Comment # 12 on bug 894636 from Marcos Mello

(In reply to Marcos Mello from comment #6)
> The service file is fine. I do not know if there is a guideline that define
> preference between systemctl, kill (or systemctl kill...), or even directly
> daemon binary invocation where applicable in logrotate snippets.
> 
> "-k rotate" will find squid's main process via its pid file and send SIGUSR1.
> 
> "-k reconfigure" (SIGHUP) will also reload the configuration file.
> 
> Getting rid of those init script calls is enough to me. ;)

Well, after reading

http://article.gmane.org/gmane.comp.web.squid.general/93737

I will change my recommendation. SIGUSR1 (aka rotate) is better.

Previously, I suggested

/usr/sbin/squid -k rotate 2>/dev/null || :

but for a "pure" systemd setup, we can also use

systemctl --kill-who=main --signal=SIGUSR1 kill squid.service 2>/dev/null || :