A concise summary of the role of 28 special characters in a shell script _linux shell

1. #

annotation function, #! Except
In addition, in the parameter replacement echo ${path#*:} here does not represent a comment, a numbering conversion, does not represent a comment echo $ ((2#101011))

2.;

Command line separator, you can write multiple commands on one line. echo Hello; echo there

3.;;

Terminate case option

Copy Code code as follows:

Case "$variable" in
ABC) echo "\ $variable = abc";;
XYZ) echo "\ $variable = xyz";;
Esac

4..
Hide file Prefix
. command is equivalent to source
. Represents the current directory ... Indicates a previous level of directory
Match as a single character in a regular expression

5. "", "

Double quotes, single quotes, where you can reference variables in double quotes, but not in single quotes, which function as organization special characters

6. \

Escape character

7./

File name Separator, Division action

8. ' (ESC button below)

Post reference, command substitution

9.:

Null command, equivalent to "NOP"
can also be considered to be the same as the Shell built-in command true, for example: Dead loop

Copy Code code as follows:

While:
Todo
echo "1"
Done

Equivalent to:

Copy Code code as follows:

While True
Todo
echo "1"
Done

To make a placeholder in a if/then statement:

Copy Code code as follows:

If condition

Then: # do nothing, draw branches

Else

Do other thing

Fi

Provides a placeholder in a 2-dollar command.

When used with the > (redirection operator), empty the file without modifying the file's permissions, such as: (if the file does not exist, the file will be created)

: > Data.log equivalent to

Cat/dev/null > Data.log

10.!

The inverse operator!= is not equal to

11. *

Universal matching character, in regular expressions
Math multiplication
* * Power operation

12.?

Test actions
Regular expression,? Match any single character

13. $

Variable symbol
Regular expression row end character
${} parameter substitution
$*,$@ Position Parameters
$? Exit status
$$ Process ID

14. ()

Command group, (A=hello Echo $a), and the list of commands in () will run as a child shell. The variables in (), because they are in the child shell, are not available for the remainder of the script.
Array initialization: array= (element1, Element2, Element3)

{XXX,YYY,ZZZ}

Curly braces Extension
Cat {File1,file2,file3} > Combined_file, merge file1,file2,file3 together and redirect to Commbined_file. Cannot have spaces in curly braces

16. {}

code block. In fact, this structure creates an anonymous function. But unlike a function, the variable declared in it is visible to the code for the rest of the script.

Copy Code code as follows:

# The content in the code block, external access, I/O redirection

1: #!/bin/bash
2:
3:file=/etc/fstab
4:
5: # in this block of code variables, external can also access
6: {
7:read line1
8:read line2
9:} < $File
10:
11:echo "The" the "a" in $File is "
12:echo "$line 1"
13:echo
14:echo "Second line in $File is"
15:echo "$line 2"
16:
17:exit 0
# Save the results of a code block to a file

17. {}\;

Path names, are generally used in the Find command, attention; –exec to end the find command sequence

18. []

Test

Array elements, such as ARRAY[1]=ABC
The range of characters used in regular expressions

19. [[]]

The test expression itself is placed in []

20. (())

Mathematical calculation extension

>& >>& >> <

redirect
ScriptName > FileName redirect script output to file, overwrite original content
Command &> filename redirect stdout and stderr to file
Command >&2 redirect stdout and stderr
ScriptName >> filename Redirection scripts are exported to the file, added to the end of the file, and if there are no files, create the file.

<< <<<

Redirect, << used in "Here Document", <<< used in "here string"

\<, \>

Word boundaries in regular expressions grep ' \<the\> ' testfile

24. |

Pipe, analyze the output of the front command, and use the output as input to the following command

>|

Force redirection

26. | |

Logical OR

&

Run the command in the background, a command followed by an ampersand, will indicate running in the background

Copy Code code as follows:

1: #!/bin/bash
2:
3:for I in 1 2 3 4 5 6 7 8 9 10
4:do
5:echo-n "$i"
6:done&

Note that the last done& of the For loop

&&

Logic and