diff --git a/bin/tests/system/zero/tests.sh b/bin/tests/system/zero/tests.sh
index b89aee223d..681ba118db 100644
--- a/bin/tests/system/zero/tests.sh
+++ b/bin/tests/system/zero/tests.sh
@@ -1,3 +1,5 @@
+#!/bin/sh
+#
 # Copyright (C) Internet Systems Consortium, Inc. ("ISC")
 #
 # This Source Code Form is subject to the terms of the Mozilla Public
@@ -7,84 +9,112 @@
 # See the COPYRIGHT file distributed with this work for additional
 # information regarding copyright ownership.
 
+#shellcheck source=conf.sh
 SYSTEMTESTTOP=..
-. $SYSTEMTESTTOP/conf.sh
+. "$SYSTEMTESTTOP/conf.sh"
 
-DIGOPTS="-p ${PORT}"
+dig_with_opts() {
+	"$DIG" -p "${PORT}" "$@"
+}
+
+wait_for_pid() (
+	for pid in "$@"; do
+		kill -0 "$pid" 2>/dev/null && return 1
+	done
+	return 0
+)
 
 status=0
 n=0
 
-n=`expr $n + 1`
+n=$((n+1))
 echo_i "check lookups against TTL=0 records ($n)"
 i=0
+ret=0
 passes=10
-$DIG $DIGOPTS @10.53.0.2 axfr example | grep -v "^ds0" |
+dig_with_opts @10.53.0.2 axfr example | grep -v "^ds0" | \
 awk '$2 == "0" { print "-q", $1, $4; print "-q", "zzz"$1, $4;}' > query.list
+
+# add 1/5 second per query
+timeout=$(($(wc -l < query.list) / 5))
 while [ $i -lt $passes ]
 do
-	ret=0
-	$DIG $DIGOPTS @10.53.0.3 -f query.list > dig.out$i.1.test$n &
-	$DIG $DIGOPTS @10.53.0.3 -f query.list > dig.out$i.2.test$n &
-	$DIG $DIGOPTS @10.53.0.3 -f query.list > dig.out$i.3.test$n &
-	$DIG $DIGOPTS @10.53.0.3 -f query.list > dig.out$i.4.test$n &
-	$DIG $DIGOPTS @10.53.0.3 -f query.list > dig.out$i.5.test$n &
-	$DIG $DIGOPTS @10.53.0.3 -f query.list > dig.out$i.6.test$n &
-	wait
-	grep "status: SERVFAIL" dig.out$i.1.test$n > /dev/null && ret=1
-	grep "status: SERVFAIL" dig.out$i.2.test$n > /dev/null && ret=1
-	grep "status: SERVFAIL" dig.out$i.3.test$n > /dev/null && ret=1
-	grep "status: SERVFAIL" dig.out$i.4.test$n > /dev/null && ret=1
-	grep "status: SERVFAIL" dig.out$i.5.test$n > /dev/null && ret=1
-	grep "status: SERVFAIL" dig.out$i.6.test$n > /dev/null && ret=1
+	(dig_with_opts @10.53.0.3 -f query.list > "dig.out$i.1.test$n") & pid1="$!"
+	(dig_with_opts @10.53.0.3 -f query.list > "dig.out$i.2.test$n") & pid2="$!"
+	(dig_with_opts @10.53.0.3 -f query.list > "dig.out$i.3.test$n") & pid3="$!"
+	(dig_with_opts @10.53.0.3 -f query.list > "dig.out$i.4.test$n") & pid4="$!"
+	(dig_with_opts @10.53.0.3 -f query.list > "dig.out$i.5.test$n") & pid5="$!"
+	(dig_with_opts @10.53.0.3 -f query.list > "dig.out$i.6.test$n") & pid6="$!"
+
+	retry_quiet "$timeout" wait_for_pid "$pid1" "$pid2" "$pid3" "$pid4" "$pid5" "$pid6" || ret=1
+	kill -TERM "$pid1" "$pid2" "$pid3" "$pid4" "$pid5" "$pid6" 2>/dev/null
+
+	wait "$pid1" || ret=1
+	wait "$pid2" || ret=1
+	wait "$pid3" || ret=1
+	wait "$pid4" || ret=1
+	wait "$pid5" || ret=1
+	wait "$pid6" || ret=1
+
+	grep "status: SERVFAIL" "dig.out$i.1.test$n" > /dev/null && ret=1
+	grep "status: SERVFAIL" "dig.out$i.2.test$n" > /dev/null && ret=1
+	grep "status: SERVFAIL" "dig.out$i.3.test$n" > /dev/null && ret=1
+	grep "status: SERVFAIL" "dig.out$i.4.test$n" > /dev/null && ret=1
+	grep "status: SERVFAIL" "dig.out$i.5.test$n" > /dev/null && ret=1
+	grep "status: SERVFAIL" "dig.out$i.6.test$n" > /dev/null && ret=1
 	[ $ret = 1 ] && break
-	i=`expr $i + 1`
+	i=$((i+1))
 	echo_i "successfully completed pass $i of $passes"
 done
 if [ $ret != 0 ]; then echo_i "failed"; fi
-status=`expr $status + $ret`
+status=$((status+ret))
 
