Explanation of special symbols in Shell

Common special symbols in shell are listed as follows:
#;.,/\ 'String' |! $ {}$? $ * "String "***? : ^ $ # $ @ 'COMMAND '{} [] [[] () | & {XX, YY, ZZ ,...}~
~ + ~ -& \ +-% =! =

Output/input redirection
>>>&> 2 &> 2> & 2

# Comments)
This is almost a full-market symbol, except for the "first line" that has been previously mentioned"
#! /Bin/bash
A well number often appears at the beginning of a line or after a complete command. This type of situation indicates that the annotation text is followed by the symbol and will not be executed.
# This line is comments.
Echo "A = $ A" # A = 0
Because of this feature, when you do not want to execute a certain line of command temporarily, you only need to add # At the beginning of the line. This is often used in the writing process.
# Echo "A = $ A" # A = 0
If it is used in an instruction, enclosed in double quotation marks, or behind the inverted slash, it becomes a general symbol and does not have the special functions described above.

~ Home Directory of the account
It is a common symbol, representing the user's home directory: Cd ~; You can also add the account name "CD ~" directly after the symbol ~ User
Or use it as a part of the path :~ /Bin ;~ + The current working directory. This symbol represents the current working directory. It serves the same purpose as the built-in command PWD.
# Echo ~ +/Var/log ~ -Last working directory. This symbol represents the last working directory.
# Echo ~ -/Etc/httpd/logs

; Semicolon (command separator)
In shell, the symbol used for the "continuous command" function is "Semicolon ". For example, Cd ~ /Backup; mkdir startup; CP ~ /. * Startup /.

; Consecutive semicolon (Terminator)
Dedicated to the case option, as the role of Terminator.
Case "$ fop" in

Help)

Echo "Usage: Command-help-version FILENAME"

;;

Version)

Echo & quot; version 0.1 & quot"

;;

Esac

. Dot)
In shell, the user should be clear that a dot represents the current directory, and two dots represent the upper directory.
Cdpath = .:~ :/Home:/home/Web:/var:/usr/local
In the upstream cdpath setting, dot after the equal sign represents the meaning of the current directory.
If the file name starts with "dot", the file is a special file, which is displayed only when the-A option is added to the LS command. In addition, in the regular expression, a dot represents matching a character.

'String' single quotes)
The content enclosed by single quotes is considered as a single string. The $ symbol representing the variable in quotation marks does not work. That is to say, it is treated as a general symbol to prevent replacement of any variable.
Heyyou = homeecho '$ heyyou' # We get $ heyyou

"String" double quotes)
The content enclosed by double quotation marks is considered as a single string. It prevents wildcard extension, but allows variable extension. This is different from how a single reference is processed.
Heyyou = homeecho "$ heyyou" # We get home

Backticks)
The preceding single double quotes enclose strings. What if the string is a command column? The answer is no execution. To handle this situation, we must use single quotes.
Fdv = 'date + % F' echo "today $ fdv"
The date + % F in the inverted quotation marks is considered as an instruction, and the execution result is included in the fdv variable.

, Comma (comma)
This symbol is often used in operations as a "segmentation" purpose. Example
#! /Bin/bash

Let "T1 = (A = 5 + 3, B = 7-1, C = 15/3 ))"

Echo "T1 = $ T1, A = $ A, B = $ B"

/Forward (forward slash)
Indicates a directory in the path.
CD/etc/rc. DCD.../. CD/
A single/usually represents the root directory. In the four arithmetic operations, it represents the division symbol.
Let "num1 = (A = 10/2, B = 25/5 ))"

\ Backslash ()
The Escape Character in interactive mode has the following functions:

Before the instruction, it can cancel the aliases;

Before a special symbol, the function of the special symbol disappears;

Put it at the end of the command, indicating that the command is connected to the next line.
# Type rmrm is aliased to 'rm-I '# \ RM./*. Log
In the above example, I added the escape character before the RM command to temporarily cancel the alias function and restore the RM command.
# Bkdir =/home # echo "backup Dir, \ $ bkdir = $ bkdir" backup Dir, $ bkdir =/home
In the previous echo example, \ $ bkdir and escape cancel the $ variable function. Therefore, $ bkdir is output, while the second $ bkdir outputs the variable content/home.

| Pipeline)
Pipeline is a basic and important concept of UNIX systems. Connect the standard output of the previous command as the standard input of the next command.
Who | WC-l
Making good use of this concept can help streamline scripts.

