Eu fiz isso funcionar assim:
check process nginx with pidfile /var/run/nginx.pid
start program = "/usr/sbin/service nginx start"
stop program = "/usr/sbin/service nginx stop"
IF does not exist THEN restart ELSE IF SUCCEEDED THEN exec "/etc/monit/slack.rb"
IF failed host 127.0.0.1 port 8080 THEN restart ELSE IF SUCCEEDED THEN exec "/etc/monit/slack.rb"