diff --git a/.github/workflows/tests.yaml b/.github/workflows/tests.yaml
new file mode 100644
index 0000000..b0ca175
--- /dev/null
+++ b/.github/workflows/tests.yaml
@@ -0,0 +1,36 @@
+name: Tests
+on:
+  push:
+  schedule:
+    - cron: "0 0 7 * *"
+
+jobs:
+  build:
+    strategy:
+      matrix:
+        image: [ "ubuntu:latest", "ubuntu:22.04", "archlinux:latest" ]
+    continue-on-error: true
+    runs-on: ubuntu-latest
+    container:
+      image: ${{ matrix.image }}
+    steps:
+    - name: Checkout prepare script
+      uses: actions/checkout@v4
+      with:
+        sparse-checkout: prepare.sh
+        sparse-checkout-cone-mode: false
+
+    - name: Prepare build environment
+      run: ./prepare.sh -u | tee -a "$GITHUB_ENV"
+
+    - name: Check out repository code
+      uses: actions/checkout@v4
+
+    - name: Correct ownership of repository
+      run: chown -R $(id -u):$(id -g) .
+
+    - name: Build
+      run: make build
+
+    - name: Check README.md
+      run: tools/is-clean --make --root=doc README.md
diff --git a/README.md b/README.md
index 2f3c82e..05c23ef 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,8 @@
 displayswitcheroo
 =================
 
+[![Tests](https://github.com/rootmos/displayswitcheroo/actions/workflows/tests.yaml/badge.svg)](https://github.com/rootmos/displayswitcheroo/actions/workflows/tests.yaml)
+
 A [Lua](https://lua.org) c-module wrapping the [XRandr](https://www.x.org/wiki/Projects/XRandR/) X11 extension,
 and a bundled command-line application with the ~~batteries~~ module included.
 
diff --git a/doc/.gitignore b/doc/.gitignore
index b0acc35..cf1adad 100644
--- a/doc/.gitignore
+++ b/doc/.gitignore
@@ -1,2 +1,3 @@
 usage.*
 output.*
+!output.list
diff --git a/doc/Makefile b/doc/Makefile
index a893689..bf92c7a 100644
--- a/doc/Makefile
+++ b/doc/Makefile
@@ -1,5 +1,5 @@
 THIS := $(shell dirname $(realpath $(firstword $(MAKEFILE_LIST))))
-ROOT ?= $(realpath $(THIS)/..)
+ROOT ?= ..
 TOOLS ?= $(ROOT)/tools
 
 export EXE ?= $(ROOT)/src/cli.exe
diff --git a/doc/README.in.md b/doc/README.in.md
index 86936ea..9b83b56 100644
--- a/doc/README.in.md
+++ b/doc/README.in.md
@@ -1,6 +1,8 @@
 displayswitcheroo
 =================
 
+[![Tests](https://github.com/rootmos/displayswitcheroo/actions/workflows/tests.yaml/badge.svg)](https://github.com/rootmos/displayswitcheroo/actions/workflows/tests.yaml)
+
 A [Lua](https://lua.org) c-module wrapping the [XRandr](https://www.x.org/wiki/Projects/XRandR/) X11 extension,
 and a bundled command-line application with the ~~batteries~~ module included.
 
