Fix unbound variable error

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## [Unreleased]
 
+- Fix "BUILD_DIR: unbound variable" error when deploying without a Gemfile with `heroku/ruby` buildpack [#1663](https://github.com/heroku/heroku-buildpack-ruby/pull/1663)
 
 ## [v328] - 2025-11-17
 

bin/detect

@@ -3,21 +3,25 @@
 set -euo pipefail
 
 APP_DIR=$1
+BIN_DIR=$(cd "$(dirname "$0")" || exit; pwd) # absolute path
 
 if [ -f "$APP_DIR/Gemfile" ]; then
   echo "Ruby"
   exit 0
 fi
 
+# shellcheck source=bin/support/bash_functions.sh
+source "$BIN_DIR/support/bash_functions.sh"
+
 output::error <<EOF
 Error: Your app is configured to use the Ruby buildpack,
-but we couldn't find any supported Ruby project files ('Gemfile.lock').
+but we couldn't find any supported Ruby project files ('Gemfile' and 'Gemfile.lock').
 
-A Ruby app on Heroku must have a 'Gemfile.lock' in the root directory of its source code.
+A Ruby app on Heroku must have a 'Gemfile' and 'Gemfile.lock' in the root directory of its source code.
 
 Currently the root directory of your app contains:
 
-$(ls -1A --indicator-style=slash "${BUILD_DIR}" || true)
+$(ls -1A --indicator-style=slash "${APP_DIR}" || true)
 
 If your app already has a package manager file, check that it:
 

bin/support/bash_functions.sh

@@ -2,14 +2,36 @@
 
 set -euo pipefail
 
+ANSI_RED='\033[1;31m'
+ANSI_RESET='\033[0m'
+
+# Output a styled multi-line error message to stderr.
+#
+# Usage:
+# ```
+# output::error <<-EOF
+# 	Error: The error summary.
+#
+# 	Detailed description.
+# EOF
+# ```
+function output::error() {
+	local line
+	echo >&2
+	while IFS= read -r line; do
+		echo -e "${ANSI_RED} !     ${line}${ANSI_RESET}" >&2
+	done
+	echo >&2
+}
+
 curl_retry_on_18() {
   local ec=18;
   local attempts=0;
   while (( ec == 18 && attempts++ < 3 )); do
     curl "$@" # -C - would return code 33 if unsupported by server
     ec=$?
   done
-  return $ec
+  return "$ec"
 }
 
 which_java()
@@ -28,7 +50,7 @@ detect_needs_java()
 
   if which_java; then
     build_data::kv_string "java_origin" "previously_installed"
-    return $skip_java_install
+    return "$skip_java_install"
   fi
 
   grep "(jruby " "$gemfile_lock" --quiet &> /dev/null

spec/hatchet/bugs_spec.rb

@@ -25,4 +25,16 @@
       end
     end
   end
+
+  it "detect fails when no Gemfile is present" do
+    Hatchet::Runner.new("default_ruby", allow_failure: true).tap do |app|
+      app.before_deploy do
+        FileUtils.rm("Gemfile")
+      end
+      app.deploy do |app|
+        expect(app.output).to include("A Ruby app on Heroku must have a 'Gemfile' and 'Gemfile.lock' in the root directory of its source code.")
+        expect(app).not_to be_deployed
+      end
+    end
+  end
 end