-n=`expr $n + 1`
+repeat_query() (
+	i=0
+	while [ "$i" -lt "$1" ]; do
+		dig_with_opts +short "@$2" "$3" | tee "dig.out$i.test$n" || return 1
+		i=$((i+1))
+	done
+)
+
+count_unique() (
+	repeat_query "$@" | sort -u | wc -l
+)
+
+n=$((n+1))
 echo_i "check repeated recursive lookups of non recurring TTL=0 responses get new values ($n)"
 ret=0
-count=`(
-$DIG $DIGOPTS +short @10.53.0.3 foo.increment
-$DIG $DIGOPTS +short @10.53.0.3 foo.increment
-$DIG $DIGOPTS +short @10.53.0.3 foo.increment
-$DIG $DIGOPTS +short @10.53.0.3 foo.increment
-$DIG $DIGOPTS +short @10.53.0.3 foo.increment
-$DIG $DIGOPTS +short @10.53.0.3 foo.increment
-$DIG $DIGOPTS +short @10.53.0.3 foo.increment
-) | sort -u | wc -l `
-if [ $count -ne 7 ] ; then echo_i "failed (count=$count)"; ret=1; fi
-status=`expr $status + $ret`
+repeats=9
+count=$(count_unique "$repeats" 10.53.0.3 foo.increment)
+if [ "$count" -ne "$repeats" ] ; then echo_i "failed (count=$count, repeats=$repeats)"; ret=1; fi
+status=$((status+ret))
 
-n=`expr $n + 1`
+n=$((n+1))
 echo_i "check lookups against TTL=1 records ($n)"
 i=0
 passes=10
+ret=0
 while [ $i -lt $passes ]
 do
-	ret=0
-	$DIG $DIGOPTS @10.53.0.3 www.one.tld > dig.out$i.1.test$n
-	$DIG $DIGOPTS @10.53.0.3 www.one.tld > dig.out$i.2.test$n
-	$DIG $DIGOPTS @10.53.0.3 www.one.tld > dig.out$i.3.test$n
-	$DIG $DIGOPTS @10.53.0.3 www.one.tld > dig.out$i.4.test$n
-	$DIG $DIGOPTS @10.53.0.3 www.one.tld > dig.out$i.5.test$n
-	$DIG $DIGOPTS @10.53.0.3 www.one.tld > dig.out$i.6.test$n
-	grep "status: SERVFAIL" dig.out$i.1.test$n > /dev/null && ret=1
-	grep "status: SERVFAIL" dig.out$i.2.test$n > /dev/null && ret=1
-	grep "status: SERVFAIL" dig.out$i.3.test$n > /dev/null && ret=1
-	grep "status: SERVFAIL" dig.out$i.4.test$n > /dev/null && ret=1
-	grep "status: SERVFAIL" dig.out$i.5.test$n > /dev/null && ret=1
-	grep "status: SERVFAIL" dig.out$i.6.test$n > /dev/null && ret=1
+	dig_with_opts @10.53.0.3 www.one.tld > "dig.out$i.1.test$n" || ret=1
+	dig_with_opts @10.53.0.3 www.one.tld > "dig.out$i.2.test$n" || ret=1
+	dig_with_opts @10.53.0.3 www.one.tld > "dig.out$i.3.test$n" || ret=1
+	dig_with_opts @10.53.0.3 www.one.tld > "dig.out$i.4.test$n" || ret=1
+	dig_with_opts @10.53.0.3 www.one.tld > "dig.out$i.5.test$n" || ret=1
+	dig_with_opts @10.53.0.3 www.one.tld > "dig.out$i.6.test$n" || ret=1
+	grep "status: SERVFAIL" "dig.out$i.1.test$n" > /dev/null && ret=1
+	grep "status: SERVFAIL" "dig.out$i.2.test$n" > /dev/null && ret=1
+	grep "status: SERVFAIL" "dig.out$i.3.test$n" > /dev/null && ret=1
+	grep "status: SERVFAIL" "dig.out$i.4.test$n" > /dev/null && ret=1
+	grep "status: SERVFAIL" "dig.out$i.5.test$n" > /dev/null && ret=1
+	grep "status: SERVFAIL" "dig.out$i.6.test$n" > /dev/null && ret=1
 	[ $ret = 1 ] && break
-	i=`expr $i + 1`
+	i=$((i+1))
 	echo_i "successfully completed pass $i of $passes"
-	$PERL -e 'select(undef, undef, undef, 0.3);'
+	sleep 1
 done
 if [ $ret != 0 ]; then echo_i "failed"; fi
-status=`expr $status + $ret`
+status=$((status+ret))
 
 echo_i "exit status: $status"
-[ $status -eq 0 ] || exit 1
+[ "$status" -eq 0 ] || exit 1