In emacs --batch (also, emacs --script),
(message "My Message")
will print "My Message" to standard error (file descriptor 2) along with a trailing newline.
What if I don't want the newline? I see that theoretically,
(princ "My Message" standard-error)`
would work, if standard-error were defined.
Confusingly, standard-output and standard-input are defined, however.
Is this even possible?
According to the manual, anything printing a message in batch mode will use stderr for this. The print/prin1/princ family will continue printing to stdout unless you pass the optional argument for picking a different char printing function, therefore the following will work:
(princ "My Message" 'external-debugging-output)
Apparently the Emacs developers consider stderr only for reporting errors and debug output ;)
princ with find-function, then searched the sources for usage of stderr. Eventually I found that special char printing function...
(defvar standard-error 'external-debugging-output)makes the above behave nicely, thanks to @wasamasa