Unix

The operating system that shaped every OS after it. Born at Bell Labs in 1969, Unix introduced the concepts of hierarchical file systems, pipes, shell scripting, and the philosophy that programs should do one thing and do it well.

What is Unix?

Unix is a family of multitasking, multiuser computer operating systems originally developed at AT&T Bell Labs by Ken Thompson, Dennis Ritchie, and others. Written in C, it became portable across hardware platforms — a revolutionary idea at the time. Today every macOS, Linux, Android, and iOS device traces its lineage to Unix.

Core Concepts

Everything is a File

Devices, sockets, pipes, and processes are all represented as files in the filesystem. One interface rules them all.

Pipes and Filters

Programs are connected via pipes (|). Small tools compose into powerful workflows without any code changes.

Hierarchical Filesystem

A single rooted tree starting at /. Directories, files, and devices all live in one unified namespace.

Shell as First-Class Language

The shell is both a command interpreter and a scripting language. Automation is built in from day one.

Multiuser & Multitasking

Multiple users share one system simultaneously. Processes are isolated with user and group permissions.

Portability

Written in C rather than assembly, Unix could be recompiled for different hardware. This made it spread everywhere.

Process Model

fork() creates a copy of a process; exec() replaces it with a new program. This elegant two-step is still used today.

Standard Streams

stdin, stdout, stderr — every process gets three open file descriptors. Redirection and piping flow from this model.

Unix Standards

POSIX

Portable Operating System Interface — IEEE standard defining the Unix API. Programs written to POSIX run on any conformant OS.

Single UNIX Specification

The Open Group's brand license. Only certified systems may call themselves "UNIX" — macOS, Solaris, AIX, HP-UX qualify.

ANSI/ISO C

Unix drove the standardization of C itself. The C language was created to write Unix, and Unix made C ubiquitous.

Unix Philosophy

Formulated by Doug McIlroy and articulated by Ken Thompson, the Unix philosophy is a set of design principles that made Unix software composable, maintainable, and powerful. Eric Raymond later distilled it into 17 rules in The Art of Unix Programming.

The Three Core Rules (McIlroy)

RULE 1

Write programs that do one thing and do it well.

Each tool has a single, clear purpose. ls lists. grep filters. wc counts. Nothing more.
RULE 2

Write programs to work together.

Tools are designed to be composed. The output of one becomes the input of the next via pipes.
RULE 3

Write programs to handle text streams, because that is a universal interface.

Plain text is human-readable, scriptable, and works across every tool, language, and protocol.

Eric Raymond's 17 Rules

Rule of Modularity
Build simple parts connected by clean interfaces.
Rule of Clarity
Clarity is better than cleverness.
Rule of Composition
Design programs to be connected with other programs.
Rule of Separation
Separate policy from mechanism; separate interfaces from engines.
Rule of Simplicity
Design for simplicity; add complexity only where you must.
Rule of Parsimony
Write a big program only when nothing else will do.
Rule of Transparency
Design for visibility to make inspection and debugging easier.
Rule of Robustness
Robustness is the child of transparency and simplicity.
Rule of Representation
Fold knowledge into data so program logic can be stupid and robust.
Rule of Least Surprise
In interface design, always do the least surprising thing.
Rule of Silence
When a program has nothing surprising to say, it should say nothing.
Rule of Repair
When you must fail, fail noisily and as soon as possible.
Rule of Economy
Programmer time is expensive; conserve it in preference to machine time.
Rule of Generation
Avoid hand-hacking; write programs to write programs when you can.
Rule of Optimization
Prototype before polishing. Get it working before you optimize it.
Rule of Diversity
Distrust all claims for one true way.
Rule of Extensibility
Design for the future, because it will be here sooner than you think.

Unix Timeline

From a discarded space travel game to the foundation of modern computing — 55+ years of Unix history.

1964

Multics

Bell Labs, MIT, and GE begin the Multics project — a time-sharing OS. Bell Labs eventually withdraws. The lessons learned directly inspire Unix.

1969

Unix Born at Bell Labs

Ken Thompson writes the first Unix on a discarded PDP-7 to run Space Travel, a game he'd written. Dennis Ritchie joins. The name "Unix" — a pun on Multics — is coined by Brian Kernighan.

1971

Unix Version 1 & The Manual

First edition of the Unix Programmer's Manual published. Unix runs on PDP-11 hardware. Used internally by Bell Labs for text processing.

1972–73

Unix Rewritten in C

Dennis Ritchie develops the C programming language. Unix is rewritten in C — making it portable across different hardware platforms for the first time.

1975

Unix Version 6 — Public Release

First Unix widely distributed outside Bell Labs. Licensed to universities for a nominal fee. UC Berkeley receives a copy and begins major modifications.

1977

BSD Begins

Bill Joy at UC Berkeley releases the first Berkeley Software Distribution (1BSD), adding the Pascal compiler and the ex editor to Unix V6.