diff --git a/doc/output.list b/doc/output.list
new file mode 100644
index 0000000..b803d4c
--- /dev/null
+++ b/doc/output.list
@@ -0,0 +1,60 @@
+screen 0: 1920x1080 (min 320x200, max 16384x16384) (531mm x 299mm) (window 1946)
+output HDMI-1
+output eDP-1: (309mm x 173mm) (fpr 0x57a9d7d6)
+   1920x1080 59.98+ 59.97d 59.96 59.93
+   1680x1050 59.95 59.88
+   1400x1050 59.98
+   1600x900 59.99d 59.94d 59.95 59.82
+   1280x1024 60.02
+   1400x900 59.96 59.88
+   1280x960 60.00
+   1440x810 60.00d 59.97d
+   1368x768 59.88 59.85
+   1280x800 59.99d 59.97d 59.81 59.91
+   1280x720 60.00d 59.99d 59.86 59.74
+   1024x768 60.04d 60.00
+   960x720 60.00d
+   928x696 60.05d
+   896x672 60.01d
+   1024x576 59.95d 59.96d 59.90 59.82
+   960x600 59.93d 60.00d
+   960x540 59.96d 59.99d 59.63 59.82
+   800x600 60.00d 60.32 56.25
+   840x525 60.01d 59.88d
+   864x486 59.92 59.57
+   700x525 59.98d
+   800x450 59.95d 59.82d
+   640x512 60.02d
+   700x450 59.96d 59.88d
+   640x480 60.00d 59.94
+   720x405 59.51 58.99
+   684x384 59.88d 59.85d
+   640x400 59.88d 59.98d
+   640x360 59.86d 59.83d 59.84 59.32
+   512x384 60.00d
+   512x288 60.00d 59.92d
+   480x270 59.63d 59.82d
+   400x300 60.32d 56.34d
+   432x243 59.92d 59.57d
+   320x240 60.05d
+   360x202 59.51d 59.13d
+   320x180 59.84d 59.32d
+output DP-2
+output HDMI-2: primary 1920x1080+0+0 (531mm x 299mm) (fpr 0x43b791a2)
+   1920x1080 60.00+* 50.00 59.94 60.00i 50.00i 59.94i
+   1680x1050 59.88
+   1600x900 60.00
+   1280x1024 75.02 60.02
+   1280x800 59.91
+   1152x864 75.00
+   1280x720 60.00 50.00 59.94
+   1024x768 75.03 60.00
+   832x624 74.55
+   800x600 75.00 60.32
+   720x576 50.00 50.00i
+   720x480 60.00 59.94 60.00i 59.94i
+   640x480 75.00 60.00 59.94
+   720x400 70.08
+output DP-1
+monitor HDMI-2: active primary automatic 1920x1080+0+0 (531mm x 299mm)
+  output HDMI-2
diff --git a/prepare.sh b/prepare.sh
new file mode 100755
index 0000000..237b806
--- /dev/null
+++ b/prepare.sh
@@ -0,0 +1,58 @@
+#!/bin/bash
+
+set -o nounset -o pipefail -o errexit
+
+SUDO=${SUDO-}
+DISTRO=${DISTRO-}
+UPDATE=${UPDATE-}
+while getopts "ud:sS:-" OPT; do
+    case $OPT in
+        d) DISTRO=$OPTARG ;;
+        u) UPDATE=1 ;;
+        s) SUDO=sudo ;;
+        S) SUDO=$OPTARG ;;
+        -) break ;;
+        ?) usage 2 ;;
+    esac
+done
+shift $((OPTIND-1))
+
+if [ -z "$DISTRO" ]; then
+    if command -v lsb_release; then
+        DISTRO=$(lsb_release -is)
+    elif command -v pacman >/dev/null; then
+        DISTRO="Arch"
+    elif command -v apt-get >/dev/null; then
+        # TODO: debian
+        DISTRO="Ubuntu"
+    else
+        echo "unable to figure out distribution: $DISTRO" 1>&2
+        exit 1
+    fi
+fi
+echo "distro: $DISTRO" 1>&2
+
+if [ "$DISTRO" = "Arch" ] || command -v pacman >/dev/null; then
+    if [ -n "$UPDATE" ]; then
+        $SUDO pacman -Sy 1>&2
+    fi
+    $SUDO pacman -S --noconfirm 1>&2 \
+        git \
+        make gcc pkgconf \
+        python gawk \
+        lua libxrandr
+elif [ "$DISTRO" = "Ubuntu" ] || command -v apt-get >/dev/null; then
+    if [ -n "$UPDATE" ]; then
+        $SUDO apt-get update 1>&2
+    fi
+    $SUDO apt-get install --yes 1>&2 \
+        --no-install-recommends --no-install-suggests \
+        git ca-certificates \
+        make gcc pkg-config \
+        python3 gawk \
+        liblua5.4-dev libxrandr-dev
+    echo "LUA_PKG=lua54"
+else
+    echo "unconfigured distribution: $DISTRO" 1>&2
+    exit 1
+fi
