diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index a633f3f..f5322bc 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -48,6 +48,7 @@ jobs:
         run: |
           sudo apt-get update -qq && sudo apt-get install -y shellcheck
           # Termux scripts use a Termux shebang that shellcheck cannot resolve,
-          # so the shell dialect is named explicitly.
+          # so the shell dialect is named explicitly. --severity=warning keeps
+          # info/style notes from failing the build across shellcheck versions.
           find . -name '*.sh' -not -path './.git/*' -print0 \
-            | xargs -0 -r shellcheck --shell=bash --external-sources
+            | xargs -0 -r shellcheck --shell=bash --severity=warning --external-sources
diff --git a/install.sh b/install.sh
index 6ba6eb3..4ed1a81 100755
--- a/install.sh
+++ b/install.sh
@@ -8,7 +8,9 @@ die() { printf 'error: %s\n' "$*" >&2; exit 1; }
 
 # --- preflight -------------------------------------------------------------
 # Fail early with a clear message rather than halfway through a package install.
-[ -n "${PREFIX:-}" ] && [ -d "$PREFIX" ] || die "this must run inside Termux (\$PREFIX not set)"
+if [ -z "${PREFIX:-}" ] || [ ! -d "$PREFIX" ]; then
+  die "this must run inside Termux (\$PREFIX not set)"
+fi
 case "$(uname -m)" in
   aarch64|arm64) : ;;
   *) die "built for aarch64 phones; this device is $(uname -m)" ;;