Get the longest non-glob string at the start of a string

Question

I'm trying to create a script to convert from $GIT_DIR/info/exclude to .gitignore files. I'd like to put the .gitignore files as close to the pattern target as possible, meaning that a glob like /a/b/*/*.c should be put in /a/b/.gitignore. To do this accurately, I need something which can return the longest substring from the start which is uniquely identified, based on man glob. Does there exist anything like this, preferably something which is portable across *nixes?

Answer 1

Found a solution - By iterating over dirname "$string" until [ -e "$string" ] succeeds. The resulting script can be found in a blog post.