selftests: mptcp: join: avoid backquotes

As explained on ShellCheck's wiki [1], it is recommended to avoid
backquotes `...` in favour of parenthesis $(...):

> Backtick command substitution `...` is legacy syntax with several
> issues.
>
> - It has a series of undefined behaviors related to quoting in POSIX.
> - It imposes a custom escaping mode with surprising results.
> - It's exceptionally hard to nest.
>
> $(...) command substitution has none of these problems, and is
> therefore strongly encouraged.

[1] https://www.shellcheck.net/wiki/SC2006Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/mptcp/mptcp_join.sh

@@ -83,7 +83,7 @@ init_partial()
 	# ns1eth4    ns2eth4
 
 	local i
-	for i in `seq 1 4`; do
+	for i in $(seq 1 4); do
 		ip link add ns1eth$i netns "$ns1" type veth peer name ns2eth$i netns "$ns2"
 		ip -net "$ns1" addr add 10.0.$i.1/24 dev ns1eth$i
 		ip -net "$ns1" addr add dead:beef:$i::1/64 dev ns1eth$i nodad
@@ -102,7 +102,7 @@ init_partial()
 init_shapers()
 {
 	local i
-	for i in `seq 1 4`; do
+	for i in $(seq 1 4); do
 		tc -n $ns1 qdisc add dev ns1eth$i root netem rate 20mbit delay 1
 		tc -n $ns2 qdisc add dev ns2eth$i root netem rate 20mbit delay 1
 	done
@@ -969,7 +969,7 @@ chk_csum_nr()
 	fi
 
 	printf "%-${nr_blank}s %s" " " "sum"
-	count=`ip netns exec $ns1 nstat -as | grep MPTcpExtDataCsumErr | awk '{print $2}'`
+	count=$(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
@@ -980,7 +980,7 @@ chk_csum_nr()
 		echo -n "[ ok ]"
 	fi
 	echo -n " - csum  "
-	count=`ip netns exec $ns2 nstat -as | grep MPTcpExtDataCsumErr | awk '{print $2}'`
+	count=$(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
@@ -1001,7 +1001,7 @@ chk_fail_nr()
 	local dump_stats
 
 	printf "%-${nr_blank}s %s" " " "ftx"
-	count=`ip netns exec $ns1 nstat -as | grep MPTcpExtMPFailTx | awk '{print $2}'`
+	count=$(ip netns exec $ns1 nstat -as | grep MPTcpExtMPFailTx | awk '{print $2}')
 	[ -z "$count" ] && count=0
 	if [ "$count" != "$fail_tx" ]; then
 		echo "[fail] got $count MP_FAIL[s] TX expected $fail_tx"
@@ -1012,7 +1012,7 @@ chk_fail_nr()
 	fi
 
 	echo -n " - failrx"
-	count=`ip netns exec $ns2 nstat -as | grep MPTcpExtMPFailRx | awk '{print $2}'`
+	count=$(ip netns exec $ns2 nstat -as | grep MPTcpExtMPFailRx | awk '{print $2}')
 	[ -z "$count" ] && count=0
 	if [ "$count" != "$fail_rx" ]; then
 		echo "[fail] got $count MP_FAIL[s] RX expected $fail_rx"
@@ -1121,7 +1121,7 @@ chk_join_nr()
 	fi
 
 	printf "%03u %-36s %s" "${TEST_COUNT}" "${title}" "syn"
-	count=`ip netns exec $ns1 nstat -as | grep MPTcpExtMPJoinSynRx | awk '{print $2}'`
+	count=$(ip netns exec $ns1 nstat -as | grep MPTcpExtMPJoinSynRx | awk '{print $2}')
 	[ -z "$count" ] && count=0
 	if [ "$count" != "$syn_nr" ]; then
 		echo "[fail] got $count JOIN[s] syn expected $syn_nr"
@@ -1132,8 +1132,8 @@ chk_join_nr()
 	fi
 
 	echo -n " - synack"
-	with_cookie=`ip netns exec $ns2 sysctl -n net.ipv4.tcp_syncookies`
-	count=`ip netns exec $ns2 nstat -as | grep MPTcpExtMPJoinSynAckRx | awk '{print $2}'`
+	with_cookie=$(ip netns exec $ns2 sysctl -n net.ipv4.tcp_syncookies)
+	count=$(ip netns exec $ns2 nstat -as | grep MPTcpExtMPJoinSynAckRx | awk '{print $2}')
 	[ -z "$count" ] && count=0
 	if [ "$count" != "$syn_ack_nr" ]; then
 		# simult connections exceeding the limit with cookie enabled could go up to
@@ -1151,7 +1151,7 @@ chk_join_nr()
 	fi
 
