Some of the supernatural powers of ${} in the shell

Suppose we define a variable as: file=/dir1/dir2/dir3/My.file.txt We can replace each other with ${} to get different values: ${file#*/}: Take out the first/its left string: Dir1/dir2/dir3/my.file.txt${file##*/}: Take out the last/and left string: My.file.txt${file#*.} : Take out the first one. And the string to the left: file.txt${file##*.} : Take out the last one. And the string to the left: TXT${file%/*}: Take off the last bar/its right string:/dir1/dir2/Dir3${file%%/*}: Take out the first one/and the string to the right: (null value) ${file%.*}: Take off the last one. And the string to the right:/dir1/dir2/dir3/My.file${file%%.*}: Take out the first one. And the string to the right:/dir1/dir2/dir3/the methods of my memory are:#is to remove the left side (on the plate # on the left of the $)% is removed to the right (on the plate%on the right of $) a single symbol is the minimum match; two symbols are the maximum match. ${file:0:5}: Extract the leftmost 5 bytes:/Dir1${file:5:5}: Extract the 5th byte to the right of the 5 consecutive bytes:/Dir2 We can also replace the string in the value of the variable: ${file/dir/path}: Change the first dir to path:/path1/dir2/dir3/My.file.txt${fileDir/path}: Change all dir to path:/path1/path2/path3/My.file.txt can also be assigned to different variable states (no setting, null value, non-null value) using ${}: ${file-My.file.txt}: If the $file is a null value, use My.file.txt as the default value. (Reservation not set and non-null value) ${file:-My.file.txt}: If the $file is not set or null, use My.file.txt as the default value. (leave non-null values) ${file+My.file.txt}: Use My.file.txt as the default value regardless of the $file value. (do not retain any values) ${file:+My.file.txt}: Use My.file.txt as the default value unless the $file is a null value. (leave null value) ${file=My.file.txt}: If $file not set, use My.file.txt as the default and define $file as a non-null value. (leave null and non-null values) ${file:=My.file.txt}: If $file is not set or null, use My.file.txt as the default and define the $file as a non-null value. (Reserved non-null value) ${file?my.file.txt}: If $file is not set, the My.file.txt output to STDERR. (leave null and non-null values)) ${file:?my.file.txt}: If the $file is not set or null, the my.file.txt output to STDERR. (leave non-null values) and, ${.#var} to calculate the length of the variable value:${#file} can get 27 because/dir1/dir2/dir3/my.file.txt is just 27 bytes ...

Some of the supernatural powers of ${} in the shell