When using tmux as a login shell, there is currently no way to specify a shell

to be used as a login shell inside tmux, so add a default-shell session option.
This sets the shell invoked as a login shell when the default-command option is
empty.

The default option value is whichever of $SHELL, getpwuid(getuid())'s pw_shell
or /bin/sh is valid first.

Based on a diff from martynas@, changed by me to be a session option rather
than a window option.

cmd-respawn-window.c

@@ -75,7 +75,8 @@ cmd_respawn_window_exec(struct cmd *self, struct cmd_ctx *ctx)
  	window_destroy_panes(w);
 	TAILQ_INSERT_HEAD(&w->panes, wp, entry);
 	window_pane_resize(wp, w->sx, w->sy);
-	if (window_pane_spawn(wp, data->arg, NULL, &env, &s->tio, &cause) != 0) {
+	if (window_pane_spawn(
+	    wp, data->arg, NULL, NULL, &env, &s->tio, &cause) != 0) {
 		ctx->error(ctx, "respawn window failed: %s", cause);
 		xfree(cause);
 		environ_free(&env);

cmd-set-option.c

@@ -55,6 +55,7 @@ const struct set_option_entry set_option_table[] = {
 	{ "buffer-limit", SET_OPTION_NUMBER, 1, INT_MAX, NULL },
 	{ "default-command", SET_OPTION_STRING, 0, 0, NULL },
 	{ "default-path", SET_OPTION_STRING, 0, 0, NULL },
+	{ "default-shell", SET_OPTION_STRING, 0, 0, NULL },
 	{ "default-terminal", SET_OPTION_STRING, 0, 0, NULL },
 	{ "display-panes-colour", SET_OPTION_COLOUR, 0, 0, NULL },
 	{ "display-panes-time", SET_OPTION_NUMBER, 1, INT_MAX, NULL },

cmd-split-window.c

@@ -18,6 +18,7 @@
 
 #include <sys/types.h>
 
+#include <paths.h>
 #include <stdlib.h>
 #include <unistd.h>
 
@@ -151,6 +152,7 @@ cmd_split_window_exec(struct cmd *self, struct cmd_ctx *ctx)
 	struct window_pane		*wp;
 	struct environ			 env;
 	char		 		*cmd, *cwd, *cause;
+	const char			*shell;
 	u_int				 hlimit;
 	int				 size;
 	enum layout_type		 type;
@@ -183,8 +185,12 @@ cmd_split_window_exec(struct cmd *self, struct cmd_ctx *ctx)
 	if (data->flag_horizontal)
 		type = LAYOUT_LEFTRIGHT;
 
+	shell = options_get_string(&s->options, "default-shell");
+	if (*shell == '\0' || areshell(shell))
+		shell = _PATH_BSHELL;
+
 	wp = window_add_pane(w, hlimit);
-	if (window_pane_spawn(wp, cmd, cwd, &env, &s->tio, &cause) != 0)
+	if (window_pane_spawn(wp, cmd, shell, cwd, &env, &s->tio, &cause) != 0)
 		goto error;
 	if (layout_split_pane(w->active, type, size, wp) != 0) {
 		cause = xstrdup("pane too small");

names.c

@@ -90,7 +90,7 @@ default_window_name(struct window *w)
 		return (xstrdup("[tmux]"));
 	if (w->active->cmd != NULL && *w->active->cmd != '\0')
 		return (parse_window_name(w->active->cmd));
-	return (parse_window_name(window_default_command()));
+	return (parse_window_name(w->active->shell));
 }
 
 char *

session.c

@@ -19,6 +19,7 @@
 #include <sys/types.h>
 #include <sys/time.h>
 
+#include <paths.h>
 #include <string.h>
 #include <stdlib.h>
 #include <unistd.h>
@@ -207,6 +208,7 @@ session_new(struct session *s,
 {
 	struct window	*w;
 	struct environ	 env;
+	const char	*shell;
 	u_int		 hlimit;
 
 	environ_init(&env);
@@ -214,9 +216,13 @@ session_new(struct session *s,
 	environ_copy(&s->environ, &env);
 	server_fill_environ(s, &env);
 
