diff --git a/check.t b/check.t
index 3e1c8aa..5336b61 100644
--- a/check.t
+++ b/check.t
@@ -1,4 +1,4 @@
-# $MirOS: src/bin/mksh/check.t,v 1.677 2015/01/25 15:23:39 tg Exp $
+# $MirOS: src/bin/mksh/check.t,v 1.678 2015/02/06 10:56:44 tg Exp $
 # -*- mode: sh -*-
 #-
 # Copyright © 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010,
@@ -30,7 +30,7 @@
 # (2013/12/02 20:39:44) http://openbsd.cs.toronto.edu/cgi-bin/cvsweb/src/regress/bin/ksh/?sortby=date
 
 expected-stdout:
-	@(#)MIRBSD KSH R50 2015/01/25
+	@(#)MIRBSD KSH R50 2015/02/06
 description:
 	Check version of shell.
 stdin:
@@ -39,7 +39,7 @@ name: KSH_VERSION
 category: shell:legacy-no
 ---
 expected-stdout:
-	@(#)LEGACY KSH R50 2015/01/25
+	@(#)LEGACY KSH R50 2015/02/06
 description:
 	Check version of legacy shell.
 stdin:
diff --git a/exec.c b/exec.c
index 4a8c735..754ee92 100644
--- a/exec.c
+++ b/exec.c
@@ -23,7 +23,7 @@
 
 #include "sh.h"
 
-__RCSID("$MirOS: src/bin/mksh/exec.c,v 1.142 2015/02/06 10:09:06 tg Exp $");
+__RCSID("$MirOS: src/bin/mksh/exec.c,v 1.143 2015/02/06 10:56:46 tg Exp $");
 
 #ifndef MKSH_DEFAULT_EXECSHELL
 #define MKSH_DEFAULT_EXECSHELL	"/bin/sh"
@@ -446,7 +446,6 @@ execute(struct op * volatile t,
 
 	case TEXEC:
 		/* an eval'd TCOM */
-		s = t->args[0];
 		up = makenv();
 		restoresigs();
 		cleanup_proc_env();
@@ -460,7 +459,7 @@ execute(struct op * volatile t,
 		if (rv == ENOEXEC)
 			scriptexec(t, (const char **)up);
 		else
-			errorf("%s: %s", s, cstrerror(rv));
+			errorf("%s: %s", t->str, cstrerror(rv));
 	}
  Break:
 	exstat = rv & 0xFF;
@@ -509,6 +508,8 @@ comexec(struct op *t, struct tbl * volatile tp, const char **ap,
 	int fcflags = FC_BI|FC_FUNC|FC_PATH;
 	struct block *l_expand, *l_assign;
 	int optc;
+	const char *exec_argv0 = NULL;
+	bool exec_clrenv = false;
 
 	/* snag the last argument for $_ */
 	if (Flag(FTALKING) && *(lastp = ap)) {
@@ -554,7 +555,22 @@ comexec(struct op *t, struct tbl * volatile tp, const char **ap,
 		} else if (tp->val.f == c_exec) {
 			if (ap[1] == NULL)
 				break;
-			ap++;
+			ksh_getopt_reset(&builtin_opt, GF_ERROR);
+			while ((optc = ksh_getopt(ap, &builtin_opt, "a:c")) != -1)
+				switch (optc) {
+				case 'a':
+					exec_argv0 = builtin_opt.optarg;
+					break;
+				case 'c':
+					exec_clrenv = true;
+					/* ensure we can actually do this */
+					resetspec = true;
+					break;
+				default:
+					rv = 2;
+					goto Leave;
+				}
+			ap += builtin_opt.optind;
 			flags |= XEXEC;
 		} else if (tp->val.f == c_command) {
 			bool saw_p = false;
@@ -633,6 +649,8 @@ comexec(struct op *t, struct tbl * volatile tp, const char **ap,
 			type_flags = LOCAL|LOCAL_COPY|EXPORT;
 	}
 	l_assign = e->loc;
+	if (exec_clrenv)
+		l_assign->flags |= BF_STOPENV;
 	if (Flag(FEXPORT))
 		type_flags |= EXPORT;
 	if (Flag(FXTRACE))
@@ -813,14 +831,24 @@ comexec(struct op *t, struct tbl * volatile tp, const char **ap,
 			break;
 		}
 
-		/* set $_ to programme's full path */
+		/* set $_ to program's full path */
 		/* setstr() can't fail here */
 		setstr(typeset("_", LOCAL | EXPORT, 0, INTEGER, 0),
 		    tp->val.s, KSH_RETURN_ERROR);
 
-		if (flags&XEXEC) {
+		/* to fork, we set up a TEXEC node and call execute */
+		texec.type = TEXEC;
+		/* for vistree/dumptree */
+		texec.left = t;
+		texec.str = tp->val.s;
+		texec.args = ap;
+
+		/* in this case we do not fork, of course */
+		if (flags & XEXEC) {
+			if (exec_argv0)
+				texec.args[0] = exec_argv0;
 			j_exit();
-			if (!(flags&XBGND)
+			if (!(flags & XBGND)
 #ifndef MKSH_UNEMPLOYED
 			    || Flag(FMONITOR)
 #endif
@@ -830,12 +858,6 @@ comexec(struct op *t, struct tbl * volatile tp, const char **ap,
 			}
 		}
 
