diff --git a/spawn.c b/spawn.c
new file mode 100644
index 00000000..b22bf670
--- /dev/null
+++ b/spawn.c
@@ -0,0 +1,437 @@
+/* $OpenBSD$ */
+
+/*
+ * Copyright (c) 2019 Nicholas Marriott <nicholas.marriott@gmail.com>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF MIND, USE, DATA OR PROFITS, WHETHER
+ * IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING
+ * OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include <sys/types.h>
+
+#include <errno.h>
+#include <paths.h>
+#include <signal.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <util.h>
+
+#include "tmux.h"
+
+/*
+ * Set up the environment and create a new window and pane or a new pane.
+ *
+ * We need to set up the following items:
+ *
+ * - history limit, comes from the session;
+ *
+ * - base index, comes from the session;
+ *
+ * - current working directory, may be specified - if it isn't it comes from
+ *   either the client or the session;
+ *
+ * - PATH variable, comes from the client if any, otherwise from the session
+ *   environment;
+ *
+ * - shell, comes from default-shell;
+ *
+ * - termios, comes from the session;
+ *
+ * - remaining environment, comes from the session.
+ */
+
+static void
+spawn_log(const char *from, struct spawn_context *sc)
+{
+	struct session		*s = sc->s;
+	struct winlink		*wl = sc->wl;
+	struct window_pane	*wp0 = sc->wp0;
+	char			 tmp[128];
+	const char		*name;
+
+	log_debug("%s: %s, flags=%#x", from, sc->item->name, sc->flags);
+
+	if (wl != NULL && wp0 != NULL)
+		xsnprintf(tmp, sizeof tmp, "wl=%d wp0=%%%u", wl->idx, wp0->id);
+	else if (wl != NULL)
+		xsnprintf(tmp, sizeof tmp, "wl=%d wp0=none", wl->idx);
+	else if (wp0 != NULL)
+		xsnprintf(tmp, sizeof tmp, "wl=none wp0=%%%u", wp0->id);
+	else
+		xsnprintf(tmp, sizeof tmp, "wl=none wp0=none");
+	log_debug("%s: s=$%u %s idx=%d", from, s->id, tmp, sc->idx);
+
+	name = sc->name;
+	if (name == NULL)
+		name = "none";
+	log_debug("%s: name=%s", from, name);
+}
+
+struct winlink *
+spawn_window(struct spawn_context *sc, char **cause)
+{
+	struct session		*s = sc->s;
+	struct window		*w;
+	struct window_pane	*wp;
+	struct winlink		*wl;
+	int			 idx = sc->idx;
+	u_int			 sx, sy;
+
+	spawn_log(__func__, sc);
+
+	/*
+	 * If the window already exists, we are respawning, so destroy all the
+	 * panes except one.
+	 */
+	if (sc->flags & SPAWN_RESPAWN) {
+		w = sc->wl->window;
+		if (~sc->flags & SPAWN_KILL) {
+			TAILQ_FOREACH(wp, &w->panes, entry) {
+				if (wp->fd != -1)
+					break;
+			}
+			if (wp != NULL) {
+				xasprintf(cause, "window %s:%d still active",
+				    s->name, sc->wl->idx);
+				return (NULL);
+			}
+		}
+
+		sc->wp0 = TAILQ_FIRST(&w->panes);
+		TAILQ_REMOVE(&w->panes, sc->wp0, entry);
+
+		layout_free(w);
+		window_destroy_panes(w);
+
+		TAILQ_INSERT_HEAD(&w->panes, sc->wp0, entry);
+		window_pane_resize(sc->wp0, w->sx, w->sy);
+
+		layout_init(w, sc->wp0);
+		window_set_active_pane(w, sc->wp0, 0);
+	}
+
+	/*
+	 * Otherwise we have no window so we will need to create one. First
+	 * check if the given index already exists and destroy it if so.
