What is a `##` means in bash variable substitution `${}`? [duplicate]

Question

I am learning the bash script materials on http://www.tldp.org/LDP/abs/html/index.html and stuck in the Example 7-7:

http://tldp.org/LDP/abs/html/comparison-ops.html#EX14

There is an ${filename##*.} != "gz", this probably means that the $filename does not end with .gz, but I do not know the meaning of ## here. Could anyone help me?

Thanks!

Answer 1

Used in a variable expansion, ${string##sub} removes the longest matching substring sub from string (# removes the shortest matching substring by contrast).

In your case, yes - this will return the string after the first . from the filename, giving the file extension.

If you search for ## in this documentation, you'll find an explanation (along with other similar commands).

Answer 2

In the context of filenames, is trying to find the extension in the variable filename

filename="*.log"
echo ${filename##*.}
log

We are attaining the part of the string filename after "*."