! Exclamation point (negate or reverse)
It usually indicates the anti-logic function, for example, in condition detection! = To indicate "not equal"
If ["$? "! = 0] thenecho "executes error" Exit 1fi
In the rule expression, she assumes the "anti-logic" role.
Ls [! 0-9]
In the preceding example, other files except A0, A1... A9 are displayed.

: Colon
In bash, This is a built-in command: "do nothing", but the returned status value is 0.
:
Echo $? # The response is 0.
:> F. $
The above line is equivalent to CAT/dev/null> F. $. Not only is the statement short, but the execution efficiency is much higher.
Sometimes, the following usage occurs:
:$ {Hostname ?} $ {User ?} $ {Mail ?}
This line is used to check whether these environment variables have been set. If they are not set, an error message is displayed as a standard error. Similar to test or if, this type of check can basically be processed, but it is not as concise and efficient as in the previous example.
In addition to the above, there is also a place where the colon must be used
Path = $ path: $ home/fbin: $ home/fperl:/usr/local/Mozilla
In the. bash_profile or any file with similar functions under the user's home directory, we use colons to set the "path" for segmentation.

? Question mark (wild card)
The role played on filename expansion matches any character but does not contain null characters.
# Ls? A1
Make good use of her features to make more accurate file name matching.

* Asterisk (wild card)
Symbol. In filename expansion, it represents any character and contains null characters.
# Ls a * A A1 access_log
During operation, it represents "multiplication ".
Let "fmult = 2*3"
In addition to the built-in command let, there is also an operation instruction expr, where the asterisk is also used as a "multiplication" role. However, be careful when using the escape character.

** Power Operation
The meaning of the two asterisks in the computing age.
Let "Sus = 2 ** 3" Echo "Sus = $ sus" # sus = 8

$ Dollar sign)
Variable Substitution.
VRS = 1, 123

Echo "VRS = $ VRS" # VRS = 123
In addition, it is defined as "end-of-line" in regular expressions ). This is often used in grep, sed, awk, and VIM (VI.
Regular Expression of $ {} variable
Bash defines a lot of usage for $. The following table is from the online description
$ {Parameter:-word }$ {parameter: = word }$ {parameter :? Word }$ {parameter: + word }$ {parameter: Offset }$ {parameter: offset: length }$ {! Prefix * }$ {# parameter }$ {parameter # Word }$ {parameter % word}
$ {Parameter/pattern/string }$ {parameter // pattern/string}
$ * Refers to the execution reference variable of the script. The algorithm of the referenced parameter is the same as that of the General Command. The command itself is 0, and the latter is 1, and so on. The reference variable is represented as follows:
$0, $1, $2, $3, $4, $5, $6, $7, $8, $9, ${10 }, ${11 }.....
For single-digit users, you can directly use a number. If the number is greater than two digits, you must use the {} symbol to enclose it.
$ * Indicates all symbols that reference variables. Double quotation marks must be added as needed.
Echo "$ *"
There is also a symbol with the same effect as $ *, but the utility and processing method are slightly different.
$ @
$ @ Has the same effect as $ *, but there is a difference between them.
Symbol $ * considers all referenced variables as a whole. But the symbol $ @ still retains the concept of block for each referenced variable.
$ #
This is also a symbol related to the referenced variable. Her role is to tell you the total number of referenced variables.
Echo "$ #"
$? Status variable)
Generally, Unix (Linux) processes end with the execution system calling exit. The return value is the status value. It is returned to the parent process to check the execution status of the child process.
If the command program is successfully executed, the return value is 0; If the command program fails, the return value is 1.
Tar cvfz dfbackup.tar.gz/home/user>/dev/nullecho "$? "$
Because the process ID is unique, it is impossible to have a repetitive PID at the same time. Sometimes, scripts need to generate temporary files to store necessary information. This script may also be used by users at the same time. In this case, the fixed file name is obviously unreliable in writing. Only dynamic file names can be generated. Symbol $ may meet this requirement. It represents the PID of the current shell.
Echo "$ hostname, $ user, $ mail"> ftmp. $
Using it as a part of a file name can avoid overwriting of the same file name at the same time.
PS: basically, the system recycles the executed PID and then allocates it as needed. Therefore, even if the temporary file is written using the dynamic file name, other problems may occur if the script is not cleared after execution.

