129 lines
4.0 KiB
Markdown
129 lines
4.0 KiB
Markdown
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---
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title: S6
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layout: default
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parent: Init Systems
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nav_order: 4
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---
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# S6
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**S6** is a process supervision suite designed for modularity, security, and performance. It's not just an init system — it's a full ecosystem of tools for service management (`s6-rc`), logging, and system initialization (`s6-linux-init`). Written by Laurent Bercot, it's the most technically sophisticated of the non-systemd inits.
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> S6 is actually three layers:
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> 1. **s6** — core process supervision (svscan, supervise, svc)
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> 2. **s6-rc** — dependency-based service manager
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> 3. **s6-linux-init** — creates the `/sbin/init` binary
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## Philosophy
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S6 takes the daemontools/runit model and extends it with:
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- **Modularity** — every piece is separate and replaceable
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- **Security** — services run with minimal privileges, no ambient authority
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- **Performance** — fully parallel startup, designed for speed
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- **Reliability** — automatic restart, logging, notification
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Unlike runit, s6 has a real dependency-based service manager (`s6-rc`) with declarative service definitions. It's the closest you can get to systemd's feature set without the bloat.
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## Who uses it?
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- **Artix Linux** — available as an alternative init
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- **Chimera Linux** — uses s6 as its init system
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- **Docker images** — s6-overlay is popular for container supervision
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- Embedded systems, minimal Linux builds
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## How it works
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1. Kernel starts `/sbin/init` (created by `s6-linux-init-maker`)
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2. `s6-linux-init` mounts tmpfs, copies early services, execs into `s6-svscan`
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3. `s6-svscan` becomes PID 1 and starts the supervision tree
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4. `rc.init` script launches `s6-rc` with the default runlevel
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5. `s6-rc` resolves dependencies and starts services in order
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## Key concepts
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| Concept | What it means |
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|---------|---------------|
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| `s6-svscan` | PID 1 — root of the supervision tree |
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| `s6-supervise` | Monitors one service directory, restarts on crash |
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| `s6-svc` | Control tool — send commands to services |
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| `s6-rc` | Dependency-based service manager (compiled database) |
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| `s6-log` | Built-in logging with auto-rotation |
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| Service directories | Like runit — each service is a directory with a `run` script |
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| Compilation | `s6-rc` compiles service definitions into a binary database |
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## Basic commands
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```bash
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# Start a service
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s6-svc -u /run/service/sshd
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# Stop a service
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s6-svc -d /run/service/sshd
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# Restart a service
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s6-svc -r /run/service/sshd
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# Check status
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s6-svc -a /run/service/sshd
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# With s6-rc (service manager):
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s6-rc -d change sshd # start
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s6-rc -d stop sshd # stop
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s6-rc -d status sshd # status
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```
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## Example service
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`/etc/s6/sv/sshd/run`:
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```bash
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#!/bin/sh
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exec /usr/bin/sshd -D
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```
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With s6-rc, you also need a service definition file:
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`/etc/s6/rc/sources/sshd/type`:
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```
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oneshot
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```
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`/etc/s6/rc/sources/sshd/up`:
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```bash
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#!/bin/sh
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s6-svc -u /run/service/sshd
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```
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## Comparison with OpenRC
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| | OpenRC | S6 |
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|---|--------|-----|
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| Service format | Shell scripts | `run` scripts + s6-rc definitions |
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| Dependencies | Manual (`need`/`use`) | Automatic via s6-rc |
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| Supervision | No built-in | Yes — full supervision tree |
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| Logging | syslog or separate | Built-in per-service (`s6-log`) |
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| Boot speed | Fast | Very fast |
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| Complexity | Low | Medium-High |
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| Flexibility | Moderate | Very high (modular) |
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## Should you use it?
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Pick S6 if:
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- You want the most powerful non-systemd init
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- You need dependency-based service management
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- You like the idea of a compiled service database
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- You value process supervision and auto-restart
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- You're comfortable with a steeper learning curve
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Stick with OpenRC if:
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- You want something simple and well-documented
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- You need shell-script-based services you can debug easily
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- You don't need process supervision or dependency resolution
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## Learn more
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- [S6 official site](https://skarnet.org/software/s6/)
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- [S6 overview](https://skarnet.org/software/s6/overview.html)
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- [s6-linux-init](https://skarnet.org/software/s6-linux-init/)
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- [Artix S6 page](https://wiki.artixlinux.org/Main/S6)
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- [S6 on Gentoo Wiki](https://wiki.gentoo.org/wiki/S6)
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