Iterate through every file in one directory

As others have said, Dir::foreach is a good option here. However, note that Dir::foreach and Dir::entries will always include . and .. (the current and parent directories). You will generally not want to work on them, so you can use Dir::each_child or Dir::children (as suggested by ma11hew28) or do something like this:

Dir.foreach('/path/to/dir') do |filename|
  next if filename == '.' or filename == '..'
  # Do work on the remaining files & directories
end

Dir::foreach and Dir::entries (as well as Dir::each_child and Dir::children) also include hidden files & directories. Often this is what you want, but if it isn’t, you need to do something to skip over them.

Alternatively, you might want to look into Dir::glob which provides simple wildcard matching:

Dir.glob('/path/to/dir/*.rb') do |rb_filename|
  # Do work on files & directories ending in .rb
end