Tip: capture standard and error output

What if we want to capture standard (and/or error) output in order to ignore it or post-process it ? It’s very simple, a little search and we’re good:

(let ((*standard-output* (make-string-output-stream))
      (*error-output* (make-string-output-stream)))
  (apply function args) ;; anything
  (setf standard-output (get-output-stream-string *standard-output*)))
(print-results standard-output))

and now in print-results we can print to standard output without being intercepted (and in our case, we’ll highlight some user-defined keywords).

Above, just don’t forget to get the output content with (get-output-stream-string *standard-output*).

A thing to note is that if your app printed stuff on error output and standard output consecutively, now it will print all standard output as a single block.

(edit) Of course, with-output-to-string is simpler to capture one stream:

(setf standard-output (with-output-to-string (*standard-output*)
                        (apply function args)))

edit 2, thanks to redditers:

Don’t bind standard-output directly; bind the string stream to a lexical, then bind *standard-output* to that:

(with-output-to-string (s)
  (let ((*standard-output* s)) (write-string "abc")))
-> "abc"

Now, let’s bind both *standard-output* and *standard-error* to s:

(with-output-to-string (s)
  (let ((*standard-output* s)
        (*standard-error* s))
    (write-string "abc")
    (write-string "def" *standard-error*)))
-> "abcdef"

Eliminate s and just bind *standard-output* and then tie *standard-error* to the same stream:

(with-output-to-string (*standard-output*)
  (let ((*standard-error* *standard-output*))
    (write-string "abc")
    (write-string "def" *standard-error*)))
--> "abcdef"

The conclusion stays: it’s handy and easy :)