Index: etc/mtree/BSD.tests.dist
===================================================================
--- etc/mtree/BSD.tests.dist
+++ etc/mtree/BSD.tests.dist
@@ -821,6 +821,8 @@
             ..
             link
             ..
+            mixer
+            ..
             mkdir
             ..
             mkfifo
Index: usr.sbin/mixer/Makefile
===================================================================
--- usr.sbin/mixer/Makefile
+++ usr.sbin/mixer/Makefile
@@ -1,6 +1,11 @@
 # $FreeBSD$
 
+.include <src.opts.mk>
+
 PROG=	mixer
 MAN=	mixer.8
+
+HAS_TESTS=
+SUBDIR.${MK_TESTS}+= tests
 
 .include <bsd.prog.mk>
Index: usr.sbin/mixer/mixer.c
===================================================================
--- usr.sbin/mixer/mixer.c
+++ usr.sbin/mixer/mixer.c
@@ -241,6 +241,11 @@
 		lrel = rrel = 0;
 		if (argc > 1) {
 			m = sscanf(argv[1], "%7[^:]:%7s", lstr, rstr);
+			if (m == -1) {
+				warnx("invalid value: %s", argv[1]);
+				dusage = 1;
+				break;
+			}
 			if (m > 0) {
 				if (*lstr == '+' || *lstr == '-')
 					lrel = rrel = 1;
Index: usr.sbin/mixer/tests/Makefile
===================================================================
--- /dev/null
+++ usr.sbin/mixer/tests/Makefile
@@ -0,0 +1,5 @@
+# $FreeBSD$
+
+ATF_TESTS_SH+=		mixer_test
+
+.include <bsd.test.mk>
Index: usr.sbin/mixer/tests/mixer_test.sh
===================================================================
--- /dev/null
+++ usr.sbin/mixer/tests/mixer_test.sh
@@ -0,0 +1,123 @@
+#
+# SPDX-License-Identifier: BSD-2-Clause-FreeBSD
+#
+# Copyright (c) 2019 Mateusz Piotrowski <0mp@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 AUTHOR 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 AUTHOR 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.
+
+# $FreeBSD$
+
+mixer_unavailable()
+{
+	! { mixer && mixer vol; } >/dev/null 2>&1
+}
+
+save_mixer_vol()
+{
+	atf_check -o match:'^[0-9]{1,3}:[0-9]{1,3}$' -o save:saved_vol \
+		-x "mixer vol | awk '{print \$7}'"
+}
+
+set_mixer_vol()
+{
+	atf_check \
+		-o match:'^Setting the mixer vol from [0-9]{1,3}:[0-9]{1,3} to 0:0\.$' \
+		mixer vol 0
+}
+
+restore_mixer_vol()
+{
+	if [ -r "saved_vol" ]; then
+		mixer vol "$(cat saved_vol)"
+	fi
+}
+
+atf_test_case s_flag cleanup
+s_flag_head()
+{
+	atf_set	"descr" "Verify that the output of the -s flag could be " \
+		"reused as command-line arguments to the mixer command"
+}
+s_flag_body()
+{
+	if mixer_unavailable; then
+		atf_skip "This test requires mixer support"
+	fi
+	save_mixer_vol
+	set_mixer_vol
+	atf_check -o inline:"vol 0:0" -o save:values mixer -s vol
+	atf_check -o inline:"Setting the mixer vol from 0:0 to 0:0.\n" \
+		mixer $(cat values)
+}
+s_flag_cleanup()
+{
+	restore_mixer_vol
+}
+
+atf_test_case S_flag cleanup
+S_flag_head()
+{
+	atf_set	"descr" "Verify that the output of the -S flag is " \
+		"matching the documented behavior"
+}
+S_flag_body()
+{
+	if mixer_unavailable; then
+		atf_skip "This test requires mixer support"
+	fi
+	save_mixer_vol
+	set_mixer_vol
+	atf_check -o inline:"vol:0:0" mixer -S vol
+}
+S_flag_cleanup()
+{
+	restore_mixer_vol
+}
+
+atf_test_case set_empty_value
+set_empty_value_head()
+{
+	atf_set	"descr" "Verify that mixer returns when the provided " \
+		"value to set is an empty string instead of a number"
+	atf_set "timeout" "1"
+}
+set_empty_value_body()
+{
+	if mixer_unavailable; then
+		atf_skip "This test requires mixer support"
+	fi
+	save_mixer_vol
+	atf_check -s exit:1 -e inline:"mixer: invalid value: \n" \
+		-o match:"^usage:" mixer vol ""
+}
+set_empty_value_cleanup()
+{
+	restore_mixer_vol
+}
+
+
+atf_init_test_cases()
+{
+	atf_add_test_case s_flag
+	atf_add_test_case S_flag
+	atf_add_test_case set_empty_value
+}