 	echo -n " - ack"
-	count=`ip netns exec $ns1 nstat -as | grep MPTcpExtMPJoinAckRx | awk '{print $2}'`
+	count=$(ip netns exec $ns1 nstat -as | grep MPTcpExtMPJoinAckRx | awk '{print $2}')
 	[ -z "$count" ] && count=0
 	if [ "$count" != "$ack_nr" ]; then
 		echo "[fail] got $count JOIN[s] ack expected $ack_nr"
@@ -1184,9 +1184,9 @@ chk_stale_nr()
 	local recover_nr
 
 	printf "%-${nr_blank}s %-18s" " " "stale"
-	stale_nr=`ip netns exec $ns nstat -as | grep MPTcpExtSubflowStale | awk '{print $2}'`
+	stale_nr=$(ip netns exec $ns nstat -as | grep MPTcpExtSubflowStale | awk '{print $2}')
 	[ -z "$stale_nr" ] && stale_nr=0
-	recover_nr=`ip netns exec $ns nstat -as | grep MPTcpExtSubflowRecover | awk '{print $2}'`
+	recover_nr=$(ip netns exec $ns nstat -as | grep MPTcpExtSubflowRecover | awk '{print $2}')
 	[ -z "$recover_nr" ] && recover_nr=0
 
 	if [ $stale_nr -lt $stale_min ] ||
@@ -1222,10 +1222,10 @@ chk_add_nr()
 	local dump_stats
 	local timeout
 
-	timeout=`ip netns exec $ns1 sysctl -n net.mptcp.add_addr_timeout`
+	timeout=$(ip netns exec $ns1 sysctl -n net.mptcp.add_addr_timeout)
 
 	printf "%-${nr_blank}s %s" " " "add"
-	count=`ip netns exec $ns2 nstat -as MPTcpExtAddAddr | grep MPTcpExtAddAddr | awk '{print $2}'`
+	count=$(ip netns exec $ns2 nstat -as MPTcpExtAddAddr | grep MPTcpExtAddAddr | awk '{print $2}')
 	[ -z "$count" ] && count=0
 
 	# if the test configured a short timeout tolerate greater then expected
@@ -1239,7 +1239,7 @@ chk_add_nr()
 	fi
 
 	echo -n " - echo  "
-	count=`ip netns exec $ns1 nstat -as | grep MPTcpExtEchoAdd | awk '{print $2}'`
+	count=$(ip netns exec $ns1 nstat -as | grep MPTcpExtEchoAdd | awk '{print $2}')
 	[ -z "$count" ] && count=0
 	if [ "$count" != "$echo_nr" ]; then
 		echo "[fail] got $count ADD_ADDR echo[s] expected $echo_nr"
@@ -1251,7 +1251,7 @@ chk_add_nr()
 
 	if [ $port_nr -gt 0 ]; then
 		echo -n " - pt "
-		count=`ip netns exec $ns2 nstat -as | grep MPTcpExtPortAdd | awk '{print $2}'`
+		count=$(ip netns exec $ns2 nstat -as | grep MPTcpExtPortAdd | awk '{print $2}')
 		[ -z "$count" ] && count=0
 		if [ "$count" != "$port_nr" ]; then
 			echo "[fail] got $count ADD_ADDR[s] with a port-number expected $port_nr"
@@ -1262,8 +1262,8 @@ chk_add_nr()
 		fi
 
 		printf "%-${nr_blank}s %s" " " "syn"
-		count=`ip netns exec $ns1 nstat -as | grep MPTcpExtMPJoinPortSynRx |
-			awk '{print $2}'`
+		count=$(ip netns exec $ns1 nstat -as | grep MPTcpExtMPJoinPortSynRx |
+			awk '{print $2}')
 		[ -z "$count" ] && count=0
 		if [ "$count" != "$syn_nr" ]; then
 			echo "[fail] got $count JOIN[s] syn with a different \
@@ -1275,8 +1275,8 @@ chk_add_nr()
 		fi
 
