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Introduction

During development, it's common to want to monitor file changes and automatically run a specific command when a change is detected.
In this article, I’ll introduce a useful tool for that purpose: the entr command.

What is the entr Command?

entr is a UNIX utility that monitors file changes and executes a specified command when a change is detected.
It reads a list of files from standard input and executes the given command if any of those files are modified.

Official Site
https://eradman.com/entrproject/

GitHub
https://github.com/eradman/entr

Installation

On macOS, you can use Homebrew, and on Debian-based systems, you can install it with apt.

# macOS (Homebrew)
brew install entr

# Ubuntu/Debian
apt install entr

Basic Usage

entr reads a list of files from standard input and monitors them for changes.

# Basic usage
echo "filename" | entr command

# Example: monitor changes to hoge.txt and run echo
## Terminal A
$ ls
hoge.txt
$ echo abc > hoge.txt

## Terminal B
$ echo hoge.txt | entr echo fuga
fuga
# After running 'echo abc > hoge.txt' in Terminal A,
# 'echo fuga' is executed and printed again.
fuga

Practical Use Cases

Run Tests When Source or Test Files Change

# Monitor test files and run tests
find . -name "*.test.js" | entr npm test

# Monitor TypeScript files and build
find src -name "*.ts" | entr npm run build

# Monitor config file changes
echo "config.yml" | entr -r ./restart-service.sh

Using find is common here, especially when you want to build code while watching for changes.
I found entr when I wanted to combine it with the sbt-native-packager's sbt stage command.

Useful Options

-r: Restart the process

# Start the server and restart it on file change
find . -name "*.js" | entr -r node server.js

-c: Clear the terminal before execution

# Clear terminal before running tests
find . -name "*.go" | entr -c go test ./...

-d: Watch for directory changes as well

# Monitor directory changes
find . -type d | entr -d echo "Directory changed"

Conclusion

The entr command is a simple yet powerful tool that automatically runs commands when files change.
It's extremely helpful for automating tasks like test execution, builds, or server restarts during development.
With its minimalistic design and ease of use, it’s an excellent choice when no other tool fits your watch-and-run use case.

This article is a translated version of my original Japanese post.

https://zenn.dev/ara_ta3/articles/entr-watch-files-and-run-commands