Replace legacy syntax as advised by shellcheck

No functional changes intended

Author: vmcj

debian/src/dj_upgrade_chroot

@@ -7,7 +7,7 @@ CHROOTDIR=/var/lib/domjudge/chroot
 # To prevent (libc6) upgrade questions:
 export DEBIAN_FRONTEND=noninteractive
 
-if [ `id -u` != 0 ]; then
+if [ "$(id -u)" != 0 ]; then
     echo "Warning: you probably need to run this program as root."
 fi
 

docker-contributor/Dockerfile

@@ -49,7 +49,7 @@ RUN ln -sf /dev/stdout /var/log/nginx/access.log \
 # Set up users
 RUN useradd -m domjudge \
   && groupadd domjudge-run \
-  && for id in `seq 0 4`; do useradd -d /nonexistent -g nogroup -s /bin/false domjudge-run-$id; done
+  && for id in $(seq 0 4); do useradd -d /nonexistent -g nogroup -s /bin/false domjudge-run-$id; done
 
 # Install composer
 RUN apt update && \

docker-contributor/scripts/start.sh

@@ -75,7 +75,7 @@ do
   if ! mysqlshow -u"${MYSQL_USER}" -p"${MYSQL_PASSWORD}" -h"${MYSQL_HOST}" "${MYSQL_DATABASE}" > /dev/null 2>&1
   then
     echo "MySQL database ${MYSQL_DATABASE} not yet found on host ${MYSQL_HOST};"
-    let "DB_UP--"
+    (( DB_UP-- ))
     sleep 30s
   else
     DB_UP=0

docker/domserver/scripts/start.d/50-domjudge.sh

@@ -198,7 +198,7 @@ do
 	if ! mysqlshow -u"${MYSQL_USER}" -p"${MYSQL_PASSWORD}" -h"${MYSQL_HOST}" "${MYSQL_DATABASE}" > /dev/null 2>&1
 	then
 		echo "MySQL database ${MYSQL_DATABASE} not yet found on host ${MYSQL_HOST};"
-		let "DB_UP--"
+		(( DB_UP-- ))
 		sleep 10s
 	else
 		DB_UP=0