Similar to check() in pm_netlink.sh, add a new helper check_output()
in mptcp_join.sh to check the output of the given commands.

Signed-off-by: Geliang Tang <geliang.tang@linux.dev>
---
 .../testing/selftests/net/mptcp/mptcp_join.sh | 27 +++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/tools/testing/selftests/net/mptcp/mptcp_join.sh b/tools/testing/selftests/net/mptcp/mptcp_join.sh
index 915ec79f5030..f4043baa59c6 100755
--- a/tools/testing/selftests/net/mptcp/mptcp_join.sh
+++ b/tools/testing/selftests/net/mptcp/mptcp_join.sh
@@ -21,6 +21,7 @@ cinfail=""
 cinsent=""
 tmpfile=""
 cout=""
+check_output_err=""
 capout=""
 ns1=""
 ns2=""
@@ -186,6 +187,7 @@ init() {
 	cout=$(mktemp)
 	evts_ns1=$(mktemp)
 	evts_ns2=$(mktemp)
+	check_output_err=$(mktemp)
 
 	trap cleanup EXIT
 
@@ -199,6 +201,7 @@ cleanup()
 	rm -f "$sin" "$sout" "$cinsent" "$cinfail"
 	rm -f "$tmpfile"
 	rm -rf $evts_ns1 $evts_ns2
+	rm -f $check_output_err
 	cleanup_partial
 }
 
@@ -3358,6 +3361,30 @@ userspace_pm_rm_sf()
 	wait_rm_sf $1 "${cnt}"
 }
 
+check_output()
+{
+	local cmd="$1"
+	local expected="$2"
+	local msg="$3"
+	local out=`$cmd 2>$check_output_err`
+	local cmd_ret=$?
+
+	printf "%-42s" "$msg"
+	if [ $cmd_ret -ne 0 ]; then
+		mptcp_lib_print_err "[ FAIL ] command execution '$cmd' stderr "
+		cat $check_output_err
+		ret=${KSFT_FAIL}
+		return $cmd_ret
+	elif [ "$out" = "$expected" ]; then
+		mptcp_lib_print_ok "[ OK ]"
+		return 0
+	else
+		mptcp_lib_print_err "[ FAIL ] expected '$expected' got '$out'"
+		ret=${KSFT_FAIL}
+		return 1
+	fi
+}
+
 userspace_tests()
 {
 	# userspace pm type prevents add_addr
-- 
2.39.2