+	shell = options_get_string(&s->options, "default-shell");
+	if (*shell == '\0' || areshell(shell))
+		shell = _PATH_BSHELL;
+
 	hlimit = options_get_number(&s->options, "history-limit");
 	w = window_create(
-	    name, cmd, cwd, &env, &s->tio, s->sx, s->sy, hlimit, cause);
+	    name, cmd, shell, cwd, &env, &s->tio, s->sx, s->sy, hlimit, cause);
 	if (w == NULL) {
 		environ_free(&env);
 		return (NULL);

tmux.1

@@ -1143,13 +1143,31 @@ maintain this maximum length.
 .It Ic default-command Ar command
 Set the command used for new windows (if not specified when the window is
 created) to
-.Ar command .
+.Ar command ,
+which may be any
+.Xr sh 1
+command.
 The default is an empty string, which instructs
 .Nm
-to create a login shell using the
+to create a login shell using the value of the
+.Ic default-shell
+option.
+.It Ic default-shell Ar path
+Specify the default shell.
+This is used as the login shell for new windows when the
+.Ic default-command
+option is set to empty, and must be the full path of the executable.
+When started
+.Nm
+tries to set a default value from the first suitable of the
 .Ev SHELL
-environment variable or, if it is unset, the user's shell returned by
+environment variable, the shell returned by
-.Xr getpwuid 3 .
+.Xr getpwuid 3 ,
+or
+.Pa /bin/sh .
+This option should be configured when
+.Nm
+is used as a login shell.
 .It Ic default-path Ar path
 Set the default working directory for processes created from keys, or
 interactively from the prompt.

tmux.c

@@ -175,6 +175,50 @@ sigreset(void)
 		fatal("sigaction failed");
 }
 
+const char *
+getshell(void)
+{
+	struct passwd	*pw;
+	const char	*shell;
+
+	shell = getenv("SHELL");
+	if (checkshell(shell))
+		return (shell);
+
+	pw = getpwuid(getuid());
+	if (pw != NULL && checkshell(pw->pw_shell))
+		return (pw->pw_shell);
+
+	return (_PATH_BSHELL);
+}
+
+int
+checkshell(const char *shell)
+{
+	if (shell == NULL || *shell == '\0' || areshell(shell))
+		return (0);
+	if (access(shell, X_OK) != 0)
+		return (0);
+	return (1);
+}
+
+int
+areshell(const char *shell)
+{
+	const char	*progname, *ptr;
+
+	if ((ptr = strrchr(shell, '/')) != NULL)
+		ptr++;
+	else
+		ptr = shell;
+	progname = __progname;
+	if (*progname == '-')
+		progname++;
+	if (strcmp(ptr, progname) == 0)
+		return (1);
+	return (0);
+}
+
 char *
 makesockpath(const char *label)
 {
@@ -346,6 +390,8 @@ main(int argc, char **argv)
 	options_set_number(&global_s_options, "bell-action", BELL_ANY);
 	options_set_number(&global_s_options, "buffer-limit", 9);
 	options_set_string(&global_s_options, "default-command", "%s", "");
+	options_set_string(
+	    &global_s_options, "default-shell", "%s", getshell());
 	options_set_string(&global_s_options, "default-terminal", "screen");
 	options_set_number(&global_s_options, "display-panes-colour", 4);
 	options_set_number(&global_s_options, "display-panes-time", 1000);

tmux.h

@@ -675,6 +675,7 @@ struct window_pane {
 #define PANE_REDRAW 0x1
 
 	char		*cmd;
+	char		*shell;
 	char		*cwd;
 
 	pid_t		 pid;
@@ -1117,6 +1118,9 @@ void		 logfile(const char *);
 void		 siginit(void);
 void		 sigreset(void);
 void		 sighandler(int);
+const char	*getshell(void);
+int		 checkshell(const char *);
+int		 areshell(const char *);
 
 /* cfg.c */
 int		 load_cfg(const char *, struct cmd_ctx *, char **);
@@ -1582,7 +1586,6 @@ int	 screen_check_selection(struct screen *, u_int, u_int);
 
 /* window.c */
 extern struct windows windows;
-const char	*window_default_command(void);
 int		 window_cmp(struct window *, struct window *);
 int		 winlink_cmp(struct winlink *, struct winlink *);
 RB_PROTOTYPE(windows, window, entry, window_cmp);
@@ -1600,8 +1603,8 @@ void		 winlink_stack_remove(struct winlink_stack *, struct winlink *);
 int	 	 window_index(struct window *, u_int *);
 struct window	*window_create1(u_int, u_int);
 struct window	*window_create(const char *, const char *, const char *,
-    		     struct environ *, struct termios *, u_int, u_int, u_int,
+		     const char *, struct environ *, struct termios *,
-		     char **);
+    		     u_int, u_int, u_int, char **);
 void		 window_destroy(struct window *);
 void		 window_set_active_pane(struct window *, struct window_pane *);
 struct window_pane *window_add_pane(struct window *, u_int);
@@ -1614,7 +1617,8 @@ void		 window_destroy_panes(struct window *);
 struct window_pane *window_pane_create(struct window *, u_int, u_int, u_int);
 void		 window_pane_destroy(struct window_pane *);
 int		 window_pane_spawn(struct window_pane *, const char *,
-		     const char *, struct environ *, struct termios *, char **);
+		     const char *, const char *, struct environ *,
+		     struct termios *, char **);
 void		 window_pane_resize(struct window_pane *, u_int, u_int);
 int		 window_pane_set_mode(
 		     struct window_pane *, const struct window_mode *);

window.c

@@ -58,38 +58,6 @@ struct windows windows;
 
