CLI Tools: entr

Although popular compilers and bundlers like esbuild commonly include --watch, -w commands, not all tools offer a convenient file system observer. Luckily, in true UNIX fashion there’s a great tool I recently discovered that just watches for file changes and runs scripts: entr.

For example (lifted from the documentation), you can replace a package like the popular npm package nodemon with this simple command.

$ ls *.js | entr -r node app.js

The -r flag tells entr to reload the child process any time the output from ls changes, in this case node app.js.

Examples

I’ve recently started playing around with the PlayDate SDK, which compiles Lua code into a package that runs in the PlayDateSimulator app. The pdc utility doesn’t have a watch command, but with entr it easily can.

$ ls *.lua | entr -r pdc Source/ MyGame.pdx

After a file save I just have to hop over to the simulator and hit CMD+R to reload, just like (old school) web development.

Go, Rust or Zig all come with testing libraries, but if you’re used to popular JavaScript libraries like Jest or Vitest, having a --watch flag is a nice feature. With entr it’s an easy add:

$ find *.go | entr -r go test
$ find src/*.rs | entr -r cargo test

Wrapping Up

I really enjoy little utilities like entr. It isn’t a drop-in replacement for great, fully featured libraries like air or bacon, but for smaller scale projects it’s a wonderful tool.

Notes

• Use -rz if you want to exit entr after a process has completed.