diff --git a/.github/workflows/check.yaml b/.github/workflows/check.yaml
index 5e89d02..10c743f 100644
--- a/.github/workflows/check.yaml
+++ b/.github/workflows/check.yaml
@@ -69,7 +69,7 @@ jobs:
           fi
           read -ra packages <<<"$APT_PACKAGES"
           sudo apt-get update
-          sudo apt-get install -y "${packages[@]}"
+          sudo apt-get install -y ${packages[@]+"${packages[@]}"}
       - name: 🐍 Install uv and Python
         uses: astral-sh/setup-uv@07a047e608db84b3d8c06cd9a7c7dcc9c355c18e # v10.3.2
         with:
@@ -91,8 +91,8 @@ jobs:
           while IFS= read -r line; do
             if [[ -n "$line" ]]; then options+=(--exclude-newer-package "$line"); fi
           done <<<"$EXCLUDE_NEWER_PACKAGE"
-          echo "Installing ${spec[*]} with uploads older than $EXCLUDE_NEWER"
-          uv tool install --python-preference only-managed --python 3.15 "${options[@]}" "${spec[@]}"
+          echo "Installing ${spec[*]+"${spec[*]}"} with uploads older than $EXCLUDE_NEWER"
+          uv tool install --python-preference only-managed --python 3.15 "${options[@]}" ${spec[@]+"${spec[@]}"}
       - name: 🐍 Install Python
         id: python
         background: true
diff --git a/changelog.d/+bash32-empty-arrays.bugfix.md b/changelog.d/+bash32-empty-arrays.bugfix.md
new file mode 100644
index 0000000..bdba86d
--- /dev/null
+++ b/changelog.d/+bash32-empty-arrays.bugfix.md
@@ -0,0 +1,3 @@
+Actions and the `check` workflow no longer fail with `unbound variable` on macOS runners when an input leaves an array
+empty, such as the `targets` input of `install-locked-tools`. macOS runs `shell: bash` steps with Bash 3.2, which
+rejects `"${array[@]}"` on an empty array under `set -u`.
diff --git a/cooled-prek/action.yml b/cooled-prek/action.yml
index 809f822..3f94b3d 100644
--- a/cooled-prek/action.yml
+++ b/cooled-prek/action.yml
@@ -14,4 +14,4 @@ runs:
       run: |
         set -euo pipefail
         read -ra arguments <<<"$ARGS"
-        "$GITHUB_ACTION_PATH/../tools/cooled_prek.sh" "${arguments[@]}"
+        "$GITHUB_ACTION_PATH/../tools/cooled_prek.sh" ${arguments[@]+"${arguments[@]}"}
diff --git a/install-locked-tools/action.yml b/install-locked-tools/action.yml
index f7a27c3..b34f14c 100644
--- a/install-locked-tools/action.yml
+++ b/install-locked-tools/action.yml
@@ -50,7 +50,7 @@ runs:
         groups=()
         while IFS= read -r line; do
           read -ra selected <<<"$line"
-          groups+=("${selected[@]}")
+          groups+=(${selected[@]+"${selected[@]}"})
         done <<<"$DEPENDENCY_GROUPS"
         if ((${#groups[@]})); then
           echo "Selected lock groups: ${groups[*]} (from the groups input), installing into $VENV from pylock.toml"
@@ -82,7 +82,7 @@ runs:
         fi
         digest=$(
           sha256sum "$BUILD_LOCK"
-          printf '%s\n' "$TARGET_PLATFORM" "$TARGETS" "${args[@]}"
+          printf '%s\n' "$TARGET_PLATFORM" "$TARGETS" ${args[@]+"${args[@]}"}
         )
         digest=$(sha256sum <<<"$digest")
         output="$RUNNER_TEMP/build-constraints-${digest%% *}.txt"
@@ -90,7 +90,7 @@ runs:
           echo "Reusing build constraints from an earlier step (same lock and targets): $output"
         else
           uv run --no-project --python "$RUNNER_TEMP/lock-reader" python \
-            "$GITHUB_ACTION_PATH/../tools/build_constraints.py" "${args[@]}" "$output" "$BUILD_LOCK"
+            "$GITHUB_ACTION_PATH/../tools/build_constraints.py" ${args[@]+"${args[@]}"} "$output" "$BUILD_LOCK"
         fi
         echo "path=$output" >>"$GITHUB_OUTPUT"
     - name: 📦 Install selected groups
diff --git a/publish-post-release-pr/action.yml b/publish-post-release-pr/action.yml
index 1f752cb..3ccd94f 100644
--- a/publish-post-release-pr/action.yml
+++ b/publish-post-release-pr/action.yml
@@ -41,7 +41,7 @@ runs:
         while IFS= read -r path || [[ -n "$path" ]]; do
           trusted_paths+=("$path")
         done < <(printf '%s' "$ALLOWED_PATHS")
-        printf '%s\n' "${trusted_paths[@]}" | LC_ALL=C sort -u >"$RUNNER_TEMP/allowed-paths"
+        printf '%s\n' ${trusted_paths[@]+"${trusted_paths[@]}"} | LC_ALL=C sort -u >"$RUNNER_TEMP/allowed-paths"
         paths=()
         while IFS=$'\t' read -r path blob; do
           case "$path" in
@@ -51,7 +51,8 @@ runs:
             ;;
           esac
           if ! grep -Fqx -- "$path" "$RUNNER_TEMP/allowed-paths"; then
-            echo "::error::prepared path $path is not in the writer's allowed-paths: ${trusted_paths[*]}" >&2
