diff --git a/.travis.yml b/.travis.yml
index ec0f110..6b793dd 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -8,6 +8,7 @@ before_script:
 
 script:
   - shellcheck -s bash clipmenu clipmenud
+  - tests/test-clipmenu
 
 matrix:
   fast_finish: true
diff --git a/README.md b/README.md
index c1b13b4..360eb74 100644
--- a/README.md
+++ b/README.md
@@ -1,11 +1,38 @@
 clipmenu is a simple clipboard manager using [dmenu][] and [xsel][].
 
-To use it, start the `clipmenud` daemon, and then call `clipmenu` to launch
-`dmenu`. Upon choosing an entry, it is copied to the clipboard.
+# Usage
+
+Start `clipmenud`, then run `clipmenu` to select something to put on the
+clipboard.
+
+A systemd user service for starting clipmenud is included at
+[init/clipmenud.service](https://github.com/cdown/clipmenu/blob/develop/init/clipmenud.service).
 
 All args passed to clipmenu are transparently dispatched to dmenu. That is, if
 you usually call dmenu with args to set colours and other properties, you can
-invoke clipmenu in exactly the same way to get the same effect.
+invoke clipmenu in exactly the same way to get the same effect, like so:
+
+    clipmenu -i -fn Terminus:size=8 -nb '#002b36' -nf '#839496' -sb '#073642' -sf '#93a1a1'
+
+# How does it work?
+
+The code is fairly simple and easy to follow, you may find it easier to read
+there, but it basically works like this:
+
+## clipmenud
+
+1. `clipmenud` polls the clipboard every 0.5 seconds (or another interval as
+   configured with the `CLIPMENUD_SLEEP` environment variable). Unfortunately
+   there's no interface to subscribe for changes in X11, so we must poll.
+2. If `clipmenud` detects changes to the clipboard contents, it writes them out
+   to the cache directory.
+
+## clipmenu
+
+1. `clipmenu` reads the cache directory to find all available clips.
+2. `dmenu` is executed to allow the user to select a clip.
+3. After selection, the clip is put onto the PRIMARY and CLIPBOARD X
+   selections.
 
 [dmenu]: http://tools.suckless.org/dmenu/
 [xsel]: http://www.vergenet.net/~conrad/software/xsel/
diff --git a/clipmenu b/clipmenu
index 05ad197..5b5f873 100755
--- a/clipmenu
+++ b/clipmenu
@@ -1,15 +1,17 @@
 #!/bin/bash
 
+major_version=3
+
 shopt -s nullglob
 
-cache_dir=/tmp/clipmenu.$USER
+cache_dir=/tmp/clipmenu.$major_version.$USER
 cache_file=$cache_dir/line_cache
 
 # It's okay to hardcode `-l 8` here as a sensible default without checking
 # whether `-l` is also in "$@", because the way that dmenu works allows a later
 # argument to override an earlier one. That is, if the user passes in `-l`, our
 # one will be ignored.
-chosen_line=$(tac "$cache_file" | uniq | dmenu -l 8 "$@")
+chosen_line=$(tac "$cache_file" | awk '!seen[$0]++' | dmenu -l 8 "$@")
 
 [[ $chosen_line ]] || exit 1
 
@@ -22,9 +24,5 @@ if ! [[ -f "$file" ]]; then
 fi
 
 for selection in clipboard primary; do
-    if type -p xsel >/dev/null 2>&1; then
-        xsel --logfile /dev/null -i --"$selection" < "$file"
-    else
-        xclip -sel "$selection" < "$file"
-    fi
+    xsel --logfile /dev/null -i --"$selection" < "$file"
 done
diff --git a/clipmenud b/clipmenud
index 879f18c..8eca3a3 100755
--- a/clipmenud
+++ b/clipmenud
@@ -1,5 +1,9 @@
 #!/bin/bash
 
+major_version=3
+cache_dir=/tmp/clipmenu.$major_version.$USER/
+cache_file=$cache_dir/line_cache
+
 get_first_line() {
     # Args:
     # - $1, the file or data
@@ -37,9 +41,6 @@ debug() {
     fi
 }
 
-cache_dir=/tmp/clipmenu.$USER/
-cache_file=$cache_dir/line_cache
-
 # It's ok that this only applies to the final directory.
 # shellcheck disable=SC2174
 mkdir -p -m0700 "$cache_dir"
@@ -48,15 +49,8 @@ declare -A last_data
 declare -A last_filename
 
 while sleep "${CLIPMENUD_SLEEP:-0.5}"; do
-
     for selection in clipboard primary; do
-        if type -p xsel >/dev/null 2>&1; then
-            debug 'Using xsel'
-            data=$(xsel --logfile /dev/null -o --"$selection"; printf x)
-        else
-            debug 'Using xclip'
-            data=$(xclip -o -sel "$selection"; printf x)
-        fi
+        data=$(xsel --logfile /dev/null -o --"$selection"; printf x)
 
         debug "Data before stripping: $data"
 