diff --git a/tools/is-clean b/tools/is-clean
new file mode 100755
index 0000000..d86219d
--- /dev/null
+++ b/tools/is-clean
@@ -0,0 +1,113 @@
+#!/usr/bin/env python3
+
+import argparse
+import hashlib
+import os
+import subprocess
+import sys
+import tempfile
+
+whoami = "is-clean"
+env_prefix = whoami.upper().replace("-", "_") + "_"
+def env(var, default=None):
+    return os.environ.get(env_prefix + var, default)
+
+import logging
+logger = logging.getLogger(whoami)
+
+def is_clean(path, make=False, root=None, target=None, show_diff=False):
+    logger.info(f"checking: {path}")
+
+    digest = "sha256"
+    with open(path, "rb") as f:
+        current_digest = hashlib.new(digest, f.read()).hexdigest()
+    logger.info(f"current; {digest.upper()}: {current_digest}")
+
+    if make:
+        root = root or os.path.dirname(path) or os.getcwd()
+        logger.debug(f"root: {root}")
+
+        target = target or os.path.relpath(path, root)
+        logger.debug(f"target: {target}")
+
+        cmdline = [ os.environ.get("MAKE", "make") ]
+        cmdline += [ "-C", root ]
+        cmdline += [ target ]
+        logger.debug(f"running: {cmdline}")
+        subprocess.check_call(cmdline)
+
+        with open(path, "rb") as f:
+            generated_digest = hashlib.new(digest, f.read()).hexdigest()
+        logger.info(f"generated; {digest.upper()}: {generated_digest}")
+        current_digest = generated_digest
+
+    with tempfile.TemporaryDirectory(prefix=f"{whoami}-") as tmp:
+        head = os.path.join(tmp, "head")
+
+        with open(head, "xb") as f:
+            rel = os.path.relpath(path, start=root)
+            cmdline = ["git", "show", "HEAD:./" + rel]
+            logger.debug(f"running: {cmdline}")
+            bs = subprocess.check_output(cmdline, cwd=root)
+            head_digest = hashlib.new(digest, bs).hexdigest()
+            f.write(bs)
+
+        logger.info(f"HEAD; {digest.upper()}: {head_digest}")
+
+        if current_digest == head_digest:
+            return True
+
+        if show_diff:
+            subprocess.run(["diff", path, head])
+
+        return False
+
+def setup_logger(level, logger=None):
+    level = level.upper()
+    if logger is None:
+        logger = logging.getLogger()
+    logger.setLevel(level)
+
+    handler = logging.StreamHandler()
+    handler.setLevel(level)
+    logger.addHandler(handler)
+
+    fmt = logging.Formatter(fmt="%(asctime)s:%(name)s:%(levelname)s %(message)s", datefmt="%Y-%m-%dT%H:%M:%S%z")
+    handler.setFormatter(fmt)
+
+def parse_args():
+    parser = argparse.ArgumentParser(
+        description="Freshness checker",
+        formatter_class=argparse.ArgumentDefaultsHelpFormatter,
+    )
+
+    parser.add_argument("--log", default=env("LOG_LEVEL", "INFO"), help="set log level")
+
+    parser.add_argument("-m", "--make", action="store_true")
+    parser.add_argument("-r", "--root", metavar="PATH")
+    parser.add_argument("-t", "--target", metavar="TARGET")
+
+    parser.add_argument("-D", "--no-diff", action="store_true")
+
+    parser.add_argument("path", metavar="PATH")
+
+    return parser.parse_args()
+
+def main():
+    args = parse_args()
+    setup_logger(args.log, logger)
+    logger.debug(f"args: {args}")
+
+    r = is_clean(
+        path = args.path,
+        make = args.make,
+        root = args.root,
+        target = args.target,
+        show_diff = not args.no_diff,
+    )
+
+    if not r:
+        sys.exit(1)
+
+if __name__ == "__main__":
+    main()