Two new options:

- server option "exit-unattached" makes the server exit when no clients
  are attached, even if sessions are present;

- session option "destroy-unattached" destroys a session once no clients
  are attached to it.

These are useful for preventing tmux remaining in the background where
it is undesirable and when using tmux as a login shell to keep a limit
on new sessions.

cmd-set-option.c

@@ -75,6 +75,7 @@ const char *set_option_bell_action_list[] = {
 
 const struct set_option_entry set_option_table[] = {
 	{ "escape-time", SET_OPTION_NUMBER, 0, INT_MAX, NULL },
+	{ "exit-unattached", SET_OPTION_FLAG, 0, 0, NULL },
 	{ "quiet", SET_OPTION_FLAG, 0, 0, NULL },
 	{ NULL, 0, 0, 0, NULL }
 };
@@ -87,6 +88,7 @@ const struct set_option_entry set_session_option_table[] = {
 	{ "default-path", SET_OPTION_STRING, 0, 0, NULL },
 	{ "default-shell", SET_OPTION_STRING, 0, 0, NULL },
 	{ "default-terminal", SET_OPTION_STRING, 0, 0, NULL },
+	{ "destroy-unattached", SET_OPTION_FLAG, 0, 0, NULL },
 	{ "detach-on-destroy", SET_OPTION_FLAG, 0, 0, NULL },
 	{ "display-panes-colour", SET_OPTION_COLOUR, 0, 0, NULL },
 	{ "display-panes-active-colour", SET_OPTION_COLOUR, 0, 0, NULL },

cmd-switch-client.c

@@ -152,6 +152,7 @@ cmd_switch_client_exec(struct cmd *self, struct cmd_ctx *ctx)
 	c->session = s;
 
 	recalculate_sizes();
+	server_check_unattached();
 	server_redraw_client(c);
 
 	return (0);

server-client.c

@@ -185,6 +185,7 @@ server_client_lost(struct client *c)
 	c->flags |= CLIENT_DEAD;
 
 	recalculate_sizes();
+	server_check_unattached();
 	server_update_socket();
 }
 

server-fn.c

@@ -404,6 +404,25 @@ server_destroy_session(struct session *s)
 	recalculate_sizes();
 }
 
+void
+server_check_unattached (void)
+{
+	struct session	*s;
+	u_int		 i;
+
+	/*
+	 * If any sessions are no longer attached and have destroy-unattached
+	 * set, collect them.
+	 */
+	for (i = 0; i < ARRAY_LENGTH(&sessions); i++) {
+		s = ARRAY_ITEM(&sessions, i);
+		if (s == NULL || !(s->flags & SESSION_UNATTACHED))
+			continue;
+		if (options_get_number (&s->options, "destroy-unattached"))
+			session_destroy(s);
+	}
+}
+
 void
 server_set_identify(struct client *c)
 {

server.c

@@ -222,15 +222,17 @@ server_loop(void)
 	}
 }
 
-/* Check if the server should be shutting down (no more clients or windows). */
+/* Check if the server should be shutting down (no more clients or sessions). */
 int
 server_should_shutdown(void)
 {
 	u_int	i;
 
-	for (i = 0; i < ARRAY_LENGTH(&sessions); i++) {
+	if (!options_get_number(&global_options, "exit-unattached")) {
-		if (ARRAY_ITEM(&sessions, i) != NULL)
+		for (i = 0; i < ARRAY_LENGTH(&sessions); i++) {
-			return (0);
+			if (ARRAY_ITEM(&sessions, i) != NULL)
+				return (0);
+		}
 	}
 	for (i = 0; i < ARRAY_LENGTH(&clients); i++) {
 		if (ARRAY_ITEM(&clients, i) != NULL)

tmux.1

@@ -1587,6 +1587,9 @@ Set the time in milliseconds for which
 waits after an escape is input to determine if it is part of a function or meta
 key sequences.
 The default is 500 milliseconds.
+.It Ic exit-unattached
+If enabled, the server will exit when there are no attached clients, rather
+than when there are no attached sessions.
 .It Ic quiet
 Enable or disable the display of various informational messages (see also the
 .Fl q
@@ -1659,6 +1662,9 @@ to work correctly, this
 be set to
 .Ql screen
 or a derivative of it.
+.It Ic destroy-unattached
+If enabled and the session is no longer attached to any clients, it is
+destroyed.
 .It Ic detach-on-destroy
 If on (the default), the client is detached when the session it is attached to
 is destroyed.

tmux.c

@@ -331,6 +331,7 @@ main(int argc, char **argv)
 	oo = &global_options;
 	options_set_number(oo, "quiet", quiet);
 	options_set_number(oo, "escape-time", 500);
+	options_set_number(oo, "exit-unattached", 0);
 
 	options_init(&global_s_options, NULL);
 	so = &global_s_options;
@@ -341,6 +342,7 @@ main(int argc, char **argv)
 	options_set_string(so, "default-path", "%s", "");
 	options_set_string(so, "default-shell", "%s", getshell());
 	options_set_string(so, "default-terminal", "screen");
+	options_set_number(so, "destroy-unattached", 0);
 	options_set_number(so, "detach-on-destroy", 1);
 	options_set_number(so, "display-panes-active-colour", 1);
 	options_set_number(so, "display-panes-colour", 4);

tmux.h

@@ -1661,6 +1661,7 @@ void	 server_unlink_window(struct session *, struct winlink *);
 void	 server_destroy_pane(struct window_pane *);
 void	 server_destroy_session_group(struct session *);
 void	 server_destroy_session(struct session *);
+void	 server_check_unattached (void);
 void	 server_set_identify(struct client *);
 void	 server_clear_identify(struct client *);
 void	 server_update_event(struct client *);