1979

Unix Version 7

The last universally distributed version of Research Unix. Considered the definitive "classic" Unix. Includes C, awk, sed, make, and the Bourne shell.

1983

4.2BSD & TCP/IP

Berkeley releases 4.2BSD with a complete TCP/IP stack implementation. This becomes the networking foundation of the early internet.

1983

System V Release 1

AT&T releases Unix System V — the official commercial Unix. System V and BSD become the two main Unix branches, diverging significantly.

1984

GNU Project Founded

Richard Stallman launches GNU — a free Unix-compatible OS. GNU tools (gcc, glibc, bash, etc.) become the userland of Linux years later.

1987

MINIX

Andrew Tanenbaum creates MINIX, a Unix-like OS for educational use. It directly inspires Linus Torvalds to create Linux four years later.

1988

POSIX Standard

IEEE publishes POSIX — a standard that unifies Unix interfaces and allows software to be portable across Unix variants.

1991

Linux 0.01

Linus Torvalds releases Linux 0.01 on a Usenet post: "Just a hobby, won't be big and professional." It becomes the most deployed OS kernel in history.

1993

FreeBSD, NetBSD, OpenBSD

The legal dispute between AT&T and BSDI is settled. FreeBSD 1.0 and NetBSD 0.8 are released. BSD splits into three major open source projects.

1999

Unix Turns 30

Thompson and Ritchie receive the National Medal of Technology from President Clinton for their work on Unix and C.

2001

Mac OS X

Apple releases Mac OS X, built on a Unix foundation (Darwin/XNU, derived from NeXTSTEP and BSD). It is certified UNIX in 2007.

2007

iOS — Unix in Your Pocket

Apple's iPhone runs a Unix kernel. Android (2008) runs Linux. Together they put Unix in billions of pockets worldwide.

2011

Dennis Ritchie Dies

Dennis Ritchie, creator of C and co-creator of Unix, dies at age 70. His contributions underpin virtually all modern computing.

2019

Unix Turns 50

Five decades since that PDP-7 experiment. Unix runs satellites, stock exchanges, the ISS, submarines, and every smartphone on earth.

Unix Commands

The original Unix command set, refined over 55 years. These tools form the foundation of every Unix-like system in the world.

File System

List directory contents

ls -lah /etc

Change directory

cd /var/log

pwd

Print working directory

pwd

mkdir

Make directories

mkdir -p a/b/c

rmdir

Remove empty directories

rmdir tmp/

Copy files and directories

cp -r src/ dst/

Move or rename files

mv old.txt new.txt

Remove files

rm -rf build/

find

Search for files

find . -name "*.log"

Create hard/soft links

ln -s /usr/bin/foo foo

chmod

Change file permissions

chmod 755 script.sh

chown

Change file owner

chown root:root file

stat

Display file status

stat /etc/passwd

Disk usage of files

du -sh /var

Disk free space

df -h

mount

Mount a filesystem

mount /dev/sdb1 /mnt

Text Processing

cat

Concatenate and print files

cat /etc/hosts

grep

Search text with patterns

grep -r "error" /var/log

sed

Stream editor

sed 's/foo/bar/g' file

awk

Pattern-action text processing

awk '{print $1}' log

sort

Sort lines of text

sort -n numbers.txt

uniq

Filter duplicate lines

sort file | uniq -c

cut

Cut sections from lines

cut -d: -f1 /etc/passwd

paste

Merge lines of files

paste file1 file2

Translate characters

tr 'a-z' 'A-Z'

Word, line, char count

wc -l file.txt

head

First N lines of file

head -20 log

tail

Last N lines of file

tail -f /var/log/syslog

diff

Compare files line by line

diff old.txt new.txt

tee

Write to stdout and files

cmd | tee output.log

xargs

Build and run commands

find . | xargs grep foo

Process Management

List running processes

ps aux

top

Live process monitor

top

kill

Send signal to process

kill -9 1234

killall

Kill by process name

killall nginx

nice

Set scheduling priority

nice -n 10 cmd

nohup

Run immune to hangups

nohup ./server &

wait

Wait for job completion

wait $!

fg / bg

Foreground / background job

bg %1

Networking

ssh

Secure remote shell

ssh user@host

scp

Secure copy over SSH

scp file user@host:/tmp

curl

Transfer data via URLs

curl -s https://api.io

wget

Download files from web

wget https://example.com

ping

ICMP reachability test

ping google.com

netstat

Network connections & stats

netstat -tlnp

traceroute

Trace packet route

traceroute google.com

nslookup

DNS lookup utility

nslookup ximg.app

System Information

uname

OS name and version

uname -a

whoami

Current user name

whoami

User and group IDs

id username

uptime

How long system has run

uptime

date

Print or set date/time

date '+%Y-%m-%d'

env

Print environment variables

env | grep PATH

which

Find command in PATH

which python3

man

Manual pages

man grep

Unix Family Tree

Unix spawned a sprawling family of operating systems. Every major OS in use today — from macOS to Android to the world's servers — descends from or was inspired by the original 1969 Bell Labs system.

