diff --git a/tools/testing/selftests/net/mptcp/mptcp_join.sh b/tools/testing/selftests/net/mptcp/mptcp_join.sh
index f5391d027af2..7314257d248a 100755
--- a/tools/testing/selftests/net/mptcp/mptcp_join.sh
+++ b/tools/testing/selftests/net/mptcp/mptcp_join.sh
@@ -1,6 +1,11 @@
 #!/bin/bash
 # SPDX-License-Identifier: GPL-2.0
 
+# Double quotes to prevent globbing and word splitting is recommended in new
+# code but we accept it, especially because there were too many before having
+# address all other issues detected by shellcheck.
+#shellcheck disable=SC2086
+
 ret=0
 sin=""
 sinfail=""
@@ -76,7 +81,7 @@ init_partial()
 	validate_checksum=$checksum
 	FAILING_LINKS=""
 
-	#  ns1              ns2
+	#  ns1         ns2
 	# ns1eth1    ns2eth1
 	# ns1eth2    ns2eth2
 	# ns1eth3    ns2eth3
@@ -288,12 +293,11 @@ check_transfer()
 	local in=$1
 	local out=$2
 	local what=$3
+	local i a b
 
 	local line
-	cmp -l "$in" "$out" | while read line; do
-		local arr=($line)
-
-		let sum=0${arr[1]}+0${arr[2]}
+	cmp -l "$in" "$out" | while read -r i a b; do
+		local sum=$((0${a} + 0${b}))
 		if [ $check_invert -eq 0 ] || [ $sum -ne $((0xff)) ]; then
 			echo "[ FAIL ] $what does not match (in, out):"
 			print_file_err "$in"
@@ -302,7 +306,7 @@ check_transfer()
 
 			return 1
 		else
-			echo "$what has inverted byte at ${arr[0]}"
+			echo "$what has inverted byte at ${i}"
 		fi
 	done
 
@@ -315,8 +319,7 @@ do_ping()
 	local connector_ns="$2"
 	local connect_addr="$3"
 
-	ip netns exec ${connector_ns} ping -q -c 1 $connect_addr >/dev/null
-	if [ $? -ne 0 ] ; then
+	if ! ip netns exec ${connector_ns} ping -q -c 1 $connect_addr >/dev/null; then
 		echo "$listener_ns -> $connect_addr connectivity [ FAIL ]" 1>&2
 		fail_test
 	fi
@@ -423,26 +426,26 @@ pm_nl_add_endpoint()
 	local nr=2
 
 	local p
-	for p in $@
+	for p in "${@}"
 	do
 		if [ $p = "flags" ]; then
 			eval _flags=\$"$nr"
-			[ ! -z $_flags ]; flags="flags $_flags"
+			[ -n "$_flags" ]; flags="flags $_flags"
 		fi
 		if [ $p = "dev" ]; then
 			eval _dev=\$"$nr"
-			[ ! -z $_dev ]; dev="dev $_dev"
+			[ -n "$_dev" ]; dev="dev $_dev"
 		fi
 		if [ $p = "id" ]; then
 			eval _id=\$"$nr"
-			[ ! -z $_id ]; id="id $_id"
+			[ -n "$_id" ]; id="id $_id"
 		fi
 		if [ $p = "port" ]; then
 			eval _port=\$"$nr"
-			[ ! -z $_port ]; port="port $_port"
+			[ -n "$_port" ]; port="port $_port"
 		fi
 
-		let nr+=1
+		nr=$((nr + 1))
 	done
 
 	if [ $ip_mptcp -eq 1 ]; then
@@ -525,18 +528,18 @@ pm_nl_check_endpoint()
 	while [ -n "$1" ]; do
 		if [ $1 = "flags" ]; then
 			_flags=$2
-			[ ! -z $_flags ]; flags="flags $_flags"
+			[ -n "$_flags" ]; flags="flags $_flags"
 			shift
 		elif [ $1 = "dev" ]; then
-			[ ! -z $2 ]; dev="dev $1"
+			[ -n "$2" ]; dev="dev $1"
 			shift
 		elif [ $1 = "id" ]; then
 			_id=$2
-			[ ! -z $_id ]; id="id $_id"
+			[ -n "$_id" ]; id="id $_id"
 			shift
 		elif [ $1 = "port" ]; then
 			_port=$2
-			[ ! -z $_port ]; port=" port $_port"
+			[ -n "$_port" ]; port=" port $_port"
 			shift
 		fi
 