@@ -109,11 +103,7 @@ while sleep "${CLIPMENUD_SLEEP:-0.5}"; do
             # We can't colocate this with the above copying code because
             # https://github.com/cdown/clipmenu/issues/34 requires knowing if
             # we would skip first.
-            if type -p xsel >/dev/null 2>&1; then
-                xsel --logfile /dev/null -o --"$selection" | xsel -i --"$selection"
-            else
-                xclip -o -sel "$selection" | xclip -i -sel "$selection"
-            fi
+            xsel --logfile /dev/null -o --"$selection" | xsel -i --"$selection"
         fi
     done
 done
diff --git a/init/clipmenud.service b/init/clipmenud.service
new file mode 100644
index 0000000..029ad0c
--- /dev/null
+++ b/init/clipmenud.service
@@ -0,0 +1,31 @@
+[Unit]
+Description=Clipmenu daemon
+
+[Service]
+ExecStart=/usr/bin/clipmenud
+Restart=always
+RestartSec=0
+Environment=DISPLAY=:0
+
+SystemCallFilter=@basic-io @default @io-event @ipc @network-io @process \
+                 brk fadvise64 getegid geteuid getgid getgroups getpgrp \
+                 getpid getppid getrlimit getuid ioctl mprotect rt_sigaction \
+                 rt_sigprocmask setitimer setsid sysinfo umask uname wait4
+
+# @file-system will handle this once v233 is released, see
+# http://bit.ly/2l1r8Ah for more details.
+SystemCallFilter=access chdir close faccessat fcntl fstat getcwd mkdir mmap \
+                 munmap open stat statfs unlink
+
+MemoryDenyWriteExecute=yes
+NoNewPrivileges=yes
+ProtectControlGroups=yes
+ProtectKernelTunables=yes
+RestrictAddressFamilies=
+RestrictRealtime=yes
+
+ProtectSystem=strict
+ReadWritePaths=/tmp
+
+[Install]
+WantedBy=default.target
diff --git a/tests/test-clipmenu b/tests/test-clipmenu
new file mode 100755
index 0000000..3da51aa
--- /dev/null
+++ b/tests/test-clipmenu
@@ -0,0 +1,81 @@
+#!/bin/bash
+
+set -x
+set -e
+set -o pipefail
+
+major_version=3
+dir=/tmp/clipmenu.$major_version.$USER
+cache_file=$dir/line_cache
+
+if [[ $0 == /* ]]; then
+    location=${0%/*}
+else
+    location=$PWD/${0#./}
+    location=${location%/*}
+fi
+
+cat - "$location/../clipmenu" > /tmp/clipmenu << 'EOF'
+#!/bin/bash
+
+shopt -s expand_aliases
+
+shim() {
+    printf '%s args:' "$1" >&2
+    printf ' %q' "${@:2}" >&2
+    printf '\n' >&2
+
+    i=0
+
+    while IFS= read -r line; do
+        let i++
+        printf '%s line %d stdin: %s\n' "$1" "$i" "$line" >&2
+    done
+
+    if [[ -v SHIM_STDOUT ]]; then
+        printf '%s\n' "$SHIM_STDOUT"
+    fi
+}
+
+alias dmenu='SHIM_STDOUT="Selected text. (2 lines)" shim dmenu'
+alias xsel='shim xsel'
+alias xclip='shim xclip'
+EOF
+
+chmod a+x /tmp/clipmenu
+
+rm -rf "$dir"
+mkdir -p "$dir"
+
+cat > "$cache_file" << 'EOF'
+Selected text. (2 lines)
+Selected text 2. (2 lines)
+EOF
+
+cat > "$dir/$(cksum <<< 'Selected text. (2 lines)')" << 'EOF'
+Selected text.
+Yes, it's selected text.
+EOF
+
+### TESTS ###
+
+output=$(/tmp/clipmenu --foo bar 2>&1)
+
+temp=$(mktemp)
+trap 'rm -f -- "$temp"' EXIT
+
+printf '%s\n' "$output" > "$temp"
+
+# Arguments are transparently passed to dmenu
+grep -Fxq 'dmenu args: -l 8 --foo bar' "$temp"
+
+# Output from cache file should get to dmenu, reversed
+grep -Fxq 'dmenu line 1 stdin: Selected text 2. (2 lines)' "$temp"
+grep -Fxq 'dmenu line 2 stdin: Selected text. (2 lines)' "$temp"
+
+# xsel should copy both to clipboard *and* primary
+grep -Fxq 'xsel args: --logfile /dev/null -i --clipboard' "$temp"
+grep -Fxq 'xsel args: --logfile /dev/null -i --primary' "$temp"
+
+grep -Fxq 'xsel line 1 stdin: Selected text.' "$temp"
+grep -Fxq "xsel line 2 stdin: Yes, it's selected text." "$temp"
diff --git a/test/test-perf b/tests/test-perf
similarity index 97%
rename from test/test-perf
rename to tests/test-perf
index fca39ac..a393de6 100755
--- a/test/test-perf
+++ b/tests/test-perf
@@ -1,10 +1,12 @@
 #!/bin/bash
 
+major_version=3
+
 msg() {
     printf '>>> %s\n' "$@" >&2
 }
 
-dir=/tmp/clipmenu.$USER
+dir=/tmp/clipmenu.$major_version.$USER
 cache_file=$dir/line_cache
 
 log=$(mktemp)
@@ -35,7 +37,6 @@ shopt -s expand_aliases
 
 alias dmenu=:
 alias xsel=:
-alias xclip=:
 
 EOF