Use of $ {},#and % in Shell

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 one. and the string on the left: file.txt $ {file ##*.}: Delete the last one. and the left string: TXT $ {file %/*}: Delete the last/and the right string:/dir1/dir2/dir3 $ {file % /*}: delete the first/and its right string: (null) $ {file %. *}: Delete the last one. and its right string:/dir1/dir2/dir3/My. file $ {file %. *}: Delete the first one. and its right string:/dir1/dir2/dir3/My
The memory method is as follows: # Remove the left side (on the keyboard # on the left side of $) % remove the right side (on the keyboard % on the right side of $). A single symbol is the minimum match; the two symbols are the maximum match $ {file: 0: 5}: extract the leftmost five Bytes:/dir1 $ {file: 5: 5 }: extract 5 consecutive bytes to 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): Invalid variable 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. (When the value is not null, It is not processed.) When file1_my.file.txt}: If $ file is set to a null value or a non-null value, my.file.txt is used as the return value. (Not processed when not set) $ {file: Your my.file.txt}: If $ file is not a null value, use my.file.txt as the callback value. (When no value is set and null values are not processed) when using file=my.file.txt}: If $ file is not set, use my.file.txt as the response value. At the same time, set $ file to my.file.txt. (Null and non-null values are not processed) $ {file: Your my.file.txt}: If $ file is not set or is null, use my.file.txt as the callback value, at the same time, the value of $ file encoding is 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.
From: http://space.baidu.com.cn/ugo5/blog/item/c550bbc54d1644079c163dbd.html

Use of $ {},#and % in Shell