@@ -675,7 +678,7 @@ do_transfer()
 					./mptcp_connect -t ${timeout_poll} -p $port -s ${cl_proto} \
 						$extra_args $connect_addr > "$cout" &
 	else
-		cat "$cinfail" | tee "$cinsent" | \
+		tee "$cinsent" < "$cinfail" | \
 			timeout ${timeout_test} \
 				ip netns exec ${connector_ns} \
 					./mptcp_connect -t ${timeout_poll} -p $port -s ${cl_proto} \
@@ -685,12 +688,13 @@ do_transfer()
 
 	# let the mptcp subflow be established in background before
 	# do endpoint manipulation
-	[ $addr_nr_ns1 = "0" -a $addr_nr_ns2 = "0" ] || sleep 1
+	if [ $addr_nr_ns1 != "0" ] || [ $addr_nr_ns2 != "0" ]; then
+		sleep 1
+	fi
 
 	if [ $addr_nr_ns1 -gt 0 ]; then
 		local counter=2
-		local add_nr_ns1
-		let add_nr_ns1=addr_nr_ns1
+		local add_nr_ns1=${addr_nr_ns1}
 		while [ $add_nr_ns1 -gt 0 ]; do
 			local addr
 			if is_v6 "${connect_addr}"; then
@@ -699,21 +703,21 @@ do_transfer()
 				addr="10.0.$counter.1"
 			fi
 			pm_nl_add_endpoint $ns1 $addr flags signal
-			let counter+=1
-			let add_nr_ns1-=1
+			counter=$((counter + 1))
+			add_nr_ns1=$((add_nr_ns1 - 1))
 		done
 	elif [ $addr_nr_ns1 -lt 0 ]; then
-		local rm_nr_ns1
-		let rm_nr_ns1=-addr_nr_ns1
+		local rm_nr_ns1=$((-addr_nr_ns1))
 		if [ $rm_nr_ns1 -lt 8 ]; then
 			local counter=0
 			local line
-			pm_nl_show_endpoints ${listener_ns} | while read line; do
+			pm_nl_show_endpoints ${listener_ns} | while read -r line; do
+				# shellcheck disable=SC2206 # we do want to split per word
 				local arr=($line)
 				local nr=0
 
 				local i
-				for i in ${arr[@]}; do
+				for i in "${arr[@]}"; do
 					if [ $i = "id" ]; then
 						if [ $counter -eq $rm_nr_ns1 ]; then
 							break
@@ -722,9 +726,9 @@ do_transfer()
 						rm_addr=$(rm_addr_count ${connector_ns})
 						pm_nl_del_endpoint ${listener_ns} $id
 						wait_rm_addr ${connector_ns} ${rm_addr}
-						let counter+=1
+						counter=$((counter + 1))
 					fi
-					let nr+=1
+					nr=$((nr + 1))
 				done
 			done
 		elif [ $rm_nr_ns1 -eq 8 ]; then
@@ -742,11 +746,10 @@ do_transfer()
 
 	# if newly added endpoints must be deleted, give the background msk
 	# some time to created them
-	[ $addr_nr_ns1 -gt 0 -a $addr_nr_ns2 -lt 0 ] && sleep 1
+	[ $addr_nr_ns1 -gt 0 ] && [ $addr_nr_ns2 -lt 0 ] && sleep 1
 
