entr

A lightweight command‑line utility watches files for changes and runs a user‑specified command when a modification is detected. It relies on the operating system’s event notification interfaces—kqueue on BSD/macOS and inotify on Linux—so it avoids continuous polling and reacts promptly to filesystem events.

Typical use cases include rebuilding a project after source edits, restarting a development server, or re‑executing a query whenever a script file is updated. The tool can be invoked with simple pipelines, for example feeding a list of files to `entr` and specifying the command with `-s`, `-r`, `-c`, or `-d` options to control execution, reloading, and directory monitoring behavior.

`entr` runs on BSD, macOS, and Linux, with optional environment variables to adjust symlink handling and to work around incomplete inotify support in WSL or Docker for Mac. It is distributed as a stable, CLI‑only package and can be built from source using the standard `./configure && make && make install` workflow.