Index: Makefile.inc1
===================================================================
--- Makefile.inc1
+++ Makefile.inc1
@@ -1979,13 +1979,28 @@
 # bootstrap-tool.
 _config_deps= lib/libnv usr.bin/lex/lib
 
+.if !empty(BUILD_WITH_OPIPEFAIL) && empty(.MAKEFLAGS:M-n)
+# The bash -o pipefail option will cause the whole pipeline to fail if one
+# of the commands returned a non-zero exit code. This is useful to find
+# missing/incompatible commands in the bootstrap $PATH since the default shell
+# will just ignore the exit code.
+_pipefail_shell=tools/build/bash-with-pipefail
+# Note: we can't set __MAKE_SHELL to bash-with-pipefail since otherwise bmake
+# will complain about an unknown shell in sys.mk (.SHELL: path=${__MAKE_SHELL})
+# To work around this bash-with-pipefail will be installed as
+# ${WORLDTMP}/legacy/bin/sh. This also has the advantage that make rules using
+# sh will now use bash-with-pipefail instead and get the additional checks.
+CROSSENV+=	__MAKE_SHELL=${WORLDTMP}/legacy/bin/sh
+BMAKEENV+=	__MAKE_SHELL=${WORLDTMP}/legacy/bin/sh
+.endif
+
 legacy: .PHONY
 .if ${BOOTSTRAPPING} < ${MINIMUM_SUPPORTED_OSREL} && ${BOOTSTRAPPING} != 0
 	@echo "ERROR: Source upgrades from versions prior to ${MINIMUM_SUPPORTED_REL} are not supported."; \
 	false
 .endif
 
-.for _tool in tools/build ${_elftoolchain_libs} ${_config_deps}
+.for _tool in tools/build ${_elftoolchain_libs} ${_config_deps} ${_pipefail_shell}
 	${_+_}@${ECHODIR} "===> ${_tool} (obj,includes,all,install)"; \
 	    cd ${.CURDIR}/${_tool}; \
 	    if [ -z "${NO_OBJWALK}" ]; then ${MAKE} DIRPRFX=${_tool}/ obj; fi; \
Index: tools/build/bash-with-pipefail/Makefile
===================================================================
--- /dev/null
+++ tools/build/bash-with-pipefail/Makefile
@@ -0,0 +1,9 @@
+# $FreeBSD$
+
+PROG=	bash-with-pipefail
+SRCS=	bash-with-pipefail.c
+BINDIR=	/bin
+
+LINKS=	${BINDIR}/bash-with-pipefail ${BINDIR}/sh
+
+.include <bsd.prog.mk>
Index: tools/build/bash-with-pipefail/bash-with-pipefail.c
===================================================================
--- /dev/null
+++ tools/build/bash-with-pipefail/bash-with-pipefail.c
@@ -0,0 +1,67 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
+ *
+ * Copyright 2018 Alex Richardson <arichardson@FreeBSD.org>
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <err.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <sysexits.h>
+
+/*
+ * Start bash with -o pipefail to catch errors.
+ * This is used when building world on a non-FreeBSD host to ensure that we
+ * don't depend on binaries that don't exist and are used in pipeline.
+ * Without -o pipefail missing commands in a pipeline are ignored and can
+ * cause empty input files to be generated.
+ */
+int main(int argc, char** argv) {
+	char** new_argv;
+	char* bash_path;
+
+	new_argv = calloc(argc + 3, sizeof(char*));
+	new_argv[0] = "bash"; /* bash won't support -o pipefail in sh mode */
+	new_argv[1] = "-eo";
+	new_argv[2] = "pipefail";
+
+	for (int i = 1; i < argc; i++)
+		new_argv[i + 2] = argv[i];
+	new_argv[argc + 2] = NULL;
+
+	/*
+	 * Since we are running with a restricted $PATH, we will not be able
+	 * to find bash in $PATH. Try /usr/local/bin and /bin first
+	 */
+	if (access("/usr/local/bin/bash", F_OK) == 0)
+		bash_path = "/usr/local/bin/bash";
+	else if (access("/bin/bash", F_OK) == 0)
+		bash_path = "/bin/bash";
+	else
+		errx(EX_OSFILE, "Could not infer path to bash");
+
+	if (execvp(bash_path, new_argv) != 0)
+		err(EX_UNAVAILABLE, "Could not execute %s", bash_path);
+}