 	if [ $addr_nr_ns2 -gt 0 ]; then
-		local add_nr_ns2
-		let add_nr_ns2=addr_nr_ns2
+		local add_nr_ns2=${addr_nr_ns2}
 		local counter=3
 		while [ $add_nr_ns2 -gt 0 ]; do
 			local addr
@@ -756,20 +759,21 @@ do_transfer()
 				addr="10.0.$counter.2"
 			fi
 			pm_nl_add_endpoint $ns2 $addr flags $flags
-			let counter+=1
-			let add_nr_ns2-=1
+			counter=$((counter + 1))
+			add_nr_ns2=$((add_nr_ns2 - 1))
 		done
 	elif [ $addr_nr_ns2 -lt 0 ]; then
-		local rm_nr_ns2
+		local rm_nr_ns2=$((-addr_nr_ns2))
 		if [ $rm_nr_ns2 -lt 8 ]; then
 			local counter=0
 			local line
-			pm_nl_show_endpoints ${connector_ns} | while read line; do
+			pm_nl_show_endpoints ${connector_ns} | while read -r line; do
+				# shellcheck disable=SC2206 # we do want to split per word
 				local arr=($line)
 				local nr=0
 
 				local i
-				for i in ${arr[@]}; do
+				for i in "${arr[@]}"; do
 					if [ $i = "id" ]; then
 						if [ $counter -eq $rm_nr_ns2 ]; then
 							break
@@ -781,9 +785,9 @@ do_transfer()
 						rm_addr=$(rm_addr_count ${listener_ns})
 						pm_nl_del_endpoint ${connector_ns} $id
 						wait_rm_addr ${listener_ns} ${rm_addr}
-						let counter+=1
+						counter=$((counter + 1))
 					fi
-					let nr+=1
+					nr=$((nr + 1))
 				done
 			done
 		elif [ $rm_nr_ns2 -eq 8 ]; then
@@ -799,23 +803,24 @@ do_transfer()
 		fi
 	fi
 
-	if [ ! -z $sflags ]; then
+	if [ -n "${sflags}" ]; then
 		sleep 1
 
 		local netns
 		for netns in "$ns1" "$ns2"; do
 			local line
-			pm_nl_show_endpoints $netns | while read line; do
+			pm_nl_show_endpoints $netns | while read -r line; do
+				# shellcheck disable=SC2206 # we do want to split per word
 				local arr=($line)
 				local nr=0
 				local id
 
 				local i
-				for i in ${arr[@]}; do
+				for i in "${arr[@]}"; do
 					if [ $i = "id" ]; then
 						id=${arr[$nr+1]}
 					fi
-					let nr+=1
+					nr=$((nr + 1))
 				done
 				pm_nl_change_endpoint $netns $id $sflags
 			done
@@ -909,14 +914,14 @@ run_tests()
 		make_file "$cinfail" "client" $size
 	# create the input file for the failure test when
 	# the first failure test run
-	elif [ "$test_linkfail" -ne 0 -a -z "$cinfail" ]; then
+	elif [ "$test_linkfail" -ne 0 ] && [ -z "$cinfail" ]; then
 		# the client file must be considerably larger
 		# of the maximum expected cwin value, or the
 		# link utilization will be not predicable
 		size=$((RANDOM%2))
 		size=$((size+1))
 		size=$((size*8192))
-		size=$((size + ( $RANDOM % 8192) ))
+		size=$((size + ( RANDOM % 8192) ))
 
