Character unfold detailed

Special character Encyclopedia in Bash
By expanding, the character you enter will expand into another character before the shell works on it. Such as:

[Sl@sl ~]$ Echo *
Desktop Pic Txt.save

The shell expands the name of the file under the current working directory before the Echo command is executed.

Path name Expansion

The working mechanism that wildcards rely on is called the path name expansion.

echo * Current directory all non-hidden files
Echo. *                            current directory All files with.
ls-d. * | less ls-d.
[!.]? *

Wavy Line Expansion

Wavy line characters ("~") have special meanings. When it is used at the beginning of a word, it expands to the home directory name of the specified user and, if no user name is specified, expands to the home directory of the current user.

[Sl@sl ~]$ echo ~
/HOME/SL

an arithmetic expression expands

Format: $ ((expression))

Echo $ ((2 + 2))
4
echo $ ((5**2) * 3))
75

Curly Braces Expand

The curly brace expression itself contains a list of strings separated by commas, or a series of integers, or a single string. This mode cannot embed whitespace characters.

echo front-{a,b,c}-back
front-a-back front-b-back front-c-back

Echo number_{1..5}
number_1  number_ 2  number_3  number_4  number_5

echo {Z. A}
Z Y X W V U T S R Q P O N M L K J I H G F E D C B A

echo a{a{1,2},b{3,4}}b
aa1b aa2b ab3b ab4b

mkdir {2007..2009}-0{1..9} {2007..2009}-{10..12}

parameter Expansion

$ variable invocation symbol
Usage: $ variable
Mechanism: Call variable to get ' value of variable '

Echo $USER

command Substitution

Linux command substitution and redirection are somewhat similar, but the difference is that the command substitution is the output of one command as a parameter of another Linux command
echo $ (LS)
Desktop Documents ls-output.txt Music Pictures public Templates Videos can
also use inverted quotes instead of $ ()
ls-l ' which CP '
-rwxr-xr-x 1 root 71516 2007-12-05 08:5 8/bin/cp

Word Segmentation

By default, the word segmentation mechanism looks for spaces, tabs, and line breaks in a word, and sees them as the defining character between words. This means that no referenced spaces, tabs, and newline characters are part of the text and are used only as delimiters.

The echo this are a test this is
 a test

Reference

The shell provides a mechanism called a reference to selectively prohibit unwanted expansion.

"" Double quote

It contains the content as a normal character, but $,\ (backslash), and ' (inverted quotes).
This means that Word segmentation, path name expansion, wavy line expansion, and braces expansion will fail, but the parameter expansion, arithmetic expansion, and command substitution are still valid.

"Single quote

Take what it contains as a normal character, without exception

echo text ~/*.txt {a,b} $ (echo foo) $ ((2+2)) $USER
text/home/me/ls-output.txt a b foo 4 SL 

The wavy line expands in double quotes, the path name expands, the curly braces are expanded, the parameters expand, the arithmetic expands, and the command substitution is still valid.

Echo ' text ~/*.txt {a,b} $ (echo foo) $ ((2+2)) $USER '
text ~/*.txt   {a,b} foo 4 me   

Fail all in single quotation marks

Echo ' text ~/*.txt {a,b} $ (echo foo) $ ((2+2)) $USER '
text ~/*.txt  {a,b} $ (echo foo) $ ((2+2)) $USER    

Escape Character

character preceded by a backslash \