Shell variables and shell Variables

Shell variables and shell Variables

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 (not set, null, or non-null ):
Optional bytes file-my.file.txt}: If $ file is not set, use my.file.txt as the return value. (Null and non-null values are not processed)
$ {File: -my.file.txt}: If $ file is not set or is null, use my.file.txt as the callback value. (Not processed when the value is not null)
Optional values file1_my.file.txt}: If $ file is set to a null or non-null value, my.file.txt is used as the callback return value. (Not handled at the specified time)
$ {File: Invalid my.file.txt}: If $ file is not null, use my.file.txt as the callback value. (When no value is set or the null value is set, it is not processed)
When using file1_my.file.txt}: If $ file is not set, use my.file.txt as the response value, and set $ file to my.file.txt at the same time. (Null and non-null values are not processed)
$ {File: Invalid my.file.txt}: If $ file is not set or is null, use my.file.txt as the callback value. At the same time, set $ file to my.file.txt. (Not processed when the value is not null)
$ {File? My.file.txt}: If $ file is not set, export my.file.txt to STDERR. (Null and non-null values are not processed)
 

$ {File :? My.file.txt}: If $ file is not set or is null, output my.file.txt to STDERR. (Not processed when the value is not null)

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.

How does shell execute statements in variables?

This is related to the shell used.
Bash:
Test = 'echo "hello world "'
$ Test

Zsh needs to be like this:
Test = 'echo "hello world "'
$ {= Test}

Two references to a shell variable

Write the variable name directly to sed.
Sed-I "s/address. */address $ variable name/"/root/1