BASH: How to extract substring that is surrounded by specific text

Question

I am trying to extract numbers from file names that follow a specific pattern:

file-8923489_something.txt
another_file-8923489_something.txt
some-other_file-8923         


        

Answer 1

Is it possible to do this using operators only, ...

$ filename="file-8923489_something.txt"
$ file=${foo//[^0-9]/}
$ echo $file
8923489

You might want to refer to Shell Parameter Expansion.

Alternatively, you can say:

$ file=$(tr -dc '[[:digit:]]' <<< "$filename")
$ echo $file
8923489