Possible to “detach” a child process?

Question

I use browse-url/browse-url-firefox to open links in Firefox from within Emacs 24.5.1 under Linux (Fedora 23) which in essence executes the firefox executable with the URL by start-process. If there is already a Firefox instance running, this will in turn cause the URL to be opened in a new tab and terminate the firefox executable spawned from Emacs, but otherwise a new Firefox instance will be running as a process child of Emacs (for testing purposes, this is equivalent to M-! sleep 1h & RET).

If I then want to exit Emacs, it asks me "Active processes exist; kill them and exit anyway?" with the option to either kill the Firefox instance or, well, leave Emacs running. Instead I would like to "detach" the firefox process from the Emacs parent so that I can exit Emacs while keeping the Firefox instance running.

Is it possible to spawn processes from Emacs that "survive" exiting Emacs, or must all spawned processes die when Emacs exits?

Answer 1

here's solution, shorting @abo-abo's answer.

(cond
     ((string-equal system-type "windows-nt") ; Windows
      // ...
      )
     ((string-equal system-type "gnu/linux")
      (start-process "my-browse"
                     nil "setsid"
                     "firefox"
                     (concat "file://" buffer-file-name ))

      ;; (browse-url ξurl)
      )
     ((string-equal system-type "darwin") ; Mac
                // ...
                ))

Note that condition is necessary, because Mac doesn't have setsid.

Answer 2

Currently the best way to achieve this (although it does not work for Windows) is by using the command call-process with the destination argument set to 0 (see https://www.gnu.org/software/emacs/manual/html_node/elisp/Synchronous-Processes.html).

E.g. to start firefox you could use:

(call-process "firefox" nil 0 nil "www.spacemacs.org")

Answer 3

Here's a command that does what you want:

(defun ora-dired-start-process (cmd &optional file-list)
  (interactive
   (let ((files (dired-get-marked-files
                 t current-prefix-arg)))
     (list
      (unless (eq system-type 'windows-nt)
        (dired-read-shell-command "& on %s: "
                                  current-prefix-arg files))
      files)))
  (if (eq system-type 'windows-nt)
      (dolist (file file-list)
        (w32-shell-execute "open" (expand-file-name file)))
    (let (list-switch)
      (start-process
       cmd nil shell-file-name
       shell-command-switch
       (format
        "nohup 1>/dev/null 2>/dev/null %s \"%s\""
        (if (and (> (length file-list) 1)
                 (setq list-switch
                       (cadr (assoc cmd ora-dired-filelist-cmd))))
            (format "%s %s" cmd list-switch)
          cmd)
        (mapconcat #'expand-file-name file-list "\" \""))))))

(define-key dired-mode-map "r" 'ora-dired-start-process)

The key thing here is to use nohup.

See the source here.