+            echo "::error::prepared path $path is not in the writer's allowed-paths:" \
+              "${trusted_paths[*]+"${trusted_paths[*]}"}" >&2
             exit 1
           fi
           case "$blob" in
diff --git a/refresh-release-lock/action.yml b/refresh-release-lock/action.yml
index 17bc3f7..496aa7e 100644
--- a/refresh-release-lock/action.yml
+++ b/refresh-release-lock/action.yml
@@ -83,7 +83,8 @@ runs:
           ;;
         esac
         cp pylock.toml "$RUNNER_TEMP/lock-before.toml"
-        uv run --no-project --python "$RUNNER_TEMP/lock-tools" nab lock --all-groups --all-extras --no-cache "${refresh[@]}"
+        uv run --no-project --python "$RUNNER_TEMP/lock-tools" nab lock --all-groups --all-extras --no-cache \
+          ${refresh[@]+"${refresh[@]}"}
         if [[ "$MODE" == check ]]; then
           python3 "$GITHUB_ACTION_PATH/../tools/check_nab_lock.py" "$RUNNER_TEMP/lock-before.toml" pylock.toml
         fi
@@ -110,15 +111,15 @@ runs:
         selected_groups=()
         while IFS= read -r line; do
           read -ra words <<<"$line"
-          platforms+=("${words[@]}")
+          platforms+=(${words[@]+"${words[@]}"})
         done <<<"$MATRIX_PLATFORMS"
         while IFS= read -r line; do
           read -ra words <<<"$line"
-          implementations+=("${words[@]}")
+          implementations+=(${words[@]+"${words[@]}"})
         done <<<"$MATRIX_IMPLEMENTATIONS"
         while IFS= read -r line; do
           read -ra words <<<"$line"
-          selected_groups+=("${words[@]}")
+          selected_groups+=(${words[@]+"${words[@]}"})
         done <<<"$ADDITIONAL_GROUPS"
         case "$ADDITIONAL_SELECTION" in
         project)
@@ -134,14 +135,15 @@ runs:
           fi
           ;;
         esac
-        echo "Secondary lock $ADDITIONAL_LOCK: $description for Python $MATRIX_PYTHON on ${platforms[*]}" \
-          "with ${implementations[*]}, ${MODE} mode"
+        echo "Secondary lock $ADDITIONAL_LOCK: $description for Python $MATRIX_PYTHON" \
+          "on ${platforms[*]+"${platforms[*]}"} with ${implementations[*]+"${implementations[*]}"}, ${MODE} mode"
         cp "$ADDITIONAL_LOCK" "$RUNNER_TEMP/additional-lock-before.toml"
-        uv run --no-project --python "$RUNNER_TEMP/lock-tools" nab lock --no-cache pyproject.toml "${selection[@]}" \
+        uv run --no-project --python "$RUNNER_TEMP/lock-tools" nab lock --no-cache pyproject.toml \
+          ${selection[@]+"${selection[@]}"} \
           --project-matrix-python "$MATRIX_PYTHON" \
-          --project-matrix-platforms "${platforms[@]}" \
-          --project-matrix-implementations "${implementations[@]}" \
-          --output "$ADDITIONAL_LOCK" "${refresh[@]}"
+          --project-matrix-platforms ${platforms[@]+"${platforms[@]}"} \
+          --project-matrix-implementations ${implementations[@]+"${implementations[@]}"} \
+          --output "$ADDITIONAL_LOCK" ${refresh[@]+"${refresh[@]}"}
         if [[ "$MODE" == check ]]; then
           python3 "$GITHUB_ACTION_PATH/../tools/check_nab_lock.py" \
             "$RUNNER_TEMP/additional-lock-before.toml" "$ADDITIONAL_LOCK"
diff --git a/select-previous-tag/action.yml b/select-previous-tag/action.yml
index a855e06..2fb8910 100644
--- a/select-previous-tag/action.yml
+++ b/select-previous-tag/action.yml
@@ -20,7 +20,7 @@ runs:
         set -euo pipefail
         args=()
         if [[ "$TAG" == */* ]]; then args+=(--package "${TAG%%/*}"); fi
-        previous=$(git tag --list | release-creator tags "$TAG" --previous "${args[@]}")
+        previous=$(git tag --list | release-creator tags "$TAG" --previous ${args[@]+"${args[@]}"})
         if [[ -n "$previous" ]]; then
           echo "Previous release tag: $previous (newest release tag before $TAG${args[*]:+ for package ${TAG%%/*}})"
         else
diff --git a/tests/bash32_arrays/test_empty_arrays.py b/tests/bash32_arrays/test_empty_arrays.py
new file mode 100644
index 0000000..ea6c2a8
--- /dev/null
+++ b/tests/bash32_arrays/test_empty_arrays.py
@@ -0,0 +1,50 @@
+import re
+from collections.abc import Iterator
+from pathlib import Path
+
+from conftest import WORKFLOWS, at, load_yaml, mapping, step_list, text
+
+
+def test_unguarded_array_expansions_are_provably_non_empty() -> None:
+    # Bash 3.2, the /bin/bash of macOS runners, aborts on "${arr[@]}" of an empty array under set -u. Bash 5 does
+    # not, so CI only sees this through the scan. Each allowed name is an array that cannot be empty where it expands.
