are many interpreters , Use different shell, Its internal instructions 、 There are some differences in command prompt and so on , adopt /etc/shells The file can understand what the current system supports shell Types of scripts .

shell The type of     shell The function of
/bin/sh     yes shell Command soft link （ Has been /bin/bash Replaced by ）
/bin/bash     Base on FNU Developed under the framework of shell
/usr/bin/sh     Has been /bin/bash Replace
/usr/bin/bash    centos and redhat By default, the system uses bash
/bin/tcsh    csh Enhanced Edition , And csh Fully compatible with , Integrated csh, More features
/usr/sbin/nologin     Make users unable to log in to the host
/sbin/nologin     Soft connection

3、shell What is a script ？

shell The script is to put the original linux Put the command or statement in a file , And then through this program file to execute , Let's call this program shell Or scripts .
Save the commands you need to execute to a file , Execute in order , It doesn't need to compile , It's interpretive .

4、shell What can we do （ Preliminary understanding ）

Automate the installation and deployment of software
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

Application scenarios *
Repetitive operations 、 Interactive tasks 、 Batch transactions 、 Service running status monitoring 、 Scheduled task execution 、......

Two 、shell Script

1、Shell The composition of the script

1. Script statement （ Interpreter ）： If the first act “#!/bin/bash”, This line of code is represented by /bin/bash Program to explain the execution of ,#!/bin/bash Is the default interpreter . There are other types of interpreters , such as #!/usr/bin/python、#!/usr/bin/expect.
2. Annotation information ： With “#” The opening statement is expressed as a comment message , The commented statement will not be executed while the script is running .
3. Executable statement ： such as echo command , For output " " String between

2、Shell Script execution

1、 Path execution

Command to specify the path , It is required that the document must have x jurisdiction .
chmod +x /opt/xg.sh
Specify the absolute path ：/opt/xg.sh
Specify the relative path ：./xg.sh（ Script environment ）

Exit script environment

2、sh or bash perform

Appoint Shell To explain the script , It is not required that the document must have x jurisdiction .
sh Script path ：sh -x xg.sh perhaps bash -x xg.sh

3、source perform

source Script path ( System environment ) Do not require the file to have Execution Authority ：. xg.sh perhaps source xg.sh

3、 Pipeline 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 .
ps aux | wc -l
echo "abc123" | passwd --stdin zhangsan

3、 ... and 、 Redirection and pipeline operation

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

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 , And cover the original content
Redirect output	>>	Put the standard output result Additional To the end of the specified file , Don't cover the original content
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 Additional 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
Mixed output	2>&1	Redirect standard error output to standard output

1、 Redirect input

Redirecting input refers to changing the way of receiving input in the command from the default keyboard to Specified file , Instead of waiting for input from the keyboard .
Redirect input using “ < ” The operator , By redirecting input, some interactive operations can be completed by reading files .
explain ： Redirect input is used , You can input the contents of a file to other places you want to receive , There is no need to enter manually .

echo "123456" > pass.txt

useradd zhangsan
passwd --stdin zhangsan < pass.txt

combination cat Use

Configure with script yum Warehouse

2、 Redirect output

Redirecting output refers to saving the normal output of a command to a specified file , Instead of directly on the screen of the monitor .
Redirect output using “ > ” or “ >> ” Operation symbol , Used to overwrite or append files, respectively .
If the target file for redirection output does not exist , The file will be created , Then save the output result of the previous command to the file , If the target file already exists , The output result is overwritten or appended to the file

And cat Use a combination of

3、 Standard error output

Error redirection refers to the error message that will appear during the execution of the command （ For example, the parameter of the option is wrong ） Save to the specified file , Instead of being displayed directly on the screen , Error redirection using “ 2> ” The operator .
effect
in application , Error redirection can be used to collect error information about program execution , Provide basis for troubleshooting .
You can also redirect unimportant error messages to an empty file /dev/null in , To keep the output of the script concise