+	 */
+	if ((~sc->flags & SPAWN_RESPAWN) && idx != -1) {
+		wl = winlink_find_by_index(&s->windows, idx);
+		if (wl != NULL && (~sc->flags & SPAWN_KILL)) {
+			xasprintf(cause, "index %d in use", idx);
+			return (NULL);
+		}
+		if (wl != NULL) {
+			/*
+			 * Can't use session_detach as it will destroy session
+			 * if this makes it empty.
+			 */
+			wl->flags &= ~WINLINK_ALERTFLAGS;
+			notify_session_window("window-unlinked", s, wl->window);
+			winlink_stack_remove(&s->lastw, wl);
+			winlink_remove(&s->windows, wl);
+
+			if (s->curw == wl) {
+				s->curw = NULL;
+				sc->flags &= ~SPAWN_DETACHED;
+			}
+		}
+	}
+
+	/* Then create a window if needed. */
+	if (~sc->flags & SPAWN_RESPAWN) {
+		if (idx == -1)
+			idx = -1 - options_get_number(s->options, "base-index");
+		if ((sc->wl = winlink_add(&s->windows, idx)) == NULL) {
+			xasprintf(cause, "couldn't add window %d", idx);
+			return (NULL);
+		}
+		default_window_size(s, NULL, &sx, &sy, -1);
+		if ((w = window_create(sx, sy)) == NULL) {
+			winlink_remove(&s->windows, sc->wl);
+			xasprintf(cause, "couldn't create window %d", idx);
+			return (NULL);
+		}
+		sc->wl->session = s;
+		winlink_set_window(sc->wl, w);
+	} else
+		w = NULL;
+	sc->flags |= SPAWN_NONOTIFY;
+
+	/* Spawn the pane. */
+	wp = spawn_pane(sc, cause);
+	if (wp == NULL) {
+		if (~sc->flags & SPAWN_RESPAWN) {
+			window_destroy(w);
+			winlink_remove(&s->windows, sc->wl);
+		}
+		return (NULL);
+	}
+
+	/* Set the name of the new window. */
+	if (~sc->flags & SPAWN_RESPAWN) {
+		if (sc->name != NULL) {
+			w->name = xstrdup(sc->name);
+			options_set_number(w->options, "automatic-rename", 0);
+		} else
+			w->name = xstrdup(default_window_name(w));
+	}
+
+	/* Switch to the new window if required. */
+	if (~sc->flags & SPAWN_DETACHED)
+		session_select(s, sc->wl->idx);
+
+	/* Fire notification if new window. */
+	if (~sc->flags & SPAWN_RESPAWN)
+		notify_session_window("window-linked", s, w);
+
+	session_group_synchronize_from(s);
+	return (sc->wl);
+}
+
+struct window_pane *
+spawn_pane(struct spawn_context *sc, char **cause)
+{
+	struct cmdq_item	 *item = sc->item;
+	struct client		 *c = item->client;
+	struct session		 *s = sc->s;
+	struct window		 *w = sc->wl->window;
+	struct window_pane	 *new_wp;
+	struct environ		 *child;
+	struct environ_entry	 *ee;
+	char			**argv, *cp, **argvp, *argv0, *cwd;
+	const char		 *cmd, *tmp;
+	int			  argc, before, full_size;
+	u_int			  idx;
+	struct termios		  now;
+	u_int			  hlimit;
+	struct winsize		  ws;
+	sigset_t		  set, oldset;
+
+	spawn_log(__func__, sc);
+
+	/* Check full size flag. */
+	if (sc->flags & SPAWN_FULLSIZE)
+		full_size = 1;
+	else
+		full_size = 0;
+
+	/*
+	 * If we are respawning then get rid of the old process. Otherwise
+	 * either create a new cell or assign to the one we are given.
