These are my unit files:

pacclean.service
[Unit]
Description=Clean-up package cache

[Service]
Type=oneshot
Nice=19
IOSchedulingClass=2
IOSchedulingPriority=7
ExecStart=/usr/bin/paccache -r
ExecStart=/usr/bin/paccache -ruk0

pacclean.timer
[Unit]
Description=Clean-up package cache

[Timer]
# run 5 minutes after boot time
OnBootSec=5min
# run 1 week after service was last started
OnUnitActiveSec=1w

[Install]
WantedBy=multi-user.target

And this is the result of systemctl status pacclean
● pacclean.service - Clean-up package cache
Loaded: loaded (/usr/local/lib/systemd/system/pacclean.service; static)
Active: failed (Result: exit-code) since Tue 2014-10-21 21:54:43 EDT; 1h 15min ago
Process: 1183 ExecStart=/usr/bin/paccache -r (code=exited, status=1/FAILURE)
Main PID: 1183 (code=exited, status=1/FAILURE)

I've implemented the feature in paccache, and I've been using it locally without any issues. The modified paccache.sh.in file is attached, and here's the git diff:

diff --git a/contrib/paccache.sh.in b/contrib/paccache.sh.in
index 71aee23..eb04e69 100644
--- a/contrib/paccache.sh.in
+++ b/contrib/paccache.sh.in
@@ -24,7 +24,7 @@ declare -r myname='paccache'
declare -r myver='@PACKAGE_VERSION@'

declare -a cachedirs=() candidates=() cmdopts=() whitelist=() blacklist=()
-declare -i delete=0 dryrun=0 filecount=0 move=0 needsroot=0 totalsaved=0 verbose=0
+declare -i delete=0 dryrun=0 filecount=0 move=0 needsroot=0 noneisok=0 totalsaved=0 verbose=0
declare delim=$'\n' keep=3 movedir= scanarch=

USE_COLOR='y'
@@ -192,6 +192,8 @@ Usage: ${myname} <operation> [options] [targets...]
delimited.
-k, --keep <num> keep "num" of each package in the cache (default: 3).
--nocolor remove color from output.
+ -n, --noneisok do report an error if there are no candidates - use
+ when checking results in another script
-u, --uninstalled target uninstalled packages.
-v, --verbose increase verbosity. specify up to 3 times.
-z, --null use null delimiters for candidate names (only with -v
@@ -205,9 +207,9 @@ version() {
echo 'Copyright (C) 2011 Dave Reisner <dreisner@archlinux.org>'
}

-OPT_SHORT=':a:c:dfhi:k:m:rsuVvz'
+OPT_SHORT=':a:c:dfhi:k:m:nrsuVvz'
OPT_LONG=('arch:' 'cachedir:' 'dryrun' 'force' 'help' 'ignore:' 'keep:' 'move'
- 'nocolor' 'remove' 'uninstalled' 'version' 'verbose' 'null')
+ 'nocolor' 'noneisok' 'remove' 'uninstalled' 'version' 'verbose' 'null')

if ! parseopts "$OPT_SHORT" "${OPT_LONG[@]}" -- "$@"; then
exit 1
@@ -252,6 +254,8 @@ while :; do
-m|--move)
move=1 movedir=$2
shift ;;
+ -n|--noneisok)
+ noneisok=1 ;;
-r|--remove)
delete=1 ;;
-u|--uninstalled)
@@ -328,7 +332,11 @@ done

if (( ! ${#candidates[*]} )); then
msg 'no candidate packages found for pruning'
- exit 1
+ if (( noneisok )); then
+ exit 0
+ else
+ exit 1
+ fi
fi

# grab this prior to signature scavenging

Send this to the pacman-dev ML if you want it in, patches in the bugtracker just hangs around with no progress.