myapp.service

# Expectations:
# 
# * application code in /srv/myapp/current
# * ruby runtime in /srv/myapp/.rubies/ruby-2.3.0
# * unicorn.rb config specifies pid file location to /srv/myapp/current/unicorn.pid and working directory to /srv/myapp/current

[Unit]
Description=Sample unicorn app

[Service]
User=myapp
WorkingDirectory=/srv/myapp/current
KillSignal=SIGQUIT
ExecStart=/srv/myapp/.rubies/ruby-2.3.0/bin/bundle exec unicorn -c /srv/myapp/current/unicorn.rb
ExecReload=/bin/kill -s USR2 $MAINPID ; /bin/sleep 1 ; /bin/kill -s QUIT $MAINPID
Environment=PATH=/srv/myapp/.rubies/ruby-2.3.0/bin:$PATH
Restart=on-abnormal
PIDFile=/srv/myapp/current/unicorn.pid

When does it start?

On systemctl start or at boot time when systemctl enable issued previously. If it depends on something, that'd be nice to describe in unit file as well.