How to go to each directory and execute a command?

Question

How do I write a bash script that goes through each directory inside a parent_directory and executes a command in each directory

Answer 1

for p in [0-9][0-9][0-9];do
    (
        cd $p
        for f in [0-9][0-9][0-9][0-9]*.txt;do
            ls $f; # Your operands
        done
    )
done