How can I use Atom as the default editor for "open"?
The open command is part of OS X and uses Finder's default “Open With…” application to open the file, just like double-clicking it in Finder. $EDITOR is used for things like fc where it is used as the default command-line editor (a replacement for vim or emacs).

To use Atom with the open command, you can select a file of the type you want in Finder, press ⌘ command+I for “Get Info”, and then change “Open With:” to Atom, and click “Change All…”.

Alternatively you can define an alias like

alias e="atom -nw"


in your .bashrc and then run e file-to-edit.txt whenever you want to edit a file in Atom.