 		echo -n " - synack"
-		count=`ip netns exec $ns2 nstat -as | grep MPTcpExtMPJoinPortSynAckRx |
-			awk '{print $2}'`
+		count=$(ip netns exec $ns2 nstat -as | grep MPTcpExtMPJoinPortSynAckRx |
+			awk '{print $2}')
 		[ -z "$count" ] && count=0
 		if [ "$count" != "$syn_ack_nr" ]; then
 			echo "[fail] got $count JOIN[s] synack with a different \
@@ -1288,8 +1288,8 @@ chk_add_nr()
 		fi
 
 		echo -n " - ack"
-		count=`ip netns exec $ns1 nstat -as | grep MPTcpExtMPJoinPortAckRx |
-			awk '{print $2}'`
+		count=$(ip netns exec $ns1 nstat -as | grep MPTcpExtMPJoinPortAckRx |
+			awk '{print $2}')
 		[ -z "$count" ] && count=0
 		if [ "$count" != "$ack_nr" ]; then
 			echo "[fail] got $count JOIN[s] ack with a different \
@@ -1301,8 +1301,8 @@ chk_add_nr()
 		fi
 
 		printf "%-${nr_blank}s %s" " " "syn"
-		count=`ip netns exec $ns1 nstat -as | grep MPTcpExtMismatchPortSynRx |
-			awk '{print $2}'`
+		count=$(ip netns exec $ns1 nstat -as | grep MPTcpExtMismatchPortSynRx |
+			awk '{print $2}')
 		[ -z "$count" ] && count=0
 		if [ "$count" != "$mis_syn_nr" ]; then
 			echo "[fail] got $count JOIN[s] syn with a mismatched \
@@ -1314,8 +1314,8 @@ chk_add_nr()
 		fi
 
 		echo -n " - ack   "
-		count=`ip netns exec $ns1 nstat -as | grep MPTcpExtMismatchPortAckRx |
-			awk '{print $2}'`
+		count=$(ip netns exec $ns1 nstat -as | grep MPTcpExtMismatchPortAckRx |
+			awk '{print $2}')
 		[ -z "$count" ] && count=0
 		if [ "$count" != "$mis_ack_nr" ]; then
 			echo "[fail] got $count JOIN[s] ack with a mismatched \
@@ -1361,7 +1361,7 @@ chk_rm_nr()
 	fi
 
 	printf "%-${nr_blank}s %s" " " "rm "
-	count=`ip netns exec $addr_ns nstat -as | grep MPTcpExtRmAddr | awk '{print $2}'`
+	count=$(ip netns exec $addr_ns nstat -as | grep MPTcpExtRmAddr | awk '{print $2}')
 	[ -z "$count" ] && count=0
 	if [ "$count" != "$rm_addr_nr" ]; then
 		echo "[fail] got $count RM_ADDR[s] expected $rm_addr_nr"
@@ -1372,7 +1372,7 @@ chk_rm_nr()
 	fi
 
 	echo -n " - rmsf  "
-	count=`ip netns exec $subflow_ns nstat -as | grep MPTcpExtRmSubflow | awk '{print $2}'`
+	count=$(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}')
@@ -1414,7 +1414,7 @@ chk_prio_nr()
 	local dump_stats
 
 	printf "%-${nr_blank}s %s" " " "ptx"
-	count=`ip netns exec $ns1 nstat -as | grep MPTcpExtMPPrioTx | awk '{print $2}'`
+	count=$(ip netns exec $ns1 nstat -as | grep MPTcpExtMPPrioTx | awk '{print $2}')
 	[ -z "$count" ] && count=0
 	if [ "$count" != "$mp_prio_nr_tx" ]; then
 		echo "[fail] got $count MP_PRIO[s] TX expected $mp_prio_nr_tx"
@@ -1425,7 +1425,7 @@ chk_prio_nr()
 	fi
 
 	echo -n " - prx   "
-	count=`ip netns exec $ns1 nstat -as | grep MPTcpExtMPPrioRx | awk '{print $2}'`
+	count=$(ip netns exec $ns1 nstat -as | grep MPTcpExtMPPrioRx | awk '{print $2}')
 	[ -z "$count" ] && count=0
 	if [ "$count" != "$mp_prio_nr_rx" ]; then
 		echo "[fail] got $count MP_PRIO[s] RX expected $mp_prio_nr_rx"
@@ -1444,8 +1444,10 @@ chk_link_usage()
 	local link=$2
 	local out=$3
 	local expected_rate=$4
-	local tx_link=`ip netns exec $ns cat /sys/class/net/$link/statistics/tx_bytes`
-	local tx_total=`ls -l $out | awk '{print $5}'`
+
+	local tx_link tx_total
+	tx_link=$(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))
 	local tolerance=5