+	 */
+	hlimit = options_get_number(s->options, "history-limit");
+	if (sc->flags & SPAWN_RESPAWN) {
+		if (sc->wp0->fd != -1 && (~sc->flags & SPAWN_KILL)) {
+			window_pane_index(sc->wp0, &idx);
+			xasprintf(cause, "pane %s:%d.%u still active",
+			    s->name, sc->wl->idx, idx);
+			return (NULL);
+		}
+		if (sc->wp0->fd != -1) {
+			bufferevent_free(sc->wp0->event);
+			close(sc->wp0->fd);
+		}
+		window_pane_reset_mode_all(sc->wp0);
+		screen_reinit(&sc->wp0->base);
+		input_init(sc->wp0);
+		new_wp = sc->wp0;
+		new_wp->flags &= ~(PANE_STATUSREADY|PANE_STATUSDRAWN);
+	} else if (sc->lc == NULL) {
+		new_wp = window_add_pane(w, NULL, hlimit, sc->flags);
+		layout_init(w, new_wp);
+	} else {
+		new_wp = window_add_pane(w, sc->wp0, hlimit, sc->flags);
+		layout_assign_pane(sc->lc, new_wp);
+	}
+
+	/*
+	 * Now we have a pane with nothing running in it ready for the new
+	 * process. Work out the command and arguments.
+	 */
+	if (sc->argc == 0) {
+		cmd = options_get_string(s->options, "default-command");
+		if (cmd != NULL && *cmd != '\0') {
+			argc = 1;
+			argv = (char **)&cmd;
+		} else {
+			argc = 0;
+			argv = NULL;
+		}
+	} else {
+		argc = sc->argc;
+		argv = sc->argv;
+	}
+
+	/*
+	 * Replace the stored arguments if there are new ones. If not, the
+	 * existing ones will be used (they will only exist for respawn).
+	 */
+	if (argc > 0) {
+		cmd_free_argv(new_wp->argc, new_wp->argv);
+		new_wp->argc = argc;
+		new_wp->argv = cmd_copy_argv(argc, argv);
+	}
+
+	/*
+	 * Work out the current working directory. If respawning, use
+	 * the pane's stored one unless specified.
+	 */
+	if (sc->cwd != NULL)
+		cwd = format_single(item, sc->cwd, c, s, NULL, NULL);
+	else if (~sc->flags & SPAWN_RESPAWN)
+		cwd = xstrdup(server_client_get_cwd(c, s));
+	else
+		cwd = NULL;
+	if (cwd != NULL) {
+		free(new_wp->cwd);
+		new_wp->cwd = cwd;
+	}
+
+	/* Create an environment for this pane. */
+	child = environ_for_session(s, 0);
+	environ_set(child, "TMUX_PANE", "%%%u", new_wp->id);
+
+	/*
+	 * Then the PATH environment variable. The session one is replaced from
+	 * the client if there is one because otherwise running "tmux new
+	 * myprogram" wouldn't work if myprogram isn't in the session's path.
+	 */
+	if (c != NULL && c->session == NULL) { /* only unattached clients */
+		ee = environ_find(c->environ, "PATH");
+		if (ee != NULL)
+			environ_set(child, "PATH", "%s", ee->value);
+	}
+	if (environ_find(child, "PATH") == NULL)
+		environ_set(child, "%s", _PATH_DEFPATH);
+
+	/* Then the shell. If respawning, use the old one. */
+	if (~sc->flags & SPAWN_RESPAWN) {
+		tmp = options_get_string(s->options, "default-shell");
+		if (*tmp == '\0' || areshell(tmp))
+			tmp = _PATH_BSHELL;
+		free(new_wp->shell);
+		new_wp->shell = xstrdup(tmp);
+	}
+	environ_set(child, "SHELL", "%s", new_wp->shell);
+
+	/* Log the arguments we are going to use. */
+	cp = cmd_stringify_argv(new_wp->argc, new_wp->argv);
+	log_debug("%s: shell=%s", __func__, new_wp->shell);
