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(elisp)Sentinels

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Sentinels: Detecting Process Status Changes
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   A "process sentinel" is a function that is called whenever the
associated process changes status for any reason, including signals
(whether sent by Emacs or caused by the process's own actions) that
terminate, stop, or continue the process.  The process sentinel is also
called if the process exits.  The sentinel receives two arguments: the
process for which the event occurred, and a string describing the type
of event.

   The string describing the event looks like one of the following:

   * `"finished\n"'.

   * `"exited abnormally with code EXITCODE\n"'.

   * `"NAME-OF-SIGNAL\n"'.

   * `"NAME-OF-SIGNAL (core dumped)\n"'.

   A sentinel runs only while Emacs is waiting (e.g., for terminal
input, or for time to elapse, or for process output).  This avoids the
timing errors that could result from running them at random places in
the middle of other Lisp programs.  A program can wait, so that
sentinels will run, by calling `sit-for' or `sleep-for' (Note:
Waiting), or `accept-process-output' (Note: Accepting Output).
Emacs also allows sentinels to run when the command loop is reading
input.

   Quitting is normally inhibited within a sentinel--otherwise, the
effect of typing `C-g' at command level or to quit a user command would
be unpredictable.  If you want to permit quitting inside a sentinel,
bind `inhibit-quit' to `nil'.  Note: Quitting.

   A sentinel that writes the output into the buffer of the process
should check whether the buffer is still alive.  If it tries to insert
into a dead buffer, it will get an error.  If the buffer is dead,
`(buffer-name (process-buffer PROCESS))' returns `nil'.

   If an error happens during execution of a sentinel, it is caught
automatically, so that it doesn't stop the execution of whatever
programs was running when the sentinel was started.  However, if
`debug-on-error' is non-`nil', the error-catching is turned off.  This
makes it possible to use the Lisp debugger to debug the sentinel.
Note: Debugger.

   In earlier Emacs versions, every sentinel that did regular expression
searching or matching had to explicitly save and restore the match data.
Now Emacs does this automatically for sentinels; they never need to do
it explicitly.  Note: Match Data.

 - Function: set-process-sentinel process sentinel
     This function associates SENTINEL with PROCESS.  If SENTINEL is
     `nil', then the process will have no sentinel.  The default
     behavior when there is no sentinel is to insert a message in the
     process's buffer when the process status changes.

          (defun msg-me (process event)
             (princ
               (format "Process: %s had the event `%s'" process event)))
          (set-process-sentinel (get-process "shell") 'msg-me)
               => msg-me
          (kill-process (get-process "shell"))
               -| Process: #<process shell> had the event `killed'
               => #<process shell>

 - Function: process-sentinel process
     This function returns the sentinel of PROCESS, or `nil' if it has
     none.

 - Function: waiting-for-user-input-p
     While a sentinel or filter function is running, this function
     returns non-`nil' if Emacs was waiting for keyboard input from the
     user at the time the sentinel or filter function was called, `nil'
     if it was not.