How to use lsof to identify process or processes accessing a file or filesystem

(1) e.g. List the process(es) that is(are) accessing a file

# lsof -f -- /usr/sbin/httpd
COMMAND   PID   USER FD   TYPE DEVICE SIZE/OFF   NODE NAME
httpd    2779   root txt    REG 253,0   354816 560817 /usr/sbin/httpd
httpd   77290 apache txt    REG 253,0   354816 560817 /usr/sbin/httpd
httpd   77291 apache txt    REG 253,0   354816 560817 /usr/sbin/httpd
httpd   77292 apache txt    REG 253,0   354816 560817 /usr/sbin/httpd
httpd   77293 apache txt    REG 253,0   354816 560817 /usr/sbin/httpd
httpd   77294 apache txt    REG 253,0   354816 560817 /usr/sbin/httpd
httpd   77295 apache txt    REG 253,0   354816 560817 /usr/sbin/httpd
httpd   77296 apache txt    REG 253,0   354816 560817 /usr/sbin/httpd
httpd   77297 apache txt    REG 253,0   354816 560817 /usr/sbin/httpd

-f => the argument that follows is file name or file path

(2) e.g. list process(es) that is accessing a file system

# lsof +f -- /var/log
COMMAND     PID   USER   FD   TYPE DEVICE SIZE/OFF   NODE NAME
rsyslogd   1857   root    1w   REG 253,6      3275 40978 /var/log/messages
rsyslogd   1857   root    2w   REG 253,6     56974 40976 /var/log/cron
rsyslogd   1857   root    4w   REG 253,6      5847 40979 /var/log/secure
mcelog     2282   root    3w   REG 253,6         0     31 /var/log/mcelog
abrt-dump 2492   root    4r   REG 253,6      3275 40978 /var/log/messages
httpd      2779   root    2w   REG 253,6       245 155654 /var/log/httpd/error_log
httpd      2779   root    5w   REG 253,6 116277457     32 /var/log/modsec_audit.log
httpd      2779   root   12w   REG 253,6     77964 155652 /var/log/httpd/access_log
httpd     77297 apache    5w   REG 253,6 116277457     32 /var/log/modsec_audit.log
httpd     77297 apache   12w   REG 253,6     77964 155652 /var/log/httpd/access_log

+f => the argument is treated as file system

where /var/log is one of the filesystems mounted

# df -h
Filesystem            Size Used Avail Use% Mounted on
/dev/mapper/vg00-lvroot
                       15G 8.0G 5.6G 59% /
tmpfs                 1.9G   12K 1.9G   1% /dev/shm
/dev/sda1             504M 103M 377M 22% /boot
/dev/mapper/vg00-lvhome
                     1008M   56M 902M   6% /home
/dev/mapper/vg00-lvtmp
                      5.0G 2.8G 1.9G 60% /tmp
/dev/mapper/vg00-lvvar
                      9.9G 1.4G 8.0G 15% /var
/dev/mapper/vg00-lvvarlog
                      9.9G 2.5G 7.0G 27% /var/log
/dev/mapper/vg00-lvaudit
                      5.0G 2.3G 2.4G 50% /var/log/audit
/dev/mapper/VGnpb-LVapps
                       49G   34G   13G 74% /apps