+	log_debug("%s: cmd=%s", __func__, cp);
+	log_debug("%s: cwd=%s", __func__, cwd);
+	cmd_log_argv(new_wp->argc, new_wp->argv, __func__);
+	environ_log(child, "%s: environment ", __func__);
+	free(cp);
+
+	/* Initialize the window size. */
+	memset(&ws, 0, sizeof ws);
+	ws.ws_col = screen_size_x(&new_wp->base);
+	ws.ws_row = screen_size_y(&new_wp->base);
+
+	/* Block signals until fork has completed. */
+	sigfillset(&set);
+	sigprocmask(SIG_BLOCK, &set, &oldset);
+
+	/* Fork the new process. */
+	new_wp->pid = fdforkpty(ptm_fd, &new_wp->fd, new_wp->tty, NULL, &ws);
+	if (new_wp->pid == -1) {
+		xasprintf(cause, "fork failed: %s", strerror(errno));
+		new_wp->fd = -1;
+		if (~sc->flags & SPAWN_RESPAWN) {
+			layout_close_pane(new_wp);
+			window_remove_pane(w, new_wp);
+		}
+		sigprocmask(SIG_SETMASK, &oldset, NULL);
+		return (NULL);
+	}
+
+	/* In the parent process, everything is done now. */
+	if (new_wp->pid != 0) {
+		new_wp->pipe_off = 0;
+		new_wp->flags &= ~PANE_EXITED;
+
+		sigprocmask(SIG_SETMASK, &oldset, NULL);
+		window_pane_set_event(new_wp);
+
+		if (sc->flags & SPAWN_RESPAWN)
+			return (new_wp);
+		if ((~sc->flags & SPAWN_DETACHED) || w->active == NULL) {
+			if (sc->flags & SPAWN_NONOTIFY)
+				window_set_active_pane(w, new_wp, 0);
+			else
+				window_set_active_pane(w, new_wp, 1);
+		}
+		if (~sc->flags & SPAWN_NONOTIFY)
+			notify_window("window-layout-changed", w);
+		return (new_wp);
+	}
+
+	/*
+	 * Child process. Change to the working directory or home if that
+	 * fails.
+	 */
+	if (chdir(new_wp->cwd) != 0) {
+		if ((tmp = find_home()) == NULL || chdir(tmp) != 0)
+			chdir("/");
+	}
+
+	/*
+	 * Update terminal escape characters from the session if available and
+	 * force VERASE to tmux's \177.
+	 */
+	if (tcgetattr(STDIN_FILENO, &now) != 0)
+		_exit(1);
+	if (s->tio != NULL)
+		memcpy(now.c_cc, s->tio->c_cc, sizeof now.c_cc);
+	now.c_cc[VERASE] = '\177';
+	if (tcsetattr(STDIN_FILENO, TCSANOW, &now) != 0)
+		_exit(1);
+
+	/* Clean up file descriptors and signals and update the environment. */
+	closefrom(STDERR_FILENO + 1);
+	proc_clear_signals(server_proc, 1);
+	sigprocmask(SIG_SETMASK, &oldset, NULL);
+	log_close();
+	environ_push(child);
+
+	/*
+	 * If given multiple arguments, use execvp(). Copy the arguments to
+	 * ensure they end in a NULL.
+	 */
+	if (new_wp->argc != 0 && new_wp->argc != 1) {
+		argvp = cmd_copy_argv(new_wp->argc, new_wp->argv);
+		execvp(argvp[0], argvp);
+		_exit(1);
+	}
+
+	/*
+	 * If one argument, pass it to $SHELL -c. Otherwise create a login
+	 * shell.
+	 */
+	cp = strrchr(new_wp->shell, '/');
+	if (new_wp->argc == 1) {
+		tmp = new_wp->argv[0];
+		if (cp != NULL && cp[1] != '\0')
+			xasprintf(&argv0, "%s", cp + 1);
+		else
+			xasprintf(&argv0, "%s", new_wp->shell);
+		execl(new_wp->shell, argv0, "-c", tmp, (char *)NULL);
+		_exit(1);
+	}
+	if (cp != NULL && cp[1] != '\0')
+		xasprintf(&argv0, "-%s", cp + 1);
+	else
+		xasprintf(&argv0, "-%s", new_wp->shell);
+	execl(new_wp->shell, argv0, (char *)NULL);
+	_exit(1);
+}