Unix Open files and lsof

The linux lsof when run with no options lists all open files used by all linux processes running on the linux system.
As per the lsof man page the open file can be:
"An  open  file  may be a regular file, a directory, a block special file, a character special file, an executing text reference, a library, a stream or a network file (Internet socket, NFS file or UNIX domain socket.)  A specific file or all the files in a file system may be selected by path.
"
Thus, the output of lsof would almost always be a super set of the process open file descriptors that are limited by linux kernel open files limit.

Lets take the below example for the running terminal emulator:

sherif@fingolfin:~$ ps -ef |grep terminal|grep -v grep
sherif   17612     1  0 18:45 ?        00:00:08 /usr/bin/xfce4-terminal
sherif@fingolfin:~$ lsof -p 17612 |wc -l
182
sherif@fingolfin:~$ ls -l /proc/17612/fd/* |wc -l
16
sherif@fingolfin:~$ lsof -p 17612 |egrep " [0-9]+[rwu] "
xfce4-ter 17612 sherif    0r      CHR                1,3      0t0       6 /dev/null
xfce4-ter 17612 sherif    1w      CHR                1,3      0t0       6 /dev/null
xfce4-ter 17612 sherif    2w      REG                8,1     7452 3805497 /home/sherif/.xsession-errors
xfce4-ter 17612 sherif    3u  a_inode               0,13        0    9863 [eventfd]
xfce4-ter 17612 sherif    4u  a_inode               0,13        0    9863 [eventfd]
xfce4-ter 17612 sherif    5u     unix 0x0000000000000000      0t0   59770 type=STREAM
xfce4-ter 17612 sherif    6u  a_inode               0,13        0    9863 [eventfd]
xfce4-ter 17612 sherif    7u     unix 0x0000000000000000      0t0   59772 type=STREAM
xfce4-ter 17612 sherif    8u  a_inode               0,13        0    9863 [eventfd]
xfce4-ter 17612 sherif    9u     unix 0x0000000000000000      0t0   59773 type=STREAM
xfce4-ter 17612 sherif   10r  a_inode               0,13        0    9863 inotify
xfce4-ter 17612 sherif   11u     unix 0x0000000000000000      0t0   59774 type=STREAM
xfce4-ter 17612 sherif   12u      CHR                5,2      0t0      88 /dev/ptmx
xfce4-ter 17612 sherif   13u      REG                8,1        0 2228246 /tmp/#2228246 (deleted)
xfce4-ter 17612 sherif   14u      REG                8,1        0 2241053 /tmp/#2241053 (deleted)
xfce4-ter 17612 sherif   15u      REG                8,1        0 2241055 /tmp/#2241055 (deleted)
sherif@fingolfin:~$ ls -l /proc/17612/fd/*
lr-x------ 1 sherif sherif 64 mrt 29 18:45 /proc/17612/fd/0 -> /dev/null
l-wx------ 1 sherif sherif 64 mrt 29 18:45 /proc/17612/fd/1 -> /dev/null
lr-x------ 1 sherif sherif 64 mrt 29 18:50 /proc/17612/fd/10 -> anon_inode:inotify
lrwx------ 1 sherif sherif 64 mrt 29 18:50 /proc/17612/fd/11 -> 'socket:[59774]'
lrwx------ 1 sherif sherif 64 mrt 29 18:50 /proc/17612/fd/12 -> /dev/ptmx
lrwx------ 1 sherif sherif 64 mrt 29 18:51 /proc/17612/fd/13 -> '/tmp/#2228246 (deleted)'
lrwx------ 1 sherif sherif 64 mrt 29 18:51 /proc/17612/fd/14 -> '/tmp/#2241053 (deleted)'
lrwx------ 1 sherif sherif 64 mrt 29 18:53 /proc/17612/fd/15 -> '/tmp/#2241055 (deleted)'
l-wx------ 1 sherif sherif 64 mrt 29 18:45 /proc/17612/fd/2 -> /home/sherif/.xsession-errors
lrwx------ 1 sherif sherif 64 mrt 29 18:45 /proc/17612/fd/3 -> 'anon_inode:[eventfd]'
lrwx------ 1 sherif sherif 64 mrt 29 18:45 /proc/17612/fd/4 -> 'anon_inode:[eventfd]'
lrwx------ 1 sherif sherif 64 mrt 29 18:45 /proc/17612/fd/5 -> 'socket:[59770]'
lrwx------ 1 sherif sherif 64 mrt 29 18:45 /proc/17612/fd/6 -> 'anon_inode:[eventfd]'
lrwx------ 1 sherif sherif 64 mrt 29 18:45 /proc/17612/fd/7 -> 'socket:[59772]'
lrwx------ 1 sherif sherif 64 mrt 29 18:45 /proc/17612/fd/8 -> 'anon_inode:[eventfd]'
lrwx------ 1 sherif sherif 64 mrt 29 18:50 /proc/17612/fd/9 -> 'socket:[59773]'
sherif@fingolfin:~$

As you can see, the output of un-customized lsof forms a super set of the actual file descriptors that fall under the Linux openfile limit.

For more information about lsof and how to tweak the output, please check: https://www.thegeekstuff.com/2012/08/lsof-command-examples