Linux Shell ${} simple usage

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For the sake of completeness, I'll use some examples here to illustrate some of the supernatural powers of ${}:
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%%/*}: Remove the first/its right string: (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/my

The methods of 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 the $)

The single symbol is the minimum match, and the two symbol is the maximum match.

${file:0:5}: Extract the leftmost 5 bytes:/dir1
${file:5:5}: Extracts the 5th byte to the right of 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
${file//dir/path}: Change all dir to path:/path1/path2/path3/my

Linux Shell ${} simple usage