daemontools

daemontools is a collection of small, open-source utilities for supervising and logging services on Unix-like systems. Created by Daniel J. Bernstein (djb) in the early 2000s, it provides a lightweight alternative to traditional init systems by ensuring that services are kept running and that their logs are handled in a consistent, reliable manner. The design emphasizes simplicity, robustness, and predictable failure modes, with a minimal set of concepts and little reliance on shell scripting.

Core components include svscan, supervise, runsv, runsvdir, svstat, svlogd, and multilog. svscan runs as a supervisor process that watches a service directory tree (commonly /service) and starts a per-service supervisor for each entry. Each service is defined by a run script, which should start the daemon and stay foreground via exec so that supervise can monitor it. A log subdirectory can provide logging via svlogd, optionally combined with multilog for buffering and rotation. The per-service supervisor (via runsv) restarts the service if it exits; svstat reports status information.

Usage typically involves creating a service directory with a run script and an optional log/run script, then starting svscan on the directory to enable supervision. Commands such as svc can be used to query service status or to bring services up or down. The approach yields immediate responsiveness and straightforward process lifecycle management, without relying on a custom init system.

History and influence: daemontools originated with Daniel J. Bernstein and has influenced later supervisor suites such as runit and s6. It has been adopted in various Unix-like environments and is distributed under a permissive BSD-style license.