() Command Group)
Enclose a string of continuous commands in parentheses. This is called a command group for shell. Example: (Cd ~ ; Vcgh = 'pwd'; echo $ vcgh), the Command Group has a feature, shell will generate subshell
To execute this set of commands. Therefore, the variables defined in the variable only act on the Command Group itself. Let's look at an example.
# Cat ftmp-01

#! /Bin/bash

A = HCG (A = incg; echo-e "\ N $ A \ n ")

Echo $

#./Ftmp-01

Incgfsh
In addition to the preceding Command Group, brackets are also used in the definition of array variables. They are also used in other scenarios where you may need to add escape characters, such as arithmetic expressions.
(())
Similar to let commands, these symbols are used in arithmetic operations and are built-in functions of bash. Therefore, the execution efficiency is much better than using the let Command.
#! /Bin/bash (A = 10) echo-e "in1_value, A = $ A \ n" (A ++) echo "after a ++, A = $"

{} Braces (Block of code)
Sometimes it appears in the script. the braces hold one or more segments ending with a semicolon to set the command or variable.
# Cat ftmp-02

#! /Bin/bash

A = hCG {A = inbc; echo-e "\ N $ A \ n "}

Echo $ A #./ftmp-02inbcinbc
This usage is very similar to the Command Group described above, but there is a difference that it is executed in the current shell and does not produce subshell.
Braces are also applied to the function. Broadly speaking, simply using braces only serves as a function without a specified name. Therefore, writing scripts in this way is quite good. This method can simplify the complexity of the script, especially for the output input.
In addition, braces have another usage:
{XX, YY, ZZ ,...}
This combination of braces is often used in the combination of strings. Let's look at an example.
Mkdir {usera, userb, userc}-{home, bin, data}
We can obtain usera-home, usera-bin, usera-data, userb-home, userb-bin, userb-data, userc-home, userc-bin, userc-data, these directories. This set of symbols is widely applied. If you can make good use of it, the return is simplified and efficient. For example
Chown root/usr/{UCB/{ex, edit}, lib/{ex ?.? *, How_ex }}
If this is not supported, we have to repeat several rows several times!

[] Brackets
It often appears in the process control and plays a role in the implicit model. If ["$? "! = 0] thenecho "executes error" Exit 1fi
This symbol acts as a role in a regular expression similar to "range" or "set ".
Rm-R 200 [1234]
In the preceding example, it indicates deleting directories such as 2001,200 2 and 2003,200 4.
[[]
This group of symbols basically works the same as the previous [] symbols, but she allows them to directly use | and & logical symbols.
#! /Bin/bashread Akif [[$ Ak> 5 | $ AK

| Logical symbol
This will often be seen, representing the or logic symbol.

& Logical symbols
This also often shows the symbols representing the and logic.

& Background work
A single & symbol is placed at the end of the complete command column, which means that the command column is put into the background for work.
Tar cvfz data.tar.gz DATA>/dev/null &

\ Single word boundary
These symbols are defined as "boundary" in Rule expressions. For example, if we want to find the word
Grep the filea
You will find that words like there will also be considered as matching words. Because the coincidence is a part of there. If we want to avoid this situation, we need to add the "boundary" symbol.
Grep '\ 'filea

+ Plus sign (plus)
In the formula, it is used to represent "addition ".
Expr 1 + 2 + 3
In addition, it is used to represent the meaning of the first character of "many" in the Rule expression.
# Grep '10 \ + 9' fileb000010094259425931010009 # This symbol must be prefixed with the escape character.

-Dash)
In the formula, it is used to represent "subtraction ".
Expr 10-2
It is also the option symbol of the system command.
Ls-expr 10-2
In the GNU command, if the-symbol is used separately without the name of any file to be added, it indicates the meaning of "standard input. This is a common option for GNU commands. For example
Tar xpvf-
Here, the-symbol indicates reading data from standard input.
However, CD commands are quite special.
CD-
This means to change the working directory to the "last" working directory.

% Division (Modulo)
In the formula, it is used to represent "division ".
Expr 10% 2
In addition, it is also used in the following rule expressions about variables:
$ {Parameter % word }$ {parameter % word}
One % indicates the shortest word match, and two indicates the longest word match.

 
= Equal sign (equals)
Symbol that is often seen when a variable is set.
Vara = 123 echo "Vara = $ Vara"
Or, such as path settings, or even applications for such purposes as operations or neural expressions.

= Equal sign (equals)
It is often seen in the condition criterion, indicating "equal.
If [$ Vara = $ varb]
...

! = Not equal
It is often seen in the condition criterion, which indicates the meaning of "not equal.
If [$ Vara! = $ Varb]
...
^
In a rule expression, this symbol represents the "beginning" of a row.