Brief bash expansion function

Brief Introduction of bash functions-general Linux technology-Linux programming and kernel information. The following is a detailed description. Bash provides seven types of expansion functions, namely (according to the expanded execution order): parentheses expand, wave number expand, variable expand, arithmetic expand, command replace, Word Segmentation and path name expand.

Parentheses
Brackets can be expanded in two forms:
A {B, c, d} e --> abe ace ade
A {B .. d} e --> abe ace ade
Brackets can also be nested:
1 {aa, B {c, d} 2 --> 1aa2 1bc2 1bd2

Wave expand
After the wave number is expanded, it is equivalent to specifying the user's main directory path (assuming that the current user is a user)
~ -->/Home/user
~ Root -->/root

Variable Expansion
Assume there is variable x:
X = "Hello World"
So
$ X --> Hello World
$ {X} --> Hello World
$ Xasdf --> Replace with the value of the variable named xasdf. leave it blank if it is not defined.
$ {X} asdf --> Hello Worldasdf

Arithmetic Expansion
The general form of arithmetic expansion:
$ (Expression ))
You can perform mathematical operations in arithmetic expansion:
$(100 + 9) --> 109

Command replacement
The general form of command replacement:
$ (Cmd)
'Cmd'
For example:
$ (Echo asdf) --> asdf

Word Segmentation
If Variable Expansion, arithmetic expansion, and command replacement are not enclosed in double quotation marks, bash splits the variable:
X = "Hello World", expand and get two words: "Hello", "World"
$ X --> Hello World

Expand path name
Used to match existing file names in the file system
* Match any string, including an empty string
? Match any character
[...] Match any character in []
[!...]
[^...] Does not match [!], Any character in [^ ...]

Process replacement
Process replacement requires the system to support fifo files
The general format is:
> (List) is replaced by the standard input file name of the list. Writing data to the file allows the list to be read from the standard input.
<(List) is replaced by the standard output file name of the list. You can read this file to get the standard output of the list.