Do you know the shell foundation often asked in interviews?

One 、shell summary

1.Shell What is it?

shell It's a command line interpreter , He provides users with a way to linux The kernel sends requests to run the program's interface system level program , Users can use shell To start up 、 Hang up 、 Stop even writing programs

shell It is also a powerful programming language , Easy to write , Easy to debug , More flexibility .shell It's the script language that explains the execution , stay shell You can call linux System commands

2.Shell classification

1）Bourne Shell： from 1979 rise Unix Start using Bourne Shell,Bourne Shell The main file name is sh

2）C Shell：C Shell Mainly in the BSD Version of Unix The system uses , Its grammar and C The language is very similar , So called C Shell

Shell There are two main grammatical types of Bourne and C, The two grammars are not compatible with each other .Bourne The family mainly includes sh、ksh、Bash（linux The standard shell）、psh、zsh;C The family mainly includes ：csh（unix The main use of shell）、tcsh

TIP： You can use commands ：echo ￥SHELL To output what the current system uses shell

3）Bash：Bash And sh compatible , Currently in use linux Is the use of Bash As a basic user Shell

3.Linux Supported by Shell

Two 、 Script execution

1.echo The output command

echo [ Options ] [ Output content ]

Options ：

-e: Support character conversion of backslash control

[[email protected] ~]# echo -e “\e[1;31m Hello \e[0m”

# Use red font to output Hello

#30m= black ,31m= Red ,32m= green ,33m= yellow ,34m= Blue ,35m= pink ,36m= Cyan ,37m= white

In the above line of example code \e Representative color ,[1 On behalf of winking ,31m Means I want to use red ,\e[0m Represents off color

2. The first script

1） Use command “vi hello.sh” Create a script

2） Enter... In the text ：”#!bin/bash”（ Make a statement ）

3） Text input ：echo –e “Hello World!”, Then save to exit

3. Script execution

1） Grant execution permission , Direct operation

chmod 755 hello.sh

./hello.ssh

2） adopt Bash Call the execution script

bash hello.sh

3、 ... and 、bash Basic functions of

1. Command aliases and shortcuts

alias

# View all command aliases in the system

alias Alias =’ The original order ’

# Set command alias （ It should be noted that adding aliases only with the command line is only temporary , amount to windows Temporary environment variables in ）

1） The alias is permanent

vi ~/.bashrc

# Write environment variable configuration file （ It should be noted that it will take effect only after the device is restarted after writing , If you need to take effect directly, you need to add another order source .bashrc It's equivalent to updating, so it can take effect directly ）

2） To delete an alias

unalias Alias

# To delete an alias （ Just delete the temporary alias ）

3） Order of order

The first sequence executes a command executed with an absolute path or a relative path

Second order execution alias

Third order execution bash The internal order of

The fourth order is executed according to ￥PATH The first command found in the directory lookup order defined by the environment variable

4） Common shortcut key

2. History commands

1） History commands

history [ Options ] [ The history command saves the file ]

Options ：

-c： Clear history command

-w： Writes the history command from the cache to the history command to save the file ”~/.bash_history”

history See all the historical orders ,.bash_history The history command in is written automatically when the system exits .

It should be noted that historical commands will be saved by default 1000 strip , Can be configured in the environment variables file /etc/profile Modify in process

2） Call of history command

Use the up and down arrows to call up the previous historical commands

Use ”!n” Repeat step n Historical orders

Use ”！！” Repeat the last command

Use ”！ character string ” Repeat the last command that starts with the string

3） Command and file completion

stay bash in , Command and file completion is a very convenient and commonly used function , We just enter a command or file , Press down Tab Key will automatically complete

3. Output redirection

1) Standard input and output

Standard output redirection saves the results of the command in a file （ As long as the result of the command has output, it will be saved ）

Error output redirection （ Note that there is no space on either side of the symbol ）

ls &>/dev/null Execute commands only , Do not save or display the running results ,/dev/null It can be used as a garbage collector

command >> file 2>&1

Save the correct output and the wrong output in the same file by appending 1

command &>> file

Save the correct output and the wrong output in the same file by appending 2

command >> file 1 2 >> file 2

Append the correct output to the file 1 in , Append the wrong to the file 2 in

3） Input redirection

wc [ Options ] [ file name ]

ctrl+d sign out

Use wc Count file lines 、 Number of words 、 And the number of bytes , You can also use the one in the middle < Symbols can also be counted

4. Pipe,

1） Multiple commands are executed sequentially

Multiple commands are executed in sequence

Logic and

Logic or

TIP： It can be done by analogy ：ls && echo yes || echo no This way to judge whether the command is wrong

2） Pipe,

command 1 | command 2

# command 1 The correct output for the command 2 Operation object of

for example ：

ls –l /etc | more To view the command execution results in separate screens

netstat –an | grep ESTABLISHED To monitor my server usage

netstat –an | grep ESTABLISHED | wc –l To see how many people are using our server

5. wildcard

1） wildcard

for example ：

ls test* List test All the documents at the beginning

Mainly match the file name or directory

2）Bash Other special symbols in

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