fswatch

fswatch is an open-source, cross-platform file change monitor that watches files and directories for changes and reports them to the user or to a running script. It provides a uniform interface across operating systems by using platform-specific backends such as inotify on Linux, FSEvents on macOS, kqueue on BSD variants, and ReadDirectoryChangesW on Windows.

It is designed for automation, build systems, deployment workflows, and development tasks where actions should be triggered when source files are edited or new files are added. fswatch can watch individual files or entire directory trees, and it can operate recursively.

Its features include filtering and path matching via include and exclude patterns, reporting of events such as creation, modification, deletion, and renaming, and control over the format and rate of output. The tool can emit events to stdout, and can also execute commands or scripts in response to changes, often through piping or shell integration. Output can be batched or delimited, and some options allow null-delimited output for safe parsing by other programs.

Architecture and usage: fswatch is implemented with a modular backend architecture that delegates to the underlying OS notification mechanism, while exposing a CLI interface as well as a library for other programs. This makes it possible to build custom watchers or integrate fswatch into larger automation pipelines.

See also: inotify, FSEvents, kqueue, Watchman.