File Watching for Much Win

Throughout my career as a developer I’ve used lots of different file watching tools. It’s really handy to have something invoke your code every time you save the file. Some of them work well, but most do not. I recently discovered fswatch, and I couldn’t be happier.

fswatch is a binary you can install with homebrew. Just run brew install fswatch. It’s a pretty low level tool, but using the right combination of pipes and other unix commands, you can achieve pretty much any task you need file watching for. I had quite a bit of trouble getting some of the flags working that fswatch supports; perhaps I was misunderstanding how to use them. Below are examples I’ve found to work flawlessly on my Mac.

Examples

fswatch *.go -l 0.2 | xargs -n1 go run

Watch all .go files in the current directory and go run them. The -n1 tells xargs to run for every new file name that gets piped to it. The -l 0.2 makes fswatch decrease the latency while watching. That number feels right to be, but feel free to tweak it to your liking.



fswatch . -l 0.1 | egrep --line-buffered '\.(js)$' | xargs -n1 node

Recursively watch all .js files in the current directory and run them with node.



fswatch . -l 0.1 | egrep --line-buffered '\.(js|jsx)$' | xargs -n1 -I{} echo "a javascript file was saved"

Recursively watch all .js or .jsx files in the current directory and echo every time one is modified. The -I{} allows you to ignore the name of the file that is piped to xargs. In this example we don’t care what specific file was saved.