One of the popular aliases in other IRC clients is
‘/amsg’. This commands sends its argument as a message to all the currently active channels.
An implementation of this command for ERC is:
(defun erc-cmd-AMSG (&rest args) "Send a message to all the channels of the current server." (when args (let ((message (substring (mapconcat #'identity args " ") 1)) (channel-list (and erc-server-process (erc-channel-list erc-server-process)))) (when channel-list (dolist (channel channel-list) (with-current-buffer channel (erc-send-message message)))))))
Add this to your Emacs or ERC startup file, and then you should be able to type
/amsg Hi, everyone! Good morning :-)
to send a message to all the channels you have just joined.