 RB_GENERATE(winlinks, winlink, entry, winlink_cmp);
 
-const char *
-window_default_command(void)
-{
-	const char	*shell, *ptr;
-	char		*progname;
-	struct passwd	*pw;
-
-	shell = getenv("SHELL");
-	if (shell != NULL && *shell != '\0')
-		goto found;
-
-	pw = getpwuid(getuid());
-	if (pw != NULL && pw->pw_shell != NULL && *pw->pw_shell != '\0') {
-		shell = pw->pw_shell;
-		goto found;
-	}
-
-	return (_PATH_BSHELL);
-
-found:
-	if ((ptr = strrchr(shell, '/')) != NULL)
-		ptr++;
-	else
-		ptr = shell;
-	progname = __progname;
-	if (*progname == '-')
-		progname++;
-	if (strcmp(ptr, progname) == 0)
-		return (_PATH_BSHELL);
-	return (shell);
-}
-
 int
 winlink_cmp(struct winlink *wl1, struct winlink *wl2)
 {
@@ -270,9 +238,9 @@ window_create1(u_int sx, u_int sy)
 }
 
 struct window *
-window_create(const char *name, const char *cmd, const char *cwd,
+window_create(const char *name, const char *cmd, const char *shell,
-    struct environ *env, struct termios *tio, u_int sx, u_int sy, u_int hlimit,
+    const char *cwd, struct environ *env, struct termios *tio,
-    char **cause)
+    u_int sx, u_int sy, u_int hlimit,char **cause)
 {
 	struct window		*w;
 	struct window_pane	*wp;
@@ -280,7 +248,7 @@ window_create(const char *name, const char *cmd, const char *cwd,
 	w = window_create1(sx, sy);
 	wp = window_add_pane(w, hlimit);
 	layout_init(w);
-	if (window_pane_spawn(wp, cmd, cwd, env, tio, cause) != 0) {
+	if (window_pane_spawn(wp, cmd, shell, cwd, env, tio, cause) != 0) {
 		window_destroy(w);
 		return (NULL);
 	}
@@ -423,6 +391,7 @@ window_pane_create(struct window *w, u_int sx, u_int sy, u_int hlimit)
 	wp->window = w;
 
 	wp->cmd = NULL;
+	wp->shell = NULL;
 	wp->cwd = NULL;
 
 	wp->fd = -1;
@@ -467,13 +436,15 @@ window_pane_destroy(struct window_pane *wp)
 
 	if (wp->cwd != NULL)
 		xfree(wp->cwd);
+	if (wp->shell != NULL)
+		xfree(wp->shell);
 	if (wp->cmd != NULL)
 		xfree(wp->cmd);
 	xfree(wp);
 }
 
 int
-window_pane_spawn(struct window_pane *wp, const char *cmd,
+window_pane_spawn(struct window_pane *wp, const char *cmd, const char *shell,
     const char *cwd, struct environ *env, struct termios *tio, char **cause)
 {
 	struct winsize	 	 ws;
@@ -492,6 +463,11 @@ window_pane_spawn(struct window_pane *wp, const char *cmd,
 			xfree(wp->cmd);
 		wp->cmd = xstrdup(cmd);
 	}
+	if (shell != NULL) {
+		if (wp->shell != NULL)
+			xfree(wp->shell);
+		wp->shell = xstrdup(shell);
+	}
 	if (cwd != NULL) {
 		if (wp->cwd != NULL)
 			xfree(wp->cwd);
@@ -541,12 +517,12 @@ window_pane_spawn(struct window_pane *wp, const char *cmd,
 		}
 
 		/* No command; fork a login shell. */
-		cmd = window_default_command();
+		ptr = strrchr(wp->shell, '/');
-		if ((ptr = strrchr(cmd, '/')) != NULL && *(ptr + 1) != '\0')
+		if (ptr != NULL && *(ptr + 1) != '\0')
 			xasprintf(&argv0, "-%s", ptr + 1);
 		else
-			xasprintf(&argv0, "-%s", cmd);
+			xasprintf(&argv0, "-%s", wp->shell);
-		execl(cmd, argv0, (char *) NULL);
+		execl(wp->shell, argv0, (char *) NULL);
 		fatal("execl failed");
 	}