How to know output type

Question

I've got this code:

(defun insert-random-uuid ()
  (interactive)
  (shell-command "uuidgen" t))

which produces a uuid that looks like this ffc715d2-1e10-4faf-b1f6-f85cae88a7d7. However, I believe it's throwing in a newline and that it isn't returning a string:

(stringp (insert-random-uuid))

which returns nil

So I find this at the EmacsWiki:

(defun chomp (str)
      "Chomp leading and tailing whitespace from STR."
      (while (string-match "\\`\n+\\|^\\s-+\\|\\s-+$\\|\n+\\'"
                           str)
        (setq str (replace-match "" t t str)))
      str)

which seems to handle both whitespace and \n. But of course (chomp (insert-random-uuid) gives an error, no doubt because insert-random-uuid is not returning a true string. So what is it returning, and how can I remove the newline? I tried various things to "cast" the output to string, but no luck. How can I get rid of this newline?

Answer 1

Use (type-of EXPRESSION) to determine the type of an expression.

In your case you are confusing the value being inserted into a buffer by a function with its return value.

shell-command inserts the textual output of the command, but it returns the exit status of the command (which, of course, is an integer).

The simplest solution to your problem is probably to prevent the shell from generating a newline in the output in the first place. e.g.:

(shell-command "printf %s \"$(uuidgen)\"" t)