+    expected = {
+        (".github/workflows/check.yaml", "options"): "starts with --exclude-newer",
+        ("check-nab-exceptions/action.yml", "files"): "starts with pyproject.toml",
+        ("install-locked-tools/action.yml", "groups"): "filled only when the selection found a group",
+        ("prepare-towncrier/action.yml", "args"): "set to three options before the call",
+        ("publish-github-release/action.yml", "request"): "starts with two -f pairs",
+        ("publish-post-release-pr/action.yml", "paths"): "checked non-empty before any other use",
+        ("refresh-release-lock/action.yml", "paths"): "starts with pylock.toml",
+        ("refresh-release-lock/action.yml", "requirements"): "set to one requirement in every mode",
+        ("refresh-release-lock/action.yml", "selected_groups"): "used only inside the non-empty check",
+        ("render-release-notes/action.yml", "args"): "starts with three options",
+        ("stage-jetbrains-marketplace/action.yml", "args"): "starts with the marketplace subcommand",
+        ("tools/install_live_tools.sh", "options"): "starts with --exclude-newer",
+        ("tools/install_live_tools.sh", "overrides"): "expanded only inside the :+ alternative",
+    }
+    found = {(path, name) for path, script in _run_scripts() for name in _unguarded_names(script)}
+
+    assert found == set(expected)
+
+
+def _run_scripts() -> Iterator[tuple[str, str]]:
+    root = Path(__file__).resolve().parents[2]
+    for action in sorted(root.glob("*/action.yml")):
+        for step in step_list(at(load_yaml(action), "runs", "steps")):
+            if "run" in step:
+                yield action.relative_to(root).as_posix(), text(step["run"])
+    for script in sorted((root / "tools").glob("*.sh")):
+        yield script.relative_to(root).as_posix(), script.read_text()
+    for workflow in sorted(WORKFLOWS.glob("*.y*ml")):
+        for job_value in mapping(at(load_yaml(workflow), "jobs")).values():
+            job = mapping(job_value)
+            if "${{" in text(job.get("runs-on", "")) and "steps" in job:
+                for step in step_list(job["steps"]):
+                    if "run" in step:
+                        yield workflow.relative_to(root).as_posix(), text(step["run"])
+
+
+def _unguarded_names(script: str) -> set[str]:
+    without_guards = re.sub(r'\$\{(\w+)\[[@*]\]\+"\$\{\1\[[@*]\]\}"\}', "", script)
+    return {match.group(1) for match in re.finditer(r"\$\{(\w+)\[[@*]\]\}", without_guards)}
diff --git a/tests/check_workflow/test_check.py b/tests/check_workflow/test_check.py
index bd24653..786f26e 100644
--- a/tests/check_workflow/test_check.py
+++ b/tests/check_workflow/test_check.py
@@ -179,6 +179,13 @@ def test_gate_requires_test_through_required_checks() -> None:
             + ["--exclude-newer", "7 days", "tox", "--with", "tox-uv"],
             id="default",
         ),
+        pytest.param(
+            "",
+            "",
+            ["tool", "install", "--python-preference", "only-managed", "--python", "3.15"]
+            + ["--exclude-newer", "7 days"],
+            id="empty-tox-install",
+        ),
         pytest.param(
             "tox --with .",
             "",
diff --git a/validate-release-patch/action.yml b/validate-release-patch/action.yml
index 9d31826..f259d17 100644
--- a/validate-release-patch/action.yml
+++ b/validate-release-patch/action.yml
@@ -39,7 +39,7 @@ runs:
           if [[ -n "$ALLOWED_PATHS" ]]; then
             echo "Release patch path: $status $path"
             matched=false
-            for pattern in "${patterns[@]}"; do
+            for pattern in ${patterns[@]+"${patterns[@]}"}; do
               # shellcheck disable=SC2053 # The patterns are globs, matched on purpose.
               if [[ "$path" == $pattern ]]; then
                 matched=true