 		cinfail=$(mktemp)
 		make_file "$cinfail" "client" $size
@@ -929,7 +934,7 @@ run_tests()
 			sinfail=$(mktemp)
 		fi
 		make_file "$sinfail" "server" $size
-	elif [ "$test_linkfail" -eq 2 -a -z "$sinfail" ]; then
+	elif [ "$test_linkfail" -eq 2 ] && [ -z "$sinfail" ]; then
 		size=$((RANDOM%16))
 		size=$((size+1))
 		size=$((size*2048))
@@ -971,8 +976,8 @@ chk_csum_nr()
 	printf "%-${nr_blank}s %s" " " "sum"
 	count=$(ip netns exec $ns1 nstat -as | grep MPTcpExtDataCsumErr | awk '{print $2}')
 	[ -z "$count" ] && count=0
-	if [ "$count" != $csum_ns1 -a $allow_multi_errors_ns1 -eq 0 ] ||
-	   [ "$count" -lt $csum_ns1 -a $allow_multi_errors_ns1 -eq 1 ]; then
+	if { [ "$count" != $csum_ns1 ] && [ $allow_multi_errors_ns1 -eq 0 ]; } ||
+	   { [ "$count" -lt $csum_ns1 ] && [ $allow_multi_errors_ns1 -eq 1 ]; }; then
 		echo "[fail] got $count data checksum error[s] expected $csum_ns1"
 		fail_test
 		dump_stats=1
@@ -982,8 +987,8 @@ chk_csum_nr()
 	echo -n " - csum  "
 	count=$(ip netns exec $ns2 nstat -as | grep MPTcpExtDataCsumErr | awk '{print $2}')
 	[ -z "$count" ] && count=0
-	if [ "$count" != $csum_ns2 -a $allow_multi_errors_ns2 -eq 0 ] ||
-	   [ "$count" -lt $csum_ns2 -a $allow_multi_errors_ns2 -eq 1 ]; then
+	if { [ "$count" != $csum_ns2 ] && [ $allow_multi_errors_ns2 -eq 0 ]; } ||
+	   { [ "$count" -lt $csum_ns2 ] && [ $allow_multi_errors_ns2 -eq 1 ]; }; then
 		echo "[fail] got $count data checksum error[s] expected $csum_ns2"
 		fail_test
 		dump_stats=1
@@ -1190,8 +1195,8 @@ chk_stale_nr()
 	[ -z "$recover_nr" ] && recover_nr=0
 
 	if [ $stale_nr -lt $stale_min ] ||
-	   [ $stale_max -gt 0 -a $stale_nr -gt $stale_max ] ||
-	   [ $((stale_nr - $recover_nr)) -ne $stale_delta ]; then
+	   { [ $stale_max -gt 0 ] && [ $stale_nr -gt $stale_max ]; } ||
+	   [ $((stale_nr - recover_nr)) -ne $stale_delta ]; then
 		echo "[fail] got $stale_nr stale[s] $recover_nr recover[s], " \
 		     " expected stale in range [$stale_min..$stale_max]," \
 		     " stale-recover delta $stale_delta "
@@ -1230,7 +1235,7 @@ chk_add_nr()
 
 	# if the test configured a short timeout tolerate greater then expected
 	# add addrs options, due to retransmissions
-	if [ "$count" != "$add_nr" ] && [ "$timeout" -gt 1 -o "$count" -lt "$add_nr" ]; then
+	if [ "$count" != "$add_nr" ] && { [ "$timeout" -gt 1 ] || [ "$count" -lt "$add_nr" ]; }; then
 		echo "[fail] got $count ADD_ADDR[s] expected $add_nr"
 		fail_test
 		dump_stats=1
@@ -1375,8 +1380,9 @@ chk_rm_nr()
 	count=$(ip netns exec $subflow_ns nstat -as | grep MPTcpExtRmSubflow | awk '{print $2}')
 	[ -z "$count" ] && count=0
 	if [ -n "$simult" ]; then
-		local cnt=$(ip netns exec $addr_ns nstat -as | grep MPTcpExtRmSubflow | awk '{print $2}')
-		local suffix
+		local cnt suffix
+
+		cnt=$(ip netns exec $addr_ns nstat -as | grep MPTcpExtRmSubflow | awk '{print $2}')
 
 		# in case of simult flush, the subflow removal count on each side is
 		# unreliable
@@ -1447,13 +1453,13 @@ chk_link_usage()
 
 	local tx_link tx_total
 	tx_link=$(ip netns exec $ns cat /sys/class/net/$link/statistics/tx_bytes)
-	tx_total=$(ls -l $out | awk '{print $5}')
-	local tx_rate=$((tx_link * 100 / $tx_total))
+	tx_total=$(stat --format=%s $out)
+	local tx_rate=$((tx_link * 100 / tx_total))
 	local tolerance=5
 
 	printf "%-${nr_blank}s %-18s" " " "link usage"
-	if [ $tx_rate -lt $((expected_rate - $tolerance)) -o \
-	     $tx_rate -gt $((expected_rate + $tolerance)) ]; then
+	if [ $tx_rate -lt $((expected_rate - tolerance)) ] || \
+	   [ $tx_rate -gt $((expected_rate + tolerance)) ]; then
 		echo "[fail] got $tx_rate% usage, expected $expected_rate%"
 		fail_test
 	else