Special usage of shell variables

Suppose we have defined a variable: file =/dir1/dir2/dir3/my.file.txt, which can be replaced with $ {} to get different values: $ {file #*/}: delete the first/and its left string: dir1/dir2/dir3/my.file.txt $ {file ##*/}: delete the last/and its left string: my.file.txt $ {file #*.}: delete

Suppose we define a variable:
File =/dir1/dir2/dir3/my.file.txt

You can replace $ {} with different values:
$ {File # */}: delete the first/and its left string: dir1/dir2/dir3/my.file.txt
$ {File ### */}: delete the last/and its left string: my.file.txt
$ {File # *.}: delete the first. and its left string: file.txt
$ {File ### *.}: delete the last. and its left string: txt
$ {File %/*}: delete the last/and its right string:/dir1/dir2/dir3
$ {File %/*}: deletes the first/and its right string: (null)
$ {File %. *}: delete the last. and its right string:/dir1/dir2/dir3/my. file
$ {File %. *}: delete the first. and its right string:/dir1/dir2/dir3/my

 

The memory is as follows:
# Remove the left side (on the keyboard # on the left side of $)
% Is to remove the right side (% on the keyboard is on the right side of $)
A single symbol is the minimum match; two symbols are the maximum match.
$ {File: 0: 5}: extract the leftmost five bytes:/dir1
$ {File: 5: 5}: extract 5 consecutive bytes from the right of 5th bytes:/dir2

 

You can also replace the string in the variable value:
$ {File/dir/path}: replace the first dir with path:/path1/dir2/dir3/my.file.txt.
$ {File // dir/path}: Replace all dir with path:/path1/path2/path3/my.file.txt.

 

 

You can use $ {} to assign values to different variable states (?] Set, Null, non-null ):
When using file-my.file.txt}: suppose $ file?] There are settings ,? T use my.file.txt as the return value. (Null and non-null? R is not processed)
$ {File: -my.file.txt}: suppose $ file?] Yes? Set or? Shield cover? T use my.file.txt? Idle idle (non-null? R is not processed)
When using file=my.file.txt}: suppose $ file ?? What is the upper cover ?? Using my.file.txt? Why?]? OK? R is not processed)
$ {File: export my.file.txt}: if $ file? Beam cover? T use my.file.txt? Why? Set and null? R is not processed)
When using file=my.file.txt}: if $ file?]? Yes ,? T use my.file.txt? What is the workload ?? R? $ File? Value? My.file.txt. (Null and non-null? R is not processed)
$ {File: export my.file.txt}: if $ file?]? Set or? Shield cover? T use my.file.txt? What is the workload ?? R? $ File? Value ?? Y.file.txt. (Non-null? R is not processed)
$ {File? My.file.txt}: if $ file?]? Yes ,? T? My.file.txt? Output to STDERR. (Null and non-null? R is not processed)
 

$ {File :? My.file.txt}: if $ file is not set or is null, output my.file.txt to STDERR. (Non-null? R is not processed)

The length of the variable value can be calculated from $ {# var:
 

$ {# File} can get 27 because/dir1/dir2/dir3/my.file.txt is 27 bytes.