diff --git a/.github/workflows/tests.yaml b/.github/workflows/tests.yaml
index b0ca175..b3fca03 100644
--- a/.github/workflows/tests.yaml
+++ b/.github/workflows/tests.yaml
@@ -17,6 +17,7 @@ jobs:
     - name: Checkout prepare script
       uses: actions/checkout@v4
       with:
+        fetch-depth: 1
         sparse-checkout: prepare.sh
         sparse-checkout-cone-mode: false
 
@@ -34,3 +35,31 @@ jobs:
 
     - name: Check README.md
       run: tools/is-clean --make --root=doc README.md
+
+  test:
+    needs: build
+    strategy:
+      matrix:
+        kind: [ "system", "user" ]
+    runs-on: ubuntu-latest
+    steps:
+    - name: Check out repository code
+      uses: actions/checkout@v4
+      with:
+        fetch-depth: 1
+
+    - name: Prepare build environment
+      run: ./prepare.sh -us | tee -a "$GITHUB_ENV"
+
+    - name: Install (${{ matrix.kind }})
+      run: |
+        if [ "${{ matrix.kind }}" = "user" ]; then
+            ./install.sh -u
+        elif [ "${{ matrix.kind }}" = "system" ]; then
+            SUDO=sudo ./install.sh -s
+        else
+            exit 1
+        fi
+
+    - name: Run tests
+      run: ./tests.sh
diff --git a/data/displayswitcheroo/list.lua b/data/displayswitcheroo/list.lua
index 06fe711..cd358cd 100644
--- a/data/displayswitcheroo/list.lua
+++ b/data/displayswitcheroo/list.lua
@@ -15,38 +15,48 @@ for n, o in pairs(setup.outputs) do
 
         local c = o.crtc
         local geom = c and string.format(" %dx%d+%d+%d", c.width, c.height, c.x, c.y) or ""
-        print(string.format("output %s:%s%s (%dmm x %dmm) (fpr 0x%x)", n, pri, geom, o.mmwidth, o.mmheight, o.fingerprint))
+
+        local str = string.format("output %s:%s%s", n, pri, geom)
+        if o.mmwidth and o.mmheight then
+            str = str .. string.format(" (%dmm x %dmm)", o.mmwidth, o.mmheight)
+        end
+        if o.fingerprint then
+            str = str .. string.format(" (fpr 0x%x)", o.fingerprint)
+        end
+        print(str)
 
         local s, w, h
         for _, m in ipairs(o.modes) do
-            local flags = ""
-            if m.interlace then
-                flags = flags .. "i"
-            end
+            if m.refresh_rate then
+                local flags = ""
+                if m.interlace then
+                    flags = flags .. "i"
+                end
 
-            if m.double_scan then
-                flags = flags .. "d"
-            end
+                if m.double_scan then
+                    flags = flags .. "d"
+                end
 
-            if o.modes:is_preferred(m) then
-                flags = flags .. "+"
-            end
+                if o.modes:is_preferred(m) then
+                    flags = flags .. "+"
+                end
 
-            if c and m == c.mode then
-                flags = flags .. "*"
-            end
+                if c and m == c.mode then
+                    flags = flags .. "*"
+                end
 
-            local t = string.format(" %.2f%s", m.refresh_rate, flags)
+                local t = string.format(" %.2f%s", m.refresh_rate, flags)
 
-            if w == m.width and h == m.height then
-                s = s .. t
-            else
-                if s then
-                    print(s)
+                if w == m.width and h == m.height then
+                    s = s .. t
+                else
+                    if s then
+                        print(s)
+                    end
+                    w = m.width
+                    h = m.height
+                    s = string.format("   %dx%d%s", w, h, t)
                 end
-                w = m.width
-                h = m.height
-                s = string.format("   %dx%d%s", w, h, t)
             end
         end
         if s then
diff --git a/install.sh b/install.sh
index 3c082f1..ff82a88 100755
--- a/install.sh
+++ b/install.sh
@@ -17,13 +17,15 @@ EOF
     exit "${1-0}"
 }
 
+SUDO=${SUDO-}
 DESTDIR=${DESTDIR-$HOME/.local}
+DIR_MODE=0700
 APP=${APP-displayswitcheroo}
 while getopts "dusah-" OPT; do
     case $OPT in
         d) DESTDIR=$OPTARG ;;
-        u) DESTDIR=$HOME/.local ;;
-        s) DESTDIR=/usr ;;
+        u) DESTDIR=$HOME/.local; DIR_MODE=0700;;
+        s) DESTDIR=/usr; DIR_MODE=0755 ;;
         a) APP=$OPTARG ;;
         h) usage ;;
         -) break ;;
@@ -34,9 +36,10 @@ shift $((OPTIND-1))
 
 make -C "$SCRIPT_DIR" clean build EXTRA_CFLAGS="-DXDG_APP='\"$APP\"'"
 
