Shell ##%%

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

Different values can be replaced with ${}, respectively:
${file#*/}: Delete the first/its left string: dir1/dir2/dir3/my.file.txt
${file##*/}: Remove the last/and left string: my.file.txt
${file#*.}  : Remove the first one. And the string to the left: file.txt
${file##*.}  : Remove the last one. And the string to the left: txt
${file%/*}: Remove the last/And right string:/dir1/dir2/dir3
${file%%/*}: Delete the first/its right string: (null value)
${file%.*}: Remove the last one. And the string to the right:/dir1/dir2/dir3/my.file
${file%%.*}: Remove 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 keyboard # on the left side)
% is removed to the right (on the keyboard% on the right side)
The single symbol is the minimum match; two symbols are 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

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

With ${} You can also assign values to different variable states (no settings, null values, non-null values):
${file-my.file.txt}: If $file is not set, use My.file.txt to return the value. (null and non-null values are not processed)
${file:-my.file.txt}: If $file is not set or null, use My.file.txt as the return value. (Non-null value is not processed)
${file+my.file.txt}: If the $file is set to null or non-null values, use My.file.txt as the return value. (not to be processed)
${file:+my.file.txt}: If $file is a non-null value, use My.file.txt as the return value. (No processing when not set and null value)
${file=my.file.txt}: If the $file is not set, use My.file.txt as the return value, and will $file the value of My.file.txt. (null and non-null values are not processed)
${file:=my.file.txt}: If the $file is not set or NULL, the my.file.txt is used as the return value, and the value of the $file is my.file.txt. (Non-null value is not processed)
${file?my.file.txt}: If $file is not set, the my.file.txt will be output to STDERR. (null and non-null values are not processed)

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

${#var} to calculate the length of the variable value:

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

Shell ##%%