entr — Run Arbitrary Commands When Files Change

Introduction

entr takes a list of filenames on stdin and re-runs a command whenever any of them changes. It's a 1000-line C program with one job, and it composes beautifully with find, ls, fd, and ripgrep. If you've ever written a bespoke fswatch | xargs loop, entr is probably a cleaner replacement.

What entr Does

• Uses kqueue (BSD/macOS) or inotify (Linux) to watch a file set.
• Re-executes a command when any watched file changes.
• -r kills a long-running child before restart (server reload).
• -c clears the screen between runs for clean test output.
• -s invokes via shell to support pipelines and redirection.

Architecture Overview

entr reads the file list from stdin once, opens OS-level file descriptors with kqueue/inotify, and blocks on events. On any CHANGE/DELETE, it either execs the command (default) or signals the child with SIGTERM/SIGKILL (-r mode) and re-execs. No daemon, no config file.

Self-Hosting & Configuration

• Install via brew, apt, dnf, pkg.
• ENTR_INOTIFY_WORKAROUND=1 handles editors that write via replace (VSCode/Vim).
• No config files; everything is flags.
• Plays nicely in Docker if you mount the source tree.

Key Features

• Zero dependencies, tiny binary.
• Works with any command — tests, builds, servers, rsync.
• -r gives you a poor-person's nodemon/air for any language.
• Composable: fd . src | entr -c go test ./....
• BSD-friendly; runs on OpenBSD, FreeBSD, macOS, Linux.

Comparison with Similar Tools

• nodemon — Node-specific; heavier.
• air — Go-specific reload.
• watchexec — Rust; auto-discovers files, more features.
• modd — YAML config; richer dev-server orchestration.
• fswatch — emits events; pairs with xargs — entr replaces the pipeline.

FAQ

Q: Why does my editor not trigger a reload? A: Atomic writes — set ENTR_INOTIFY_WORKAROUND=1.

Q: Max file count? A: OS-limited (inotify max_user_watches, kqueue limits).

Q: Works in Docker? A: Yes if inotify is available and the mount propagates events.

Q: Can I pass multiple commands? A: Use -s "cmd1 && cmd2".

Sources

• http://eradman.com/entrproject/
• https://github.com/eradman/entr