Use of special symbols for Linux

Linux Special symbols

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$ number

Usually represents the meaning of a variable

echo $ variable name = variable Value

= equals sign

The general definition of variables is used when

For example [[email protected] aaa]# ABC=/ETC/PASSWD

[Email protected] aaa]# echo $ABC

/etc/passwd

^

This symbol represents the "start" position of a row in a regular expression

For example [[email protected] aaa]# grep "^a" 111

ACBac0AAA

ABCWWDwww

# Number of Wells

1. Often the beginning of a line of shell script, or as a comment information, such a case that the symbol is followed by the text, will not be executed.

For example: # This is comments.

2, or to comment out the command, do not let this command execute

For example: #echo "www.baidu.com" >/root/abc

; semicolon

Command 1; Command 2: Indicates whether the preceding command executes successfully, executes the following command, which is equivalent to carriage return, and can execute multiple commands

For example: [[email protected]/]# cd/root/; ls-l

Total 84

-rw-r--r--1 root root 4 09-29 14:55 1.txt

;; Double semicolon

Dedicated in case options, assume the role of Terminator. Don't do too much explaining

&

Indicates job control, the foreground command is transferred to the background

&& symbols, logic and

Command 1 && Command 2: Command 1 on the left executes successfully and command 2 on the right is executed. If command 1 on the left does not execute successfully, command 2 on the right does not execute

For example:

[Email protected] ~]# echo "abc" >>/root/1.txt && ls-l/root/1.txt

-rw-r--r--1 root root 4 09-29 14:55/root/1.txt

If there is no 1.txt file in the directory, the command is automatically created and then added ABC to 1.txt.

| Pipeline

Command 1 | Command 2: Output of the preceding command, as input to the following command

For example: [[email protected]/]# cat/etc/passwd |wc-l

38

|| Dual pipe, logic or

Command 1 | | Command 2: Command 1 on the left executes successfully, and command 2 on the right does not execute. If command 1 on the left does not succeed, command 2 on the right does not execute

For example:

[[email protected] ~]# Ls-l Www.txt | | Touch Www.txt

Ls:www.txt: No file or directory

For example: [[email protected] ~]# Ls-l Www.txt | | echo "Yes"

-rw-r--r--1 root root 0 09-29 15:03 www.txt

' Single quotation mark

The contents of single quotes are treated as ordinary symbols, preventing any variable substitution

Example:$www =www.baidu.com

[Email protected] ~]# echo "$www"

Www.baidu.com

[Email protected] ~]# echo ' $www '

$www

"" Double quotation marks

The content enclosed in double quotes is treated as a single string, preventing wildcard expansion, but allowing variable expansion.

Example:$www =www.baidu.com

[Email protected] ~]# echo "$www"

Www.baidu.com

For example: echo "\"/root/www/"\"

"/root/www/"

"\\": Backslash converts special characters

"\"//"\" backslash

Convert Special Symbols

For example: echo "\"/root/www/"\"

"/root/www/"

"Anti-apostrophe, inverted quote = = $ ()

' Command ': The counter-apostrophe is the command to execute

For example 1, [[email protected] ~]# fdv= ' date +%f '; echo "Today $FDV"

Today 2014-09-29

Example 2, [email protected]/]# fdv=$ (date +%f); echo "Today $FDV"

Today 2014-09-29

* No.

Match any symbol of any length

Example:ls/root/*.txt

1.txt Aaa.txt Www.txt

* * No.

Represents a sub-square operation

For example: [[email protected] ~]# let b=4**2; Echo $b

16

？ Question mark

Match individual length of a single character

For example: [[email protected] ~]# ls/root/?. Txt

/root/1.txt

Comma

This symbol is often used in operations as an "isolated" use.

。 Period

Indicates a hidden file or current directory, no big use, no more explanation

: Colon

Usually in the script to do the interval effect

！ Exclamation mark

The meaning of "non" in the sense of logical operation, mainly used in awk

_ Short horizontal bar, commonly known as minus

is usually the CD command inside the back to the last working directory meaning

Or as an arithmetic symbol

~ Wave number

Generally used for CD commands

[] Brackets

[[email protected] aaa]# ls

A d f FH h R

[ABCD] There must be a character or a or B or ... is one of the characters in parentheses that matches the display

[Email protected] aaa]# ls-l [ABCD]

-rw-r--r--1 root root 0 10-02 19:49 a

-rw-r--r--1 root root 0 10-02 19:49 D

Represents a continuous combination of

[email protected] aaa]# ll *[0-9]*

-rw-r--r--1 root root 0 10-02 19:18 1

-rw-r--r--1 root root 5 10-02 19:18 111

-rw-r--r--1 root root 0 10-02 19:18 2

-rw-r--r--1 root root 0 10-02 19:28 w323ewq

[-]

Represents a continuous match from-before-to-after, if no continuation matches are not displayed next until consecutive characters end, and then the matching values are displayed

For example

[[email protected] aaa]# ll [1-3] represents all numbers between 1-3

-rw-r--r--1 root root 0 10-02 19:18 1

-rw-r--r--1 root root 0 10-02 19:19 3

[^]

