watchexec — Run Commands When Files Change, with Smart Defaults

What it is

watchexec is a cross-platform file watcher that runs a command whenever files in a directory change. Written in Rust, it provides smart defaults: respecting .gitignore, debouncing rapid changes, and restarting long-running processes automatically.

watchexec is ideal for developers who want instant feedback loops during development -- rerunning tests, rebuilding projects, or restarting servers on save without configuring complex watch tools.

How it saves time or tokens

watchexec eliminates the manual cycle of saving a file, switching to a terminal, and rerunning a command. It handles the edge cases that simple watch scripts miss: debouncing multiple rapid file saves into a single run, ignoring build artifacts via .gitignore, and cleanly restarting long-running processes rather than spawning duplicates.

How to use

1. Install watchexec:

# macOS
brew install watchexec
# Rust
cargo install watchexec-cli --locked

1. Run tests on any file change:

watchexec -- cargo test

1. Watch specific file types:

watchexec -e ts,tsx -- npm run build

1. Restart a server on changes:

watchexec -r -- ./my-server

Example

# Clear screen between runs and run pytest
watchexec -c -- pytest

# Watch only src/ directory, restart on change
watchexec -w src -r -- python app.py

# Filter by extension and ignore patterns
watchexec -e py -i '__pycache__' -- python -m myapp

# Run with shell features (pipes, redirects)
watchexec -- 'cargo build 2>&1 | head -20'

Related on TokRepo

• Automation tools — file watching and workflow automation
• AI coding tools — development productivity tools

Common pitfalls

• Forgetting the -- separator between watchexec flags and the command to run
• Not using -r for long-running processes, which causes multiple instances to stack up
• Watching too broad a directory tree, triggering runs from build output changes