I cannot remember this command (and googling was unsuccessful), but there is a way to get the list of actions performed by a process, that outputs something like
# listprocessactions -p 1234
0.321 Open "A" /var/log/nginx/supersite.log
0.322 Write to /var/log/nginx/supersite.log
0.401 Close /var/log/nginx/supersite.log
0.555 Opens TCP connection with slashdot.org
...
I'm interested in the files aspect (open / RW files).
The question is what is that command (and if possible in which package on deb / ubuntu)