diff --git a/download.sh b/download.sh
index 0494129..4e941a1 100755
--- a/download.sh
+++ b/download.sh
@@ -43,7 +43,14 @@ LOCK_HELD=0
 
 lock() {
   log "Creating lock… ($LOCK_FILE)"
-  exec ${LOCK_FD}>"$LOCK_FILE" || die "Could not open lock file $LOCK_FILE"
+  # Use a literal FD number for portability: exec 200>"$LOCK_FILE"
+  exec 200>"$LOCK_FILE" || die "Could not open lock file $LOCK_FILE"
+  if flock -n 200; then
+    LOCK_HELD=1
+  else
+    die "Lock failed — another process is running."
+  fi
+}>"$LOCK_FILE" || die "Could not open lock file $LOCK_FILE"
   if flock -n ${LOCK_FD}; then
     LOCK_HELD=1
   else
@@ -54,7 +61,12 @@ lock() {
 unlock() {
   if (( LOCK_HELD )); then
     log "Releasing lock…"
-    flock -u ${LOCK_FD} || true
+    flock -u 200 || true
+    # Close the FD to avoid lingering descriptors
+    exec 200>&- || true
+    LOCK_HELD=0
+  fi
+} || true
     # Close the FD to avoid lingering descriptors
     exec ${LOCK_FD}>&- || true
     LOCK_HELD=0
@@ -351,4 +363,3 @@ fi
 
 # cleanup happens via trap
 exit 0
-  
\ No newline at end of file