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

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.