4、 Mixed output

① Correct and error information are saved in the same file

② Redirect standard error output to standard output

Four 、shell Action type of variable

1、 Action type of variable

environment variable ： Maintained by system , Used to set up the work 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

Role of variables ： It is used to store specific parameters that the system and users need to use （ Or value ）
Variable name ： Use a fixed name , Preset by the system or defined by the user
A variable's value ： According to the user settings 、 The system changes with the change of environment

2、 Custom variable

2.1、 Definition of variables

Format ： Variable name = A variable's value （ Be careful = There should be no spaces on either side ）
Undefine variables ：unset Variable name

Variable names need to start with letters or underscores , The name should not contain special characters （ Such as ：+、-、*、/、.、?、%、&、# etc. ）
By adding a leading symbol before the variable name “ $” , You can reference the value of a variable , Use echo Command to view variables , You can also view multiple variable values at the same time .

2.2、echo Output variable value

When the variable name is easily confused with other characters immediately following it , You need to add braces “ {} ”, Enclose it , Otherwise, the correct variable cannot be determined , For undefined variables , Will be displayed as a null value .

①echo Common options

-n ： Indicates no line feed output
-e ： Output escape characters , Output the escaped content to the screen

② 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 （ Multiple spaces ）

③ Escape character \

2.3、 Single quotation marks 、 Double quotes 、 Apostrophe

① Double quotes （“”）： Allowed to pass through $ The symbol references other variable values

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, it is usually possible to omit

② Single quotation marks （‘’）： Do not reference other variable values ,$ Treat as normal characters

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 , Display what you enter ,
But when the assignment content contains single quotation marks , Need to use \ Symbols are transferred , To avoid conflict .

③ Apostrophe （``）： Command substitution , Extract the output of the command after execution ,`…` and $(…) The same effect

！！ Be careful ： The apostrophe above can be used to assign an executable command line to a variable , However, nested substitution operations cannot be formed in one line of command , At this time, you can use “ $() ” Instead of the apostrophe , To solve the problem of nesting .

2.4、 Define variables interactively （read）*

You can define a variable , Change this variable to the value entered by the user

①read Common options

Options	function
-p	Information to prompt users
-n	Define the number of characters
-s	Don't show user input , Commonly used to enter passwords
-t	Define timeout , If you haven't lost for more than how long, you will automatically quit

2.5、 Action range of variable （ Local and global variables ）

By default , The newly defined variable is only in the current shell Effective in the environment , So it's a local variable , When entering subroutine or subroutine shell Environmental time , Local variables will no longer be used .

In order to make User defined variables in all children shell Can continue to be used in the environment , Reduce repeated settings , It can be done by internal command export Set the variables as global variables everywhere , You can specify multiple variable names as parameters at the same time （ No need to use " $ " Symbol ）, Multiple variable names are separated by spaces

Format 1：export Variable name
Format 2：export Variable name = A variable's value

env View the user's current environment variables

3、 Special variables

3.1、 environment variable

Environment variables refer to those that are required by Linux A type of variable created by the system in advance , It is mainly used to set the user's working environment , It will change with the change of user state .
Use env The environment variables in the current working environment can be found by the command , For some common environment variables, we should understand their respective purposes USER： Represents the user name ;
HOME： Represents the user's Host Directory ;
LANG： Represents the language and character set ;
PWD： Indicates the current working directory ;
PATH： Indicates command search path, etc ;
RANDOM： Represents a random number , Returns the 0-32767 The integer of
Variable PATH Represents the default search path for executable programs
echo $PATH           # View the current search path
PATH="$PATH:/root"       # take /root Add directory to search path
export PATH="$PATH:/root"   # The output is a global environment variable

PATH Variable

Used to set the executable default search path , When only the file name is specified to execute the command program ,Linux The system will be in PATH Variable to find the corresponding executable file . That is to say, if there is an executive document , Want to execute this file directly , Then the path of this file needs to be in $PATH Inside , If not , Want to execute , You must add a path to execute .

