Because space has a meaning for shell, ls ab means list directories a and b not list directory "ab", which is why if you want to include spaces in the filename you either escape the spaces, e.g. a\ b or quote it, e.g. 'a b'. Otherwise the meaning is ambiguous.
Because space has a meaning for shell,
ls a bmeans list directoriesaandbnot list directory"a b", which is why if you want to include spaces in the filename you either escape the spaces, e.g.a\ bor quote it, e.g.'a b'. Otherwise the meaning is ambiguous.