$ {} Usage

We define a variable:
File =/dir1/dir2/dir3/my.file.txt
We can use $ {} to calculate different values for the token:
$ {File # */}: remove the first/and its left outer strings: dir1/dir2/dir3/my.file.txt
$ {File ### */}: remove the last/and its left string: my.file.txt
$ {File # *.}: remove the first. And its left-side string: file.txt
$ {File ### *.}: remove the last. And its left string: txt
$ {File %/*}: remove the last line/and its right string:/dir1/dir2/dir3
$ {File %/*}: remove the first/and its right-hand string: (null)
$ {File %. *}: remove the last. And its right-hand string:/dir1/dir2/dir3/My. File
$ {File %. *}: remove the first. And its right-hand string:/dir1/dir2/dir3/My
Remember the method as follows:
# Remove the left sequence (on the sequence # On the left sequence of $)
% Is to remove the right cursor (% on the right margin of $)
Single-character matching is the minimum matching. The two single-character matching is the maximum matching.

$ {File: 0: 5}: extract the leftmost five words? :/Dir1
$ {File: 5: 5}: extract 5th words? Five characters in the upper right corner? :/Dir2

We can also replace the strings in the variable value:
$ {File/DIR/path}: extract the first dir to path:/path1/dir2/dir3/my.file.txt.
$ {File // DIR/path}: Extract all dir files as path:/path1/path2/path3/my.file.txt

You can use $ {} to modify variable values for different variables (unspecified, null, or non-null ):
Optional values file-my.file.txt}: If $ file is null, use my.file.txt as the default value. (Reserved unspecified and non-null values)
$ {File: -my.file.txt}: If $ file is not set or is null, use my.file.txt as the default value. (Keep non-null values)
When using file1_my.file.txt}: regardless of the $ file value, my.file.txt is used as the default value. (No value is retained)
$ {File: Invalid my.file.txt}: Unless $ file is null, my.file.txt is used as the default value. (Retain null values)
Optional values file1_my.file.txt}: If $ file is not set, use my.file.txt as the default value. At the same time, set $ file to a non-null value. (Retain null and non-null values)
$ {File: Your my.file.txt}: If $ file is not set or is null, use my.file.txt as the default value. At the same time, set $ file to a non-null value. (Keep non-null values)
$ {File? My.file.txt}: If $ file is not set, export my.file.txt to stderr. (Retain null and non-null values ))
$ {File :? My.file.txt}: If $ file is not set or is null, my.file.txt is exported to stderr. (Keep non-null values)

You can also calculate the length of the variable value by using $ {# var:
$ {# File} can get 27, because/dir1/dir2/dir3/my.file.txt contains 27 Characters ?...

Next? Zookeeper refers to the Array Processing Method of bash.
Generally, a = "a B C Def" only converts $ A into a single string,
But to a = (a B C DEF), it is to define $ A as a group number...
The following method can be used to test Bash's Data Group replacement method:
$ {A [@]} or $ {

• } Obtain a B c def (all groups)
  $ {A [0]} can get a (first group number). $ {A [1]} is the second group number...
  $ {# A [@]} or $ {#
• } Expected 4 (all groups)
  $ {# A [0]} can get 1 (that is, the length of the first group (), $ {A [3]} returns 3 (the length of the first group (DEF)
  A [3] = xyz is to redefine 4th groups as XYZ...
  
  Why? ....