use locking by mkdir

update

@@ -4,6 +4,8 @@
 CSV_URL="https://www8.cao.go.jp/chosei/shukujitsu/syukujitsu.csv"
 
 # Internal configuration settings
+LOCK_DIR="lock"
+
 TMP_CSV_FILE="holidays-jp-tmp.csv"
 RAW_CSV_FILE="holidays-jp-raw.csv"
 CSV_FILE="holidays-jp.csv"
@@ -21,6 +23,12 @@ if ! command -v nkf >/dev/null; then
   exit $EXIT_FAILURE
 fi
 
+# Acquire the lock or exit
+if ! mkdir "$LOCK_DIR" 2>/dev/null; then
+  echo "Another process is holding the lock. Exiting."
+  exit $EXIT_FAILURE
+fi
+
 # Run
 curl -sS -L -o $TMP_CSV_FILE $CSV_URL
 if ! diff -q $RAW_CSV_FILE $TMP_CSV_FILE >/dev/null 2>&1; then
@@ -30,5 +38,8 @@ else
   rm $TMP_CSV_FILE
 fi
 
+# Release the lock
+rmdir "$LOCK_DIR"
+
 # Exit
 exit $EXIT_SUCCESS