systemd-config

A simple systemd config (de)serializer. It parses and writes unit/config files, and can compute the effective configuration of a unit combined with its drop-ins (see Drop-ins).

This project is highly inspired by go-systemd/unit. The difference is duplicate-section support: unlike go-systemd/unit, systemd-config handles repeated sections such as the [Address] sections in .network files.

Install

go get github.com/javadh75/systemd-config

Usage

package main

import (
	"fmt"
	"log"
	"os"

	systemdconfig "github.com/javadh75/systemd-config"
)

func main() {
	f, err := os.Open("eth0.network")
	if err != nil {
		log.Fatal(err)
	}
	defer f.Close()

	unit, err := systemdconfig.Deserialize(f)
	if err != nil {
		log.Fatal(err)
	}

	// Look up a value (last assignment wins, like systemd, even across
	// duplicate sections).
	if dns, ok := unit.Value("Network", "DNS"); ok {
		fmt.Println("DNS:", dns)
	}

	// Duplicate sections are preserved and addressable.
	for _, addr := range unit.SectionsByName("Address") {
		if v, ok := addr.Value("Address"); ok {
			fmt.Println("address:", v)
		}
	}

	// Edit and write back.
	unit.AddSection("Route").AddOption("Gateway", "10.0.0.1")
	if _, err := unit.WriteTo(os.Stdout); err != nil {
		log.Fatal(err)
	}
}

Drop-ins

Merge computes the effective configuration of a unit combined with its drop-ins (systemctl cat semantics): drop-ins apply in order as if appended, and an empty assignment (ExecStart=) resets every earlier occurrence of that option, as described in systemd.unit(5). Duplicate sections are never collapsed.

base, _ := systemdconfig.Deserialize(baseFile)     // nginx.service
override, _ := systemdconfig.Deserialize(dropFile) // nginx.service.d/override.conf

effective := systemdconfig.Merge(base, override)
cmd, _ := effective.Value("Service", "ExecStart")

Behavior notes

• Duplicate sections and options are preserved in order. Unit.Value follows systemd's last-assignment-wins rule across duplicate sections; Unit.Values returns every occurrence. Section.Value/Section.Values do the same within a single section.
• Comments and blank lines are not preserved: deserializing discards them, so a deserialize/serialize round trip produces a normalized file.
• Continuation lines follow systemd.syntax(7): a line ending in \ is joined with the following non-comment line and the backslash becomes a space. A value therefore cannot end in a backslash — dangling markers are dropped.
• Empty sections survive a round trip ([Install] with no options stays).
• Line length is capped at 1 MiB per line (systemd's LONG_LINE_MAX); longer lines yield ErrLineTooLong.
• Assignments before the first section header are rejected with ErrAssignmentOutsideSection, as in systemd.
• Canonical output: serializing a parsed unit yields a fixpoint — parsing and serializing the output again reproduces it byte for byte (fuzz-tested).

Development

make check     # full quality gate: tidy, fmt, vet, lint, security, tests
make coverage  # coverage report, fails below COVERAGE_MIN (see Makefile)
make hooks     # install pre-commit/pre-push git hooks (lefthook)
make fuzz      # short fuzz run of the deserializer

See CLAUDE.md for the full tooling and testing policy.

License

Apache License 2.0.