-mkdir -pm 0700 "$DESTDIR/bin"
-install -v "$SCRIPT_DIR/src/cli.exe" "$DESTDIR/bin/$APP"
 
-mkdir -pm 0700 "$DESTDIR/share/$APP"
-install -v -D "$SCRIPT_DIR/data/displayswitcheroo/list.lua" "$DESTDIR/share/$APP/list.lua"
-install -v -D "$SCRIPT_DIR/data/displayswitcheroo/displayswitcheroo.lua" "$DESTDIR/share/$APP/$APP.lua"
+$SUDO mkdir -pm "$DIR_MODE" "$DESTDIR/bin"
+$SUDO install -v "$SCRIPT_DIR/src/cli.exe" "$DESTDIR/bin/$APP"
+
+$SUDO mkdir -pm "$DIR_MODE" "$DESTDIR/share/$APP"
+$SUDO install -v -D "$SCRIPT_DIR/data/displayswitcheroo/list.lua" "$DESTDIR/share/$APP/list.lua"
+$SUDO install -v -D "$SCRIPT_DIR/data/displayswitcheroo/displayswitcheroo.lua" "$DESTDIR/share/$APP/$APP.lua"
diff --git a/prepare.sh b/prepare.sh
index 237b806..4c5fee6 100755
--- a/prepare.sh
+++ b/prepare.sh
@@ -5,12 +5,14 @@ set -o nounset -o pipefail -o errexit
 SUDO=${SUDO-}
 DISTRO=${DISTRO-}
 UPDATE=${UPDATE-}
-while getopts "ud:sS:-" OPT; do
+TESTS=${TESTS-}
+while getopts "ud:sS:t-" OPT; do
     case $OPT in
         d) DISTRO=$OPTARG ;;
         u) UPDATE=1 ;;
         s) SUDO=sudo ;;
         S) SUDO=$OPTARG ;;
+        t) TESTS=1 ;;
         -) break ;;
         ?) usage 2 ;;
     esac
@@ -18,7 +20,7 @@ done
 shift $((OPTIND-1))
 
 if [ -z "$DISTRO" ]; then
-    if command -v lsb_release; then
+    if command -v lsb_release >/dev/null; then
         DISTRO=$(lsb_release -is)
     elif command -v pacman >/dev/null; then
         DISTRO="Arch"
@@ -36,21 +38,28 @@ if [ "$DISTRO" = "Arch" ] || command -v pacman >/dev/null; then
     if [ -n "$UPDATE" ]; then
         $SUDO pacman -Sy 1>&2
     fi
-    $SUDO pacman -S --noconfirm 1>&2 \
-        git \
-        make gcc pkgconf \
-        python gawk \
-        lua libxrandr
+    PKGs=(git)
+    PKGs+=(make gcc pkgconf)
+    PKGs+=(python gawk)
+    PKGs+=(lua libxrandr)
+    if [ -n "$TESTS" ]; then
+        PKGs+=(xorg-server-xvfb)
+    fi
+    $SUDO pacman -S --noconfirm "${PKGs[@]}" 1>&2
 elif [ "$DISTRO" = "Ubuntu" ] || command -v apt-get >/dev/null; then
     if [ -n "$UPDATE" ]; then
         $SUDO apt-get update 1>&2
     fi
-    $SUDO apt-get install --yes 1>&2 \
+    PKGs=(git ca-certificates)
+    PKGs+=(make gcc pkg-config)
+    PKGs+=(python3 gawk)
+    PKGs+=(liblua5.4-dev libxrandr-dev)
+    if [ -n "$TESTS" ]; then
+        PKGs+=(xvfb)
+    fi
+    $SUDO apt-get install --yes \
         --no-install-recommends --no-install-suggests \
-        git ca-certificates \
-        make gcc pkg-config \
-        python3 gawk \
-        liblua5.4-dev libxrandr-dev
+        1>&2  "${PKGs[@]}"
     echo "LUA_PKG=lua54"
 else
     echo "unconfigured distribution: $DISTRO" 1>&2
diff --git a/src/.k b/src/.k
index 59ca2ed..8ef666e 100644
--- a/src/.k
+++ b/src/.k
@@ -4,6 +4,11 @@ cli() {
     make LOG_LEVEL=DEBUG cli.exe && ./cli.exe "${1-list}"
 }
 
+xvfb() {
+    make LOG_LEVEL=DEBUG cli.exe && xvfb-run ./cli.exe "$K_DIR/../data/displayswitcheroo/list.lua"
+}
+
+
 watch() {
     make LOG_LEVEL=DEBUG cli.exe && ./cli.exe -w go.lua
 }
diff --git a/tests.sh b/tests.sh
new file mode 100755
index 0000000..711f11a
--- /dev/null
+++ b/tests.sh
@@ -0,0 +1,9 @@
+#!/bin/bash
+
+set -o nounset -o pipefail -o errexit
+
+APP=${APP-displayswitcheroo}
+
+which "$APP"
+
+xvfb-run "$APP" list