diff --git a/.github/workflows/lint.yml b/.github/workflows/lint.yml
new file mode 100644
index 0000000..d2e5373
--- /dev/null
+++ b/.github/workflows/lint.yml
@@ -0,0 +1,41 @@
+name: Lint
+
+# theta-env has no app code of its own to unit-test (it orchestrates the
+# proxy/sso-manager-node submodules) -- this checks the one thing that can
+# actually break silently: setup.sh and bootstrap.js.
+on:
+  pull_request:
+    branches:
+      - master
+  push:
+    branches-ignore:
+      - master
+
+jobs:
+  shellcheck:
+    name: Shellcheck setup.sh
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Syntax check
+        run: bash -n setup.sh
+
+      - name: Shellcheck
+        run: shellcheck -S warning setup.sh
+
+  bootstrap-syntax:
+    name: Syntax check bootstrap.js
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: 22.x
+
+      - name: Syntax check
+        run: node --check bootstrap/bootstrap.js
diff --git a/setup.sh b/setup.sh
index 1b7651a..3ea1355 100755
--- a/setup.sh
+++ b/setup.sh
@@ -558,7 +558,7 @@ backup_before_rebuild() {
 		# Only prune real backup dirs — skip symlinks (a stray symlink could
 		# point rm at an arbitrary tree) and non-dir entries.
 		[[ -d "$BACKUP_DIR/$old" && ! -L "$BACKUP_DIR/$old" ]] || continue
-		rm -rf "$BACKUP_DIR/$old" || true
+		rm -rf "${BACKUP_DIR:?}/$old" || true
 		removed=$((removed + 1))
 	done < <(ls -1 "$BACKUP_DIR" 2>/dev/null | sort -r | tail -n +$((keep + 1)))
 	[[ "$removed" -gt 0 ]] && info "  pruned $removed old backup(s) (keeping $keep)."
@@ -572,7 +572,8 @@ backup_before_rebuild
 # hash from inside the Docker build context. Resolve it on the host (where
 # the submodule DOES resolve correctly) and pass it in as a build arg; see
 # docker-compose.yml and sso-manager-node's Dockerfile.openldap.
-export SSO_GIT_COMMIT="$(git -C sso-manager-node rev-parse --short HEAD 2>/dev/null || echo unknown)"
+SSO_GIT_COMMIT="$(git -C sso-manager-node rev-parse --short HEAD 2>/dev/null || echo unknown)"
+export SSO_GIT_COMMIT
 info "Building + starting sso-manager (first run builds the image; this takes a while)..."
 "${COMPOSE[@]}" up -d --build sso-manager
 
@@ -629,7 +630,6 @@ BOOTSTRAP_OUT=$("${COMPOSE[@]}" exec -T sso-manager node /bootstrap/bootstrap.js
 
 getval() { echo "$BOOTSTRAP_OUT" | grep -m1 "^$1=" | cut -d= -f2-; }
 CLIENT_ID=$(getval CLIENT_ID)
-CLIENT_SECRET=$(getval CLIENT_SECRET)
 ALREADY_CONFIGURED=$(getval ALREADY_CONFIGURED)
 [[ -n "$CLIENT_ID" ]] || die "bootstrap did not return CLIENT_ID:\n${BOOTSTRAP_OUT}"
 
@@ -641,7 +641,8 @@ fi
 
 # ── 6. Start the proxy, wait for health ───────────────────────────────────────
 # PROXY_GIT_COMMIT: same reasoning as SSO_GIT_COMMIT above.
-export PROXY_GIT_COMMIT="$(git -C proxy rev-parse --short HEAD 2>/dev/null || echo unknown)"
+PROXY_GIT_COMMIT="$(git -C proxy rev-parse --short HEAD 2>/dev/null || echo unknown)"
+export PROXY_GIT_COMMIT
 info "Building + starting proxy (first run builds the image; this takes a while)..."
 "${COMPOSE[@]}" up -d --build proxy