Looks great! Although I found one small issue

If you run it with multiple files and one of those doesn't exist, it differs from coreutils in erroring out.

For example, run the following and compare results between cat in shellrb and cat from coreutils

cat Gemfiles Gemfile.lock Gemfile

This will fail on the first file and not proceed further but coreutils will report error and then move onto the next params

cat Gemfile Gemfile.lock Gemfiles

This will silently fail here but coreutils will report an error,

I think you'll have to write an extra test for that scenario, but great work! I really appreciate it!