How to format (print) output of the shell-command function

Question

I have this

(defun insert-dateutc ()
  (interactive)
  (shell-command "printf %s $(date +\"%Y-%m-%dT%H:%M:%S\")" t))

which I want to use in this

(defun foo1 ()
  (format "this is the date-time %s" (insert-dateutc)))

but the output is the elisp id number of the function call (yes?)

(foo1)
==> "this is the date-time 1185"
2015-11-18T20:31:30 ; <-- inserted

with the actual desired return left outside. I need to suppress that number and get the actual date-time output. Gosh, I know it's something simple...

Answer 1

If what you want is just a timestamp, you can use

(defvar user-time-format
  (eval-when-compile (or (getenv "STRFTIME") "%Y-%m-%d %a %H:%M:%S %Z"))
  "The time format for the user to set.")
(defun user-time-format (&optional insert)
  "Return or insert the time string according to `user-time-format'."
  (interactive "P")
  (let ((str (format-time-string user-time-format)))
    (if insert (insert str)
        (if (called-interactively-p 'interactive) (message "%s" str) str))))

except you would set user-time-format to "%Y-%m-%dT%H-%M-%S".

However, your question raises an interesting issue, so let us discuss it too.

Your insert-dateutc is fine (but see below), but it inserts the date instead of returning it, so foo has to correct that:

(defun foo ()
  (with-temp-buffer
    (insert-dateutc)
    (buffer-string)))

(format "timestamp = [%s]" (foo))
==> "timestamp = [2015-11-18T16:37:12]"

Finally, take a look at C-h f shell-command RET. It says discourages the use of this function in lisp code, recommending call-process instead:

(with-temp-buffer
  (call-process "date" nil t nil "+%Y-%m-%dT%H:%M:%S")
  (buffer-substring-no-properties 1 (line-end-position 0)))
==> "2015-11-18T16:47:27"