-		/* to fork we set up a TEXEC node and call execute */
-		texec.type = TEXEC;
-		/* for tprint */
-		texec.left = t;
-		texec.str = tp->val.s;
-		texec.args = ap;
 		rv = exchild(&texec, flags, xerrok, -1);
 		break;
 	}
diff --git a/mksh.1 b/mksh.1
index 83a0a77..0449282 100644
--- a/mksh.1
+++ b/mksh.1
@@ -1,9 +1,9 @@
-.\" $MirOS: src/bin/mksh/mksh.1,v 1.348 2015/01/25 15:23:42 tg Exp $
+.\" $MirOS: src/bin/mksh/mksh.1,v 1.349 2015/02/06 10:56:47 tg Exp $
 .\" $OpenBSD: ksh.1,v 1.156 2015/01/16 15:32:32 schwarze Exp $
 .\"-
 .\" Copyright © 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009,
 .\"		2010, 2011, 2012, 2013, 2014, 2015
-.\"	Thorsten Glaser <tg@mirbsd.org>
+.\"	Thorsten “mirabilos” Glaser <tg@mirbsd.org>
 .\"
 .\" Provided that these terms and disclaimer and all copyright notices
 .\" are retained or reproduced in an accompanying document, permission
@@ -74,7 +74,7 @@
 .\" with -mandoc, it might implement .Mx itself, but we want to
 .\" use our own definition. And .Dd must come *first*, always.
 .\"
-.Dd $Mdocdate: January 25 2015 $
+.Dd $Mdocdate: February 6 2015 $
 .\"
 .\" Check which macro package we use, and do other -mdoc setup.
 .\"
@@ -3376,9 +3376,25 @@ string which the shell then parses and executes in the current environment.
 .Pp
 .It Xo
 .Ic exec
+.Op Fl a Ar argv0
+.Op Fl c
 .Op Ar command Op Ar arg ...
 .Xc
 The command is executed without forking, replacing the shell process.
+This is currently absolute, i.e.\&
+.Ic exec
+never returns, even if the
+.Ar command
+is not found.
+The
+.Fl a
+option permits setting a different
+.Li argv[0]
+value, and
+.Fl c
+clears the environment before executing the child process, except for the
+.Ev _
+variable and direct assignments.
 .Pp
 If no command is given except for I/O redirection, the I/O redirection is
 permanent and the shell is
@@ -6485,7 +6501,7 @@ $ /bin/sleep 666 && echo fubar
 .Ed
 .Pp
 This document attempts to describe
-.Nm mksh\ R50
+.Nm mksh\ R51beta
 and up,
 compiled without any options impacting functionality, such as
 .Dv MKSH_SMALL ,
diff --git a/sh.h b/sh.h
index fe4c057..c09f38f 100644
--- a/sh.h
+++ b/sh.h
@@ -169,9 +169,9 @@
 #endif
 
 #ifdef EXTERN
-__RCSID("$MirOS: src/bin/mksh/sh.h,v 1.709 2015/01/25 15:23:43 tg Exp $");
+__RCSID("$MirOS: src/bin/mksh/sh.h,v 1.710 2015/02/06 10:56:48 tg Exp $");
 #endif
-#define MKSH_VERSION "R50 2015/01/25"
+#define MKSH_VERSION "R50 2015/02/06"
 
 /* arithmetic types: C implementation */
 #if !HAVE_CAN_INTTYPES
@@ -1237,6 +1237,7 @@ struct block {
 
 /* Values for struct block.flags */
 #define BF_DOGETOPTS	BIT(0)	/* save/restore getopts state */
+#define BF_STOPENV	BIT(1)	/* do not export further */
 
 /*
  * Used by ktwalk() and ktnext() routines.
diff --git a/var.c b/var.c
index ea32740..18a1633 100644
--- a/var.c
+++ b/var.c
@@ -2,7 +2,7 @@
 
 /*-
  * Copyright (c) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010,
- *		 2011, 2012, 2013, 2014
+ *		 2011, 2012, 2013, 2014, 2015
  *	Thorsten Glaser <tg@mirbsd.org>
  *
  * Provided that these terms and disclaimer and all copyright notices
@@ -28,7 +28,7 @@
 #include <sys/sysctl.h>
 #endif
 
-__RCSID("$MirOS: src/bin/mksh/var.c,v 1.185 2014/12/15 23:18:47 tg Exp $");
+__RCSID("$MirOS: src/bin/mksh/var.c,v 1.186 2015/02/06 10:56:49 tg Exp $");
 
 /*-
  * Variables
@@ -1112,6 +1112,8 @@ makenv(void)
 				}
 				XPput(denv, vp->val.s);
 			}
+		if (l->flags & BF_STOPENV)
+			break;
 	}
 	XPput(denv, NULL);
 	return ((char **)XPclose(denv));