owner of the backup is changed when user exists

server/backup.sh

 #! /bin/bash
 BACKUP_FILE="/opt/backup/backup.tar.gz"
 BACKUP_DIR="/tmp/backup/"
+BACKUP_USER="borgbackup"
 
 rm -f $BACKUP_FILE
 rm -rdf $BACKUP_DIR
@@ -30,7 +31,12 @@ pg_dump -U map_viewer map_viewer -h localhost | gzip > "$BACKUP_DIR/tmp/map_view
 
 cd $BACKUP_DIR/tmp
 tar -zcf $BACKUP_FILE *
-chown backup $BACKUP_FILE
+
+#change owner of backup file only if user exists
+id -u $BACKUP_USER
+status=$?
+[ $status -eq 0 ] && chown $BACKUP_USER $BACKUP_FILE
+
 cd /
 rm -rf $BACKUP_DIR/tmp