sieve_extprograms plugin calling bash script

[Tom]

I receive an email a few times a week containing a URL for a live stream which starts within a minute after receiving the email. I want to automate capturing the whole stream.

I've been trying to write a sieve script which uses the sieve extprograms plugin to call a bash script that parses the URL from the email and passes it to a capture program. So far I've had no luck getting the bash script to call _any_ external programs. I can't even write a file to /tmp/, so it's not a filesystem permissions issue. The sieve script has no issues finding and executing the bash script, it just does nothing other than return an exit code.

Does sieve put the extprogram script in some kind of sandbox where it has no access to the system?

This may not be the best place to find answers to sieve plugin info. If so, can someone point me in the right direction?