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Documentation Index

Fetch the complete documentation index at: https://docs.microsandbox.dev/llms.txt

Use this file to discover all available pages before exploring further.

Before a sandbox starts doing real work, you can prepare it three ways. Scripts bundle reusable commands. Patches modify the rootfs before the VM boots. A custom init system (systemd, OpenRC, s6) can run as PID 1 instead of microsandbox’s minimal agent. All three are defined at creation time and keep the base image untouched.

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Scripts

Scripts are files mounted at /.msb/scripts/ inside the sandbox. The directory is on PATH, so each script is callable by name through exec() or shell(). It provides a clean way to bundle setup procedures or entry points with a sandbox without baking them into the image. 
use indoc::indoc;
use microsandbox::Sandbox;

let sb = Sandbox::builder("worker")
    .image("ubuntu")
    .script("setup", indoc! {"
        #!/bin/bash
        apt-get update && apt-get install -y python3 curl
    "})
    .script("start", indoc! {"
        #!/bin/bash
        exec python3 /app/main.py
    "})
    .create()
    .await?;

sb.shell("setup").await?;
let output = sb.shell("start").await?;

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Patches

Patches modify the rootfs before the VM boots. Write config files, copy directories from the host, create symlinks, append to existing files, remove things you don’t need. The base image stays untouched since patches are written to the writable layer on top. Patches are applied in order and work with OCI images and bind-mounted rootfs. They’re not supported with disk image roots (QCOW2, Raw).
By default, patching a path that already exists in the image will error. Pass replace: true on the operation to allow it. Mkdir and Remove are idempotent and won’t error either way.
use microsandbox::Sandbox;

let sb = Sandbox::builder("worker")
    .image("alpine")
    .patch(|p| p
        .text("/etc/greeting.txt", "Hello from a patched rootfs!\n", None, false)
        .text("/etc/motd", "Custom message of the day.\n", None, true) // replace existing
        .mkdir("/app", Some(0o755))
        .text("/app/config.json", r#"{"debug": true}"#, Some(0o644), false)
        .copy_file("./cert.pem", "/etc/ssl/cert.pem", None, false)
        .append("/etc/hosts", "127.0.0.1 myapp.local\n")
    )
    .create()
    .await?;

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Available operations

The patch builder appends operations in the order you call them; calls are chainable. Available operations across SDKs: text, file, mkdir, append, copyFile / copy_file, copyDir / copy_dir, symlink, remove. Full per-language signatures, parameters, and option shapes are documented in the SDK references:

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Custom init system

By default the microsandbox agent runs as PID 1 inside the guest: small, fast, minimal. For workloads that expect a real init (systemd, OpenRC, s6, runit, etc.), --init hands PID 1 over to the init binary of your choice. The handoff sequence:
  • Agent does the boot-time setup: mount filesystems, configure network, prepare runtime dirs.
  • Agent forks.
  • Parent execs your init and becomes PID 1.
  • Child agent continues serving host requests over the same channel.
Common reasons to opt in: long-lived daemons, system service tests, anything that talks to dbus or expects systemctl to work. The simplest entry point is auto, which probes a small list of well-known paths inside the guest rootfs and picks the first one that exists:
  • /sbin/init
  • /lib/systemd/systemd
  • /usr/lib/systemd/systemd
Pass an absolute path instead when you need pinning (e.g. for reproducible CI). 
use microsandbox::Sandbox;

let sb = Sandbox::builder("worker")
    .image("ghcr.io/superradcompany/debian-systemd:12")
    .memory(1024)
    .cpus(2)
    .init("auto")
    .create()
    .await?;
To verify the handoff worked, check /proc/1/comm inside the sandbox. It should print the init’s name (systemd, init, etc.): 
$ msb run ghcr.io/superradcompany/debian-systemd:12 --init auto -- cat /proc/1/comm
systemd
If --init=auto can’t find anything in its candidate list, agentd fails boot with a clear error in kernel.log listing every path it checked. Switch to an explicit path (--init=/lib/systemd/systemd) when you know exactly where the init lives, or follow the image-picking guidance below to choose an image that actually ships one.

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Argv and env

Pass extra argv and env to the init:
  • Rust / TypeScript: init_with(...) / initWith(...).
  • Python: pass an InitConfig to init=.
  • CLI: repeat --init-arg once per entry, and --init-env KEY=VAL once per env var.
Argv defaults to [<cmd>] when none is given; env is merged on top of the inherited environment. 
let sb = Sandbox::builder("worker")
    .image("ghcr.io/superradcompany/debian-systemd:12")
    .init_with("/lib/systemd/systemd", |i| i
        .args(["--unit=multi-user.target"])
        .env("container", "microsandbox"))
    .create()
    .await?;

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Picking an image

Most slim Docker base images (debian:bookworm-slim, ubuntu:24.04, python:3.12-slim) are stripped of init binaries; they’re built for “one process per container” and don’t ship systemd at all. If you point --init at a path the image doesn’t contain, the agent’s pre-flight check fails boot with a clear error in the kernel log (no kernel panic). Three ways to get an image with an init:
We maintain a small collection at superradcompany/guest-images for the cases where you specifically need a real init. Current set:
ImagePull
Debian + systemdghcr.io/superradcompany/debian-systemd:12
Ubuntu + systemdghcr.io/superradcompany/ubuntu-systemd:24.04
Fedora + systemdghcr.io/superradcompany/fedora-systemd:40
Alpine + OpenRCghcr.io/superradcompany/alpine-openrc:3.20
Multi-arch (linux/amd64 + linux/arm64), rebuilt weekly so they pick up upstream security patches, smoke-tested on every rebuild. Each image’s GHCR page documents its specific tag aliases and contents.
# Dockerfile.systemd
FROM debian:bookworm
RUN apt-get update \
    && apt-get install -y --no-install-recommends systemd \
    && rm -rf /var/lib/apt/lists/*

docker buildx build -t local-systemd:debian -f Dockerfile.systemd .
msb run local-systemd:debian --init /lib/systemd/systemd -- bash
Examples: jrei/systemd-debian:12, jrei/systemd-ubuntu:22.04. These work out of the box but are published by community maintainers, not the distros themselves. Vet them like any other third-party image before running real workloads.
For a sanity check that doesn’t need systemd, alpine:3.20 ships BusyBox at /sbin/init, which is enough to exercise the handoff mechanics: 
msb run alpine:3.20 --init /sbin/init -- sh -c "cat /proc/1/comm"
# busybox

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Shutdown semantics

Without --init, microsandbox shuts the guest down by remounting root read-only and calling reboot(RB_POWER_OFF). Typically <100 ms. With --init, the agent isn’t PID 1 anymore, so it asks your init to shut down:
  • SIGRTMIN+4 first (systemd’s poweroff signal).
  • SIGTERM fallback for inits that don’t speak it.
Your init then runs its own teardown (stop services, unmount, halt), which is slower:
InitTypical shutdown
BusyBox / s6 / runit<100 ms
OpenRC50-500 ms
systemd1-5 s
This is the price of a real init. If your test harness measures end-to-end sandbox lifecycle and you’re comparing to a no-handoff baseline, account for this.

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Init and entrypoint

--init and --entrypoint are orthogonal:
  • --init controls PID 1: what runs the system.
  • --entrypoint (and the trailing -- cmd) controls what your workload runs.
They can be combined. Boot systemd as PID 1 and have microsandbox exec calls land your shell, scripts, or app inside the systemd-managed environment.