emacs --eval '(let ((display-buffer-alist `(("^\\*magit: " display-buffer-same-window) ,display-buffer-alist))) (magit-status))'

Or with emacs client:

emacsclient -c --eval '(let ((display-buffer-alist `(("^\\*magit: " display-buffer-same-window) ,display-buffer-alist))) (magit-status))'

Found this posted here: https://www.reddit.com/r/emacs/comments/59drz2/opening_emacsclient_eval_magitstatus_opens_an/d97oz1w