sudo apt-get install auditd
sudo vi /etc/audit/audit.rules # to add files to monitor; man audit.rules to check the grammar
sudo service auditd restart
sudo ausearch -f <file-name>
# inotifywait: cannot record who accesses the files, but can notify immediately
sudo apt-get install inotify-tools
inotifywait -m /etc/cron.d # can be a directory or file
# Check opened files in a ongoing process
lsof -p <pid>
strace -p <pid>
References:
How to monitor file access on Linux with auditd
Continuously monitor files opened/accessed by a process