Under X11 (i.e. mainstream Unix), you can set GUI-related parameters via X resources. To disable toolbars:
This does not apply to a native Mac program. You can set some GUI parameters through user defaults (see examples of
defaults write org.gnu.Emacs … in the Emacs Wiki), but I don't know if there's one about the toolbar.
Looking at the Emacs startup process, if you want to execute code before the initial frame is opened, you have a few choices. None of them are really intended to do what you want to do but it should work nonetheless. All of them involve changing a file in the directory which is on the load path before your init file runs. You can add load path directories by setting the environment variable
EMACSLOADPATH. In one of those directories, edit or create
subdirs.el and add the line
(setq tool-bar-mode 0)
This does not affect existing frames, but we're doing this before any frames are started, so I think it will work (but I haven't tested). It may help to also have
(setq default-frame-alist (cons '(tool-bar-lines . 0) default-frame-alist))
(setq initial-frame-alist (cons '(tool-bar-lines . 0) initial-frame-alist))
But rather than seek this, I would just start the Emacs daemon at session startup and then open windows in the existing session as needed.