Research Unix (Bell Labs)

Research Unix V1–V7

Bell Labs 1969–79

The original. Written by Thompson, Ritchie & others. V7 (1979) is the last Research Unix — the definitive classical Unix.

Plan 9 from Bell Labs

Bell Labs 1992

The spiritual successor to Unix from the same team. Everything is truly a file, including the network. Influenced Go's design.

AT&T / System V Branch

UNIX System V

AT&T 1983

The official commercial Unix. System V Release 4 (1989) merged BSD features. Basis for Solaris, HP-UX, and AIX.

Solaris / illumos

Sun/Oracle 1992

Sun's SVR4 Unix, famous for ZFS and DTrace. Oracle acquired it in 2010. The open source fork is illumos.

HP-UX

HP 1984

Hewlett-Packard's certified UNIX, running on PA-RISC and Itanium hardware for enterprise computing.

IBM AIX

IBM 1986

IBM's enterprise Unix for POWER architecture. Still widely deployed in banking and critical infrastructure.

BSD Branch

BSD (Berkeley)

UC Berkeley 1977

Bill Joy's additions to Unix V6. Added TCP/IP stack (4.2BSD), vi editor, csh, and virtual memory.

FreeBSD

1993 → present

The most widely deployed BSD. Netflix, WhatsApp, PlayStation OS, and many CDNs run on FreeBSD.

OpenBSD

1996 → present

Security-focused BSD. Created OpenSSH (used on every server in the world). Default-secure since day one.

NetBSD

1993 → present

Portability-first BSD. Runs on more hardware platforms than any other OS — from ARM to SPARC to VAX.

macOS / iOS

Apple 2001

Built on Darwin (XNU kernel), which merges Mach microkernel with FreeBSD. Certified UNIX. Powers all Apple devices.

Linux Branch

Linux Kernel

Torvalds 1991

POSIX-compatible Unix-like kernel. Dominates servers, clouds, supercomputers (100% of Top500), and Android devices.

Debian / Ubuntu

1993 / 2004

The dominant Linux family for servers and desktops. Ubuntu made Linux accessible to millions of new users.

Red Hat / Fedora / CentOS

1994 → present

Enterprise Linux. RHEL runs vast portions of the Fortune 500. Fedora is the upstream bleeding-edge variant.

Android

Google 2008

Linux-based mobile OS. Runs on 3+ billion active devices — the most widely deployed OS variant in history.

People of Unix

The minds behind the most influential operating system ever written.

Ken Thompson

Co-creator of Unix, co-creator of C, creator of Go

Wrote the first Unix kernel in three weeks on a discarded PDP-7, building it to run a space travel game. Also designed the B programming language (predecessor to C), grep, and the UTF-8 encoding. Later created Go at Google.

Dennis Ritchie

Co-creator of Unix, creator of C

Created the C programming language, which enabled Unix to be rewritten portably. Authored "The C Programming Language" (K&R) with Kernighan. Received the Turing Award and National Medal of Technology.

Brian Kernighan

Co-authored K&R C, named Unix, co-created AWK

Coined the name "Unix" as a pun on Multics. Co-authored "The C Programming Language" and "The Unix Programming Environment." Created AWK with Al Aho and Peter Weinberger.

Doug McIlroy

Invented Unix pipes, formulated Unix philosophy

Created Unix pipes — the core mechanism that enables the Unix philosophy of composable tools. His 1964 memo proposing pipes was the seed of the Unix design philosophy.

Bill Joy

Co-founder of Sun, creator of BSD, vi, csh, NFS

At UC Berkeley, rewrote the Unix TCP/IP stack (4.2BSD) that became the internet's networking foundation. Created vi and the C shell. Later co-founded Sun Microsystems.

Linus Torvalds

Creator of Linux, creator of Git

Wrote the Linux kernel in 1991 as a hobby project inspired by MINIX. Linux now runs 100% of the world's top 500 supercomputers, most cloud servers, and every Android phone.

Richard Stallman

Founder of GNU, creator of GPL, FSF

Launched the GNU Project in 1983 to create a free Unix-compatible OS. Created GCC, Emacs, and the GPL license. His GNU tools + Linux kernel form GNU/Linux.

Andrew Tanenbaum

Creator of MINIX, microkernel pioneer

Created MINIX in 1987 for education — the OS that directly inspired Linus Torvalds to create Linux. Author of "Modern Operating Systems," a foundational CS textbook.

Theo de Raadt

Founder of OpenBSD and OpenSSH

Founded OpenBSD in 1996, creating the world's most security-focused OS. OpenSSH — used on virtually every Unix server — is an OpenBSD project.

Kirk McKusick

BSD pioneer, creator of FFS, TCP/IP contributor

Created the Fast File System (FFS/UFS) at Berkeley. Key architect of 4.3BSD. Still active in FreeBSD development. Co-authored the BSD daemon logo.