Alert script questions

I would like to use alert script to write backup results in sensu, but I could not find any documentation.
Could you explain me some points?

  1. Does script run on client or on server?
  2. What is working directory for the script?
  3. How to test it?

I have written simple script:
os.execute('touch ur_alert_script.tst')
But could not find the file on client nor server after backup. How to run external commands?

Urbackup server version is 2.4.13