When writing code which needs to be built before it can be used, whether that’s a transpile step like ES7 JavaScript to ES5, or a compile step like with Go, you’re likely going to want to do this when a file in your application tree is modified. There are a large number of project and language specific tools which were developed to tackle this problem but did you know that there are system level packages available that you can use across all your projects?

Introducing inotifywait, which is an efficient and easy to use cli tool which uses Linux’s inotify interface to watch for changes to the file system. And fswatch for those on OSX. Most of the language and project specific tools are built using wrappers around these two tools.

If you’re like me and don’t like to add more build tools and layers of abstraction than necessary then you’re probably already using Make to build and develop your application, and you’ll be happy to know that using these tools with it is trivial. Make has no way to know when a file has changed until the next time you run make so because of this many have tried something like the following which will run the build target every 2s.

watch -n 2 make build

Or they will build the loop into the makefile.

.PHONY watch
watch:
  while true; do \
    make build --silent; \
    sleep 1; \
  done

This works, but it’s performing a lot of unneccesary work being run in a loop especially since the file system is able to tell us when a file or directory has been modified using inotify. Instead of automatically looping we wait for a file system event that we’re interested in and then run our build target. In the following code we’re able to create a make target in our makefile which will watch for file changes under our specified directory recursively.

.PHONY watch
watch:
  while true; do \
    inotifywait -qr -e modify -e create -e delete -e move app/src; \
    make build; \
  done

This works by creating an infinite loop which is started when you run the watch target. Before the first loop can finish it hits inotifywait which sets up listeners on all of the files and directories in your app/src directory. These listeners are waiting for any files to be modified, created, deleted, or moved in or out of app/src/…. When a file or directory changes inotifywait lets the loop continue triggering the call to your build target. That target executes in its entirety building only the file(s) which changed (assuming you’ve properly set up your makefile) and then the loop starts again with inotifywait waiting for those files to change again.

Using this technique will allow you to create an easy and efficient file change watcher for your makefile without too many additional tools. If you have any questions or comments leave them in the comments below or mention me on twitter @fromanegg. Thanks for reading!