default data streams

They are the standard streams for I/O in UNIX/Linux systems.

They have file descriptors

0 - standard input
1 - standard output
2 - standard error

A file descriptor is a unique, non-negative integer used by an operating system kernel to identify and track an open file or input/output resource, such as a pipe or network socket

?? - Alright, why they have file descriptors? and why they are called file descriptors? Because a process needs a simple way to refer to resources it has opened, and we can represent those resources as integers. Basically, its a simple way to communicate with the kernel without the kernel knowing about the actual resource.

Main idea: The descriptors are basically the process’s handles to I/O resources managed by the kernel.

Those integers we call file descriptors. They are represented by fd

The name file descriptor comes from Unix’s design where many I/O resources are accessed using a file-like interface. A fd can refer to a regular file, terminal, pipe, socket or a device.

Every process has a list of file descriptors.

Process A

fd 0 → terminal input
fd 1 → terminal output
fd 2 → terminal errors
fd 3 → file.txt
fd 4 → socket

Process B

fd 0 → terminal input
fd 1 → terminal output
fd 2 → terminal errors
fd 3 → database.sock

So fd 3 in Process A and fd 3 in Process B can refer to completely different things.

The file descriptor number is only meaningful inside that process.

The following script will print all file descriptors of all processes.

#! /bin/bash

find /proc -maxdepth 1 -type d -regex '/proc/[0-9]+' -printf '%P\n' |
  {
    while read -r pid; do
      if  -d /proc/$pid ; then
        printf '%d:' "$pid"
        find /proc/"$pid"/fd -type l -printf ' %P' 2>/dev/null
        printf '\n'
      fi
    done
  }

what the heck is a stream ?

A stream is just a flow of data that a program can read from or write to over time. In other words: A stream is a channel through which data flows between a program and something else.

A file descriptor belongs to an OS-level I/O resource managed by the kernel. A stream is a higher-level idea: a flow of data.

stdout stream
     ↓
file descriptor 1
     ↓
kernel
     ↓
terminal

Stdout and stderr are separate so the system can distinguish normal outputs from problems.

connects the cat stdout with the grep stdin through pipe

cat file.txt | grep "docker"

we are changing the python stdout before starting it

python script.py > output.txt

Redirect the file descriptor 2 to errors.txt

2 > errors.txt

When we say a process “inherits” stdin, stdout, and stderr, we mean that when one process starts another process, the new process normally begins with access to the same open file descriptors as its parent.

Terminal
   ↓
 bash
   ↓
 python

The parent process gives the child process its initial stdin, stdout, and stderr connections.