### Unix systems use integers to track open files Process, represented by a box with a smiley face: Open foo.txt
kernel, also represented by a box with a smiley face: okay! that’s file #7 for you.

these integers are called file descriptors

lsof (list open files) will show you a process’s open files

$lsof -P 4242
(4242 is the PID we’re interested in)

FD NAME

0 /dev/pts/tty1
1 /dev/pts/tty1
2 pipe: 29174
3 /home/bork/awesome.txt 
5 /tmp/

(FD is for file descriptor)

file descriptors can refer to:

• files on disk
• pipes
• sockets (network connections)
• terminals (like xterm)
• devices (your speaker! /dev/null!)
• LOTS MORE (event fd, inotify, signalfo, epoll, etc.)

little tiny smiling stick figure: not EVERYTHING on Unix is a file, but lots of things are

When you read or write to a file/pipe/network connection you do that using a file descriptor

person: connect to google.com OS: ok! fd is 5!
person: write GET / HTTP/1.1) to fd #5
OS: done!

Let’s see how some simple Python code works under the hood:

Python:

f = open ("file.txt")
f. read lines()

Behind the scenes:
Python program: open file.txt
OS: ok! fd is 4
Python program: read from file #4
OS: here are the contents!

(almost) every process has 3 standard FDs:

• stdin: 0
• stdout: 1
• stderr: 2

“read from stdin”
means
“read from the file descriptor O”
(could be a pipe or file or terminal)