diff --git a/scripts/install_db.sh b/scripts/install_db.sh
index bc9cb2d..e5e5b9a 100755
--- a/scripts/install_db.sh
+++ b/scripts/install_db.sh
@@ -69,14 +69,20 @@ trap cleanup EXIT
 usage() {
     cat <<EOF
 Użycie:
-  $0 <MARIADB_VERSION> <PORT>
+  $0 <MARIADB_VERSION> [PORT]
 
 Przykłady:
+  $0 10.11
   $0 10.11 33033
   $0 11.4 3020
   $0 11.8 3021
 
 Dozwolone gałęzie MariaDB: 10.11, 11.4, 11.8.
+
+Jeżeli PORT zostanie pominięty, przyjmowana jest wartość domyślna 3306. W takim
+przypadku skrypt najpierw przenosi natywną instancję MariaDB zarządzaną przez
+CustomBuild na inny port (patrz scripts/setup/native_mariadb_port_takeover.sh),
+aby zwolnić port 3306 dla alt-mariadb.
 EOF
 }
 
@@ -98,19 +104,15 @@ apply_cli_args() {
     fi
 
     if [[ $# -gt 2 ]]; then
-        usage_error "Podano zbyt wiele argumentów. Oczekiwano: MARIADB_VERSION PORT."
+        usage_error "Podano zbyt wiele argumentów. Oczekiwano: MARIADB_VERSION [PORT]."
     fi
 
-    if [[ $# -lt 2 ]]; then
-        usage_error "Podaj wersję MariaDB i port TCP, np. 11.4 3020."
-    fi
-
-    if [[ $# -ge 1 && -n "${1:-}" ]]; then
-        MARIADB_VERSION="$1"
-    fi
+    MARIADB_VERSION="$1"
 
     if [[ $# -ge 2 && -n "${2:-}" ]]; then
         PORT="$2"
+    else
+        PORT="3306"
     fi
 
     MARIADB_DIR="/usr/local/mariadb-$MARIADB_VERSION"
diff --git a/tests/install_db_cli_test.sh b/tests/install_db_cli_test.sh
index 44a3608..39f6288 100755
--- a/tests/install_db_cli_test.sh
+++ b/tests/install_db_cli_test.sh
@@ -41,10 +41,20 @@ set -e
 [ "$NO_ARGS_STATUS" -eq 2 ] || fail "install_db.sh without arguments should exit with status 2, got $NO_ARGS_STATUS"
 printf '%s\n' "$NO_ARGS_OUTPUT" | grep -Fq 'Użycie:' \
   || fail "install_db.sh without arguments does not print usage"
-printf '%s\n' "$NO_ARGS_OUTPUT" | grep -Fq '<MARIADB_VERSION> <PORT>' \
-  || fail "install_db.sh usage does not show required version and port"
+printf '%s\n' "$NO_ARGS_OUTPUT" | grep -Fq '<MARIADB_VERSION> [PORT]' \
+  || fail "install_db.sh usage does not show optional port"
 if printf '%s\n' "$NO_ARGS_OUTPUT" | grep -Fq 'Ten skrypt musi zostać uruchomiony jako root'; then
   fail "install_db.sh without arguments reaches root check instead of usage"
 fi
 
+set +e
+ONE_ARG_OUTPUT="$("$SCRIPT" 10.11 2>&1)"
+ONE_ARG_STATUS=$?
+set -e
+
+[ "$ONE_ARG_STATUS" -eq 1 ] \
+  || fail "install_db.sh with only a version argument should reach the root check (status 1), got $ONE_ARG_STATUS: $ONE_ARG_OUTPUT"
+printf '%s\n' "$ONE_ARG_OUTPUT" | grep -Fq 'Ten skrypt musi zostać uruchomiony jako root' \
+  || fail "install_db.sh with only a version argument should default PORT to 3306 and reach the root check, got: $ONE_ARG_OUTPUT"
+
 echo "install_db_cli_test: OK"