To denote the meaning of inversion

[[email protected] aaa]# ll [^1-5] match all numbers except 1 to 5

-rw-r--r--1 root root 0 10-02 19:19 6

-rw-r--r--1 root root 0 10-02 19:19 7

-rw-r--r--1 root root 0 10-02 19:19 8

-rw-r--r--1 root root 0 10-02 19:19 9

() parentheses

Typically represents the beginning of a neutron shell in a shell statement

[[email protected] aaa]# awk ' {if ($0~/abc/) print $ ' 111

Abc

{} curly Braces

Middle is a combination of command blocks

Example 1,

Cat 111 |awk ' {print $} '

Example 2,

Represents a collection

[[email protected] aaa]# Touch {1..9}{a,b,c}

[[email protected] aaa]# ls

1a 1b 1c 2a 2b 2c 3a 3b 3c 4a 4b 4c 5a 5b 5c 6a 6b 6c 7a 7b 7c 8a 8b 8c 9a 9b 9c

/Slash: 1, meaning of the path to the directory + addition operator-subtraction operator \* multiplication operator /division operator % fetch operator

Example: Echo "/etc/passwd/"

/etc/passwd

For example: [[email protected] ~]# x=100

[Email protected] ~]# a=30

[Email protected] ~]# expr $x + $a

130

[Email protected] ~]# expr $x-$a

70

[Email protected] ~]# expr $x \* $a

3000

[Email protected] ~]# expr $x/$a

3

[Email protected] ~]# expr $x% $a

10

File descriptor name common abbreviation default value

0 Standard Input stdin keyboard

1 Standard output stdout screen

2 standard error Output stderr screen

When we simply use < or >, the equivalent of using 0< or 1>

1> indicates the correct output

0> indicates the correct output

> Add the output to a text file and overwrite the original

>> Appends the output content results to a text file without overwriting the original

>test.txt If you have test.txt this file, clear all the contents of the file

If you do not have a test.txt file, create a Test.txt file

< Data flow redirection: Save content in a file beforehand, specify the file path when needed, and the system can automatically detect the read

<< Input Guidance

&> or 2>&1 add the correct output and error information to the same text file and overwrite the original

&>> Add the correct information of the output to the same text file as the error message, and append it in

2> adds the output error message to the text file and overwrites the original

2>> Appends the output error message to a text file, and does not overwrite the original

echo "www" >1.txt 2>2.txt means that the correct information is entered into a 1.txt file, and the wrong information is entered into a 2.txt file

Saving the wrong information can be 2>/dev/null

Cat > www

Www.baidu.com

Www.taobao.com

Press Ctrl+d to exit

Then the information you entered will be stored in the WWW file.

Cat > www << "EOF"

Www.baidu.com

Www.taobao.com

EOF does not need to press Ctrl+d to exit

It's over now.

$# indicates the number of positional parameters in the command line

Cases:

[Email protected] ~]# vim test.sh

#!/bin/bash

#

echo "The current position variable is: $ $"

echo "The current position variable is: $"

echo "The current position variable is: $ $"

echo "The current position variable is: $4"

Echo $#

[Email protected] ~]#/test.sh a b c D

The current position variable is: a

The current position variable is: b

The current position variable is: C

The current position variable is: D

4 Statistics out the use of several positional variable parameters

$* represents the contents of all positional parameters

Cases:

[Email protected] ~]# vim test.sh

#!/bin/bash

#

echo "The current position variable is: $ $"

echo "The current position variable is: $"

echo "The current position variable is: $ $"

echo "The current position variable is: $4"

Echo $*

[Email protected] ~]#/test.sh a b c D

The current position variable is: a

The current position variable is: b

The current position variable is: C

The current position variable is: D

A b c D shows the contents of the position variable (value)

$? represents the return status of the previous command after execution, with a return value of 0 indicating that the execution is correct, and that a value other than 0 is an execution error

Example 1,

[[email protected]/]# ls www

Ls:www: No file or directory

[[email protected]/]# echo $?

2

For example: [[email protected] ~]# CAT/ETC/PASSWD

[[email protected] ~]# echo $?

0 Returns a value of 0, indicating that the command executed earlier CAT/ETC/PASSWD is correct

$ A indicates the name of the currently executing script or program

Cases:

[Email protected] ~]# vim test.sh

#!/bin/bash

#

echo "The current position variable is: $ $"

echo "The current position variable is: $"

echo "The current position variable is: $ $"

echo "The current position variable is: $4"

Echo

[Email protected] ~]#/test.sh a b c D

The current position variable is: a

The current position variable is: b

The current position variable is: C

The current position variable is: D

./test.sh Show executed scripts or programs

This article symbol is sufficient in simple or normal scripts, and there are special symbols that involve certain commands, which I'll add to the specific commands.

All right! Now probably know so many symbols, perhaps some special symbols I have not learned, if there are shortcomings in the hope that you have a lot of readers forgive, learn from each other.

Thank you for your support!

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Use of special symbols for Linux