diff --git a/README.md b/README.md
index 4302671..435f179 100644
--- a/README.md
+++ b/README.md
@@ -161,7 +161,13 @@ cd phpunit-test-runner/
 
 ## Configuring the Runner
 
-The next step will be to configure the environment. To do this, make a copy of the example file and then configure it.
+The fastest way is the setup script. It checks the required software, asks for the database credentials and the report API key, tests the database connection, and creates `.env`:
+
+```
+./host-tools/setup.sh
+```
+
+To configure the runner by hand, make a copy of the example file and then configure it.
 
 ```
 cp .env.default .env
diff --git a/host-tools/setup.sh b/host-tools/setup.sh
new file mode 100755
index 0000000..7429e30
--- /dev/null
+++ b/host-tools/setup.sh
@@ -0,0 +1,225 @@
+#!/usr/bin/env bash
+#
+# Set up the test runner: check the requirements, create .env and test the
+# database connection.
+#
+# Run this from your clone of the runner:
+#
+#   git clone https://github.com/WordPress/phpunit-test-runner.git
+#   cd phpunit-test-runner
+#   ./host-tools/setup.sh
+#
+# The script asks for each value. Press Enter to keep the value in brackets.
+# Values come from the environment first, then from the current .env (if it
+# exists), then from built-in defaults.
+#
+# Options:
+#   --non-interactive  Do not ask. Use values from the environment and the current .env.
+#                      Example: WPT_DB_NAME=wptests WPT_DB_USER=wptests WPT_DB_PASSWORD=secret \
+#                               ./host-tools/setup.sh --non-interactive
+#   --skip-checks      Do not check the required software.
+#   --skip-db-check    Do not test the database connection.
+#
+# The script does not install software, create the database, or set up cron.
+
+set -uo pipefail
+
+RUNNER_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+ENV_FILE="$RUNNER_DIR/.env"
+ENV_DEFAULT="$RUNNER_DIR/.env.default"
+
+INTERACTIVE=1
+CHECK_SOFTWARE=1
+CHECK_DB=1
+
+for arg in "$@"; do
+	case "$arg" in
+		--non-interactive) INTERACTIVE=0 ;;
+		--skip-checks) CHECK_SOFTWARE=0 ;;
+		--skip-db-check) CHECK_DB=0 ;;
+		-h|--help) sed -n '3,24p' "${BASH_SOURCE[0]}" | sed 's/^# \{0,1\}//'; exit 0 ;;
+		*) echo "Error: unknown option: $arg" >&2; exit 1 ;;
+	esac
+done
+
+if [[ ! -f "$ENV_DEFAULT" ]]; then
+	echo "Error: $ENV_DEFAULT not found. Run this script from a clone of the runner." >&2
+	exit 1
+fi
+
+if [[ $EUID -eq 0 ]]; then
+	echo "Warning: you are root. Run the tests as a non-root user (see the Requirements section in the README)." >&2
+fi
+
+# --- Requirements -----------------------------------------------------------
+
+if [[ $CHECK_SOFTWARE -eq 1 ]]; then
+	echo "Checking the required software..."
+	missing=()
+	for cmd in php git rsync wget unzip node npm; do
+		if command -v "$cmd" >/dev/null 2>&1; then
+			echo "  found:   $cmd"
+		else
+			echo "  missing: $cmd"
+			missing+=("$cmd")
+		fi
+	done
+	if command -v composer >/dev/null 2>&1; then
+		echo "  found:   composer"
+	else
+		echo "  not found: composer (optional, prepare.php downloads composer.phar)"
+	fi
+	if [[ ${#missing[@]} -gt 0 ]]; then
+		echo "Error: install the missing software, then run this script again: ${missing[*]}" >&2
+		echo "See the Requirements section in the README." >&2
+		exit 1
+	fi
+	echo "  Node.js $(node --version), npm $(npm --version)."
+	echo "  Use the versions in the \"engines\" field of wordpress-develop's package.json."
+fi
+
+# --- Current values ---------------------------------------------------------
+
+# The variables that this script sets. All other lines of .env.default are kept.
+VARS=(WPT_PREPARE_DIR WPT_TEST_DIR WPT_DB_NAME WPT_DB_USER WPT_DB_PASSWORD WPT_DB_HOST WPT_REPORT_API_KEY WPT_PHP_EXECUTABLE)
+
+# Values from the environment win over the current .env.
+for var in "${VARS[@]}"; do
+	if [[ -n "${!var+set}" ]]; then
+		printf -v "ENV_$var" '%s' "${!var}"
+	fi
+done
+if [[ -f "$ENV_FILE" ]]; then
+	# shellcheck source=/dev/null
+	source "$ENV_FILE"
+fi
+for var in "${VARS[@]}"; do
+	env_name="ENV_$var"
+	if [[ -n "${!env_name+set}" ]]; then
+		printf -v "$var" '%s' "${!env_name}"
+	fi
+done
+
+# Defaults for a local run. WPT_TEST_DIR must be the same as WPT_PREPARE_DIR when
+# the tests run on this server (check_required_env() in functions.php).