① Put the script directory in $PATH Inside as follows ：

② Put your script into $PATH In a directory in

The global configuration file of the environment variable is /etc/profile, The variables defined in this file apply to all users . Each user also has its own independent configuration file （～/.bash_profile）. It can be used to change or set an environment variable for a long time .
vim /root/.bash_profile
export HISTSIZE=200 # modify root The number of historical command records of the user

3.2、 A read-only variable

shell There is a special case in variables , Once set , Its value is unchangeable , This variable is called a read-only variable .
You can set it as a read-only property when creating , You can also set an existing variable as a read-only property , Read only variables are mainly used for variable values that cannot be modified , Cannot be deleted .

readonly Command to set read-only variables
product=benet
readonly product           # Set to read-only variable
echo $product
product=accp           # Read only variables cannot be reassigned
unset product           # Read only variables cannot be deleted ,unset The command is used to delete variables

3.3、 Positional variable

When performing command line operations , The first field represents the command name or script program name , The remaining string parameters are assigned to the position variable in order from left to right .
$n：n Is the number ,$0 For the order itself ,$1-$9 Represents taking one to the ninth parameter , More than ten parameters need to be represented by braces , For example, the tenth parameter is ${10}

3.4、 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 , You cannot create a new predefined variable , Nor can you directly predefine variable assignment , Predefined variables use “ $ ” A combination of a symbol and another symbol indicates .

Predefined variables    function
$#   Indicates the number of positional parameters in the command line
$*   Represents the contents of all locations , These contents as a whole
[email protected]   Indicates that all position parameters are listed , But it is listed in a single form
$?   Table 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 It is also often used for Shell Script return Exit function and return the exit value .
$0   Indicates the name of the currently executing script or program
$$   Indicates that the current process number is returned
$!   Returns the process number of the last background process

understand “ $ ” and “ [email protected] ” The difference between *

$*： Think of all parameters as a whole string separated by spaces （ Single string ）, representative “$1 $2 $3 $4”
[email protected] ： Separate each parameter with double quotation marks into n A list of the parameters of the , Each parameter is returned as a string , representative “$1" “$2” “$3"”$4"

5、 ... and 、 Variable operation

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

1、expr command

The operation of integer value is mainly through internal command expr Conduct , Between operators and variables , There must be at least one space , It can not only perform operations , It also supports output to the screen

Format ：expr Variable 1 Operator Variable 2 [ Operator Variable 3]
Operator ：+ Add 、- Subtraction 、\* Multiplication 、/ division 、% Remainder

2、 Operation symbol

$(()) and $ [] And echo Together with , Because it can only calculate and cannot output results , Between these two operation symbols , No need to use $ Symbols refer to variables , You can use expressions directly

2.1 $(()) Use of symbols

2.2、 $[] Use of symbols

2.3、let command

let The operation of can change the value of the variable itself , But don't show the results , need echo.

i++ amount to i=$[$i+1]
i-- amount to i=$[$i-1]
i+=2 amount to i=$[$i+2]

i++ Operation after assignment

++i Evaluate first and then assign

2.4、 bc command

Use bc Carry out operations , Support decimal operations , But it cannot be used directly in scripts , Otherwise, you will enter the interactive interface , have access to echo Use in combination with piping .

Support decimal operations , But it cannot be used directly in scripts , Otherwise, you enter the interactive interface , So in use , Need to add echo expression + Pipe, + bc . You can also use scale The command indicates how many decimal places to keep , direct scale=2, Means to keep two decimal places . Add to the expression , That is to say ： echo ”scale=2; expression “ | bc

bc Make logical judgment

2.5 Variable format

① Format ： Variable name = A variable's value

“=” Indicates assignment
Variable naming rule ： Start with a letter or underline , Case sensitive
product=benet
version=6.0
name="zhang san

② Look at the value of the variable
Format ：echo $ Variable name

name=lisi
wang=45+
echo $name
echo ${wang} ${name} View multiple variables
echo $wang1
echo ${wang}1 Braces define a variable as an integer

summary

shell The role of ： Mainly for automatic operation

1、shell The programming specification of
Be careful shell The order of , To use .sh ending , The parser needs to be injected into the script , The default parser is ：#!/bin/bash,shell There are many ways to implement , You can use absolute or relative paths to execute scripts , But this method needs to add execution permission , If you don't add execution permission , have access to bash、sh、source Order to execute

