• Q: Error message when running locate: can not open `/var/lib/mlocate/mlocate.db': Permission denied
• A: check the property of /usr/bin/mlocate

ls -lth /usr/bin/mlocate
-rwxr-sr-x 1 root syslog 43K Mar  2  2018 /usr/bin/mlocate

# Do the following
sudo chown root:mlocate /usr/bin/mlocate
sudo chmod g+sx /usr/bin/mlocate

ls -lth /usr/bin/mlocate
-rwxr-sr-x 1 root mlocate 43K Mar  2  2018 /usr/bin/mlocate