Watching for File Changes

Although it is not entirely stable, the fs module provides a useful tool for watching a file and executing a callback function when the file changes. This can be useful if you want to trigger events to occur when a file is modified but do not want to continually poll from your application directly. Watches do incur some overhead in the underlying operating system, so you should use them sparingly.

To implement a watch on a file, use the following command, passing the path to the file you want to watch:

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fs.watchFile(path, [options], callback)

You can also pass in options, which is an object that contains persistent and interval properties. The persistent property is true if you want the process to continue to run as long as files are being watched. The interval property specifies the time, in milliseconds, that you want the file to be polled for changes.

When a file change occurs, the callback function is executed and passed current and previous Stats objects.

The following code snippet monitors a file named log.txt every 5 seconds and uses the Stats object to output the current and previous times the file was modified:

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fs.watchFile("log.txt", {persistent:true, interval:5000}, function (curr, prev) {
  console.log("log.txt modified at: " + curr.mtime);
  console.log("Previous modification was: " + prev.mtime);
});