2、 Redirection and pipeline operation
Redirect

With standard input （stdin）、 standard output （stout）、 Error output （stderr）

" > "： Indicates standard redirection output , Can combine cat Enter some information manually , End of input , Will overwrite the previous content

“ >> ”： Indicates redirection output , Redirect the manually entered content to the file

“ < ” ： Indicates redirection input , combination cat When using , You can input the contents of the file into cat in , then cat Output content

“ << ” ： Indicates that it ends with a set character , Most of them use EOF

“1>&2” ： Indicates that the standard output will be , Mixed into the error output

“2>&1” ： Indicates that the error will be output , Mixed into standard output

Pipe symbol “ | ”

Indicates the execution result before the pipe symbol , Output to the pipeline symbol for execution
3、shell Variable
① Custom variable

have access to echo Add parameters to output . Sometimes variable output should be output together with ordinary content , have access to ${} Reference the variable name to distinguish .

The function of single quotation mark （ ‘ ’ ）： The contents of quotation marks are the contents to be output , Does not transform the inside of quotation marks

The function of double quotation marks （“ ”）： Indicates the contents of quotation marks for output , But the internal variables will be output ,echo The default is double quotation marks , Usual echo When there are spaces in the output , It can be marked with double quotation marks .

The function of single apostrophe (``）： Indicates the contents of the apostrophe , Must be an executable command , But this cannot be nested , So you can use $() Input nested output , Output from the inside out .

Define variables interactively ： Use read Variables can be obtained from the user's hand , Interact with users , add -p You can enter interactive prompt information ,-t, When the user enters the variable content , Don't show

** Action range of variable ： ** Variable action range , In the current shell Environment , If you use bash Turn on a dime shell, Then its parent process shell Variables of will not be available , You can use commands export Add local variables to global variables ,

② Special variables

environment variable ： The name of the variable automatically generated for the system , Different variable names represent different meanings , And change with the user state , among $ PATH Indicates the variable path that the command can generate , When we execute a command , He would search for $PATH Command below
A read-only variable ： Variables can only be read and cannot be modified or deleted , have access to readonly Add the variable name to generate
Positional variable ： Use $1 ,$2 ,$3… And so on represent the position variable ,$0 Represents the script name , The location variable needs to be input by the user when executing the script .
Predefined variables ： $#： Represents the number of location variables , ∗ ： surface in position Set up change The amount Of Inside Rong , discharge stay One rise transport Out , *： Represents the content of the location variable , Put them together and output ,∗： Represents the content of the location variable , Put them together and output ,@： Represents the content of the location variable , Show output separately ,$？： Indicates whether the last command was executed correctly ,$0： Represents the script name
4、 Variable operation
expr command ： It can not only perform operations , You can also display and output , It should be noted that , A space is required on both sides of the operator , Otherwise, an error will be reported .

Use expr command , Basically ,＋、－、*、/、%、 operation

Operation symbol operation ： $[] perhaps $ (( )) The contents of these two brackets can be calculated , But it can only perform operations , Cannot output results , So we need to combine echo For the output . And there is no need to use $ Symbols to declare variables , Variable values can be used directly .

let command ： let The command can change the value itself , It is equivalent to calculating the variable itself , Then assign it to itself ,

But no output , It also needs the help of echo For the output ,

bc command ： Support decimal operations , But it cannot be used directly in scripts , Otherwise, you enter the interactive interface , So in use , Need to add echo expression + Pipe, + bc . You can also use scale The command indicates how many decimal places to keep , direct scale=2, Means to keep two decimal places . Add to the expression , That is to say ： echo ”scale=2; expression “ | bc

原网站

版权声明
本文为[Ruthless]所创，转载请带上原文链接，感谢
https://yzsam.com/2022/209/202207281439576116.html