+WPT_PREPARE_DIR="${WPT_PREPARE_DIR:-/tmp/wp-test-runner}"
+WPT_TEST_DIR="${WPT_TEST_DIR:-$WPT_PREPARE_DIR}"
+WPT_DB_HOST="${WPT_DB_HOST:-localhost}"
+WPT_PHP_EXECUTABLE="${WPT_PHP_EXECUTABLE:-php}"
+
+# ask VAR "Question" [secret]
+ask() {
+	local var="$1" question="$2" secret="${3:-}" current="${!1:-}" shown answer
+	shown="$current"
+	if [[ -n "$secret" && -n "$current" ]]; then
+		shown="(hidden)"
+	fi
+	if [[ -n "$secret" && -t 0 ]]; then
+		read -r -s -p "$question [$shown]: " answer
+		echo
+	else
+		read -r -p "$question [$shown]: " answer
+	fi
+	if [[ -n "$answer" ]]; then
+		printf -v "$var" '%s' "$answer"
+	fi
+}
+
+if [[ $INTERACTIVE -eq 1 ]]; then
+	echo
+	echo "Enter the values for .env. Press Enter to keep the value in brackets."
+	echo "WARNING: the test suite deletes and creates tables in this database. Use a database that is only for the tests."
+	ask WPT_DB_NAME "Database name"
+	ask WPT_DB_USER "Database user"
+	ask WPT_DB_PASSWORD "Database password" secret
+	ask WPT_DB_HOST "Database host (host, host:port or localhost:/path/to/socket)"
+	ask WPT_PHP_EXECUTABLE "PHP executable"
+	ask WPT_PREPARE_DIR "Directory for the WordPress checkout"
+	WPT_TEST_DIR="$WPT_PREPARE_DIR"
+	echo "To report results to make.wordpress.org, enter the bot user and application password as user:password."
+	echo "Leave it empty to run the tests without reporting."
+	ask WPT_REPORT_API_KEY "Report API key" secret
+fi
+
+for var in WPT_DB_NAME WPT_DB_USER WPT_DB_HOST; do
+	if [[ -z "${!var:-}" ]]; then
+		echo "Error: $var is empty." >&2
+		exit 1
+	fi
+done
+
+# --- Database connection ----------------------------------------------------
+
+if [[ $CHECK_DB -eq 1 ]]; then
+	echo
+	echo "Testing the database connection..."
+	read -r -a PHP <<< "$WPT_PHP_EXECUTABLE"
+	if ! WPT_RUNNER_FUNCTIONS="$RUNNER_DIR/functions.php" WPT_DB_NAME="$WPT_DB_NAME" WPT_DB_USER="$WPT_DB_USER" WPT_DB_PASSWORD="${WPT_DB_PASSWORD:-}" WPT_DB_HOST="$WPT_DB_HOST" \
+		"${PHP[@]}" -r '
+			require getenv( "WPT_RUNNER_FUNCTIONS" );
+			if ( ! extension_loaded( "mysqli" ) ) {
+				fwrite( STDERR, "The mysqli PHP extension is not loaded.\n" );
+				exit( 1 );
+			}
+			$db = wpt_runner_parse_db_host( getenv( "WPT_DB_HOST" ) );
+			if ( false === $db ) {
+				fwrite( STDERR, "WPT_DB_HOST is not valid.\n" );
+				exit( 1 );
+			}
+			mysqli_report( MYSQLI_REPORT_OFF );
+			$link = @mysqli_connect( $db["host"], getenv( "WPT_DB_USER" ), getenv( "WPT_DB_PASSWORD" ), getenv( "WPT_DB_NAME" ), $db["port"] ? $db["port"] : 0, $db["socket"] ? $db["socket"] : "" );
+			if ( ! $link ) {
+				fwrite( STDERR, mysqli_connect_error() . "\n" );
+				exit( 1 );
+			}
+			echo "  Connected to " . mysqli_get_server_info( $link ) . " with PHP " . PHP_VERSION . ".\n";
+		' 2>&1; then
+		echo "Error: cannot connect to the database. .env was not changed." >&2
+		echo "Check the values, or use --skip-db-check to save them anyway." >&2
+		exit 1
+	fi
+fi
+
+# --- Write .env -------------------------------------------------------------
+
+if [[ -f "$ENV_FILE" ]]; then
+	backup="$ENV_FILE.bak-$(date +%Y%m%d%H%M%S)"
+	cp -p "$ENV_FILE" "$backup"
+	echo "Saved the old .env as $backup"
+fi
+
+# Quote a value for a shell file: 'value', with each ' written as '\''.
+shell_quote() {
+	printf "'%s'" "$(printf '%s' "$1" | sed "s/'/'\\\\''/g")"
+}
+
+tmp="$(mktemp "$RUNNER_DIR/.env.XXXXXX")"
+chmod 600 "$tmp"
+while IFS= read -r line || [[ -n "$line" ]]; do
+	replaced=0
+	for var in "${VARS[@]}"; do
+		if [[ "$line" == "export $var="* ]]; then
+			printf 'export %s=%s\n' "$var" "$(shell_quote "${!var:-}")"
+			replaced=1
+			break
+		fi
+	done
+	if [[ $replaced -eq 0 ]]; then
+		printf '%s\n' "$line"
+	fi
+done < "$ENV_DEFAULT" > "$tmp"
+mv "$tmp" "$ENV_FILE"
+
+echo "Saved $ENV_FILE (readable only by you)."
+echo
+echo "Next steps:"
+echo "  1. Run the tests once:"
+if [[ -f "$RUNNER_DIR/host-tools/testrunner.sh" ]]; then
+	echo "       $RUNNER_DIR/host-tools/testrunner.sh"
+else
+	echo "       cd $RUNNER_DIR && source .env && php prepare.php && php test.php; php report.php; php cleanup.php"
+fi
+echo "  2. Schedule the run, for example with cron. See \"Automatic running\" in the README."
+if [[ -z "${WPT_REPORT_API_KEY:-}" ]]; then
+	echo "  3. To report results, create a bot user. See https://make.wordpress.org/hosting/handbook/tests/"
+fi