# entr — Run Arbitrary Commands When Files Change

> entr is a tiny, dependency-free event notify-based tool for running commands when any file in a list changes, perfect for live-reload workflows with tests, linters, and builds.

## Install

Save as a script file and run:

# entr — Run Arbitrary Commands When Files Change

## Quick Use
```bash
brew install entr
# Re-run tests on save
ls *.py | entr -c pytest -x
# Clear screen + rebuild + restart server
find src -name "*.go" | entr -r -c go run ./cmd/server
# Run on any file change in repo (BSD find)
find . -type f | entr make test
```

## 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

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Source: https://tokrepo.com/en/workflows/bd0e9cda-389d-11f1-9bc6-00163e2b0d79
Author: Script Depot