Overview

UNIX is the godfather of about 90% of all modern OSes. Developed a group of computer science researchers at AT&T's Bell Labs, they developed the basic filesystem structure and many of the basic programs we use in most OSes today. Although first written in assembly for specific computers, it was eventually written in C and made portable, which was a major step towards it gaining popularity.

Derivatives of UNIX

Eventually other projects with modifications to the UNIX OS started to appear.

BSD or Berkley Software Distribution was originally just a superset of Unix and implemented the first TCP/IP protocol. It eventually became the father of two other OSes that are fairly popular in the modern age, MacOS and FreeBSD. There were some other popular OSes that came from Unix/BSD which eventually lost popularity such as SunOS and Solaris.

The Rise of Linux

The problem with UNIX and most other UNIX-like OSes is they were all paid for and proprietary software. While some projects such as BSD were already pushing to make their software completely open source, Linux came in as one of the first completely open source and free to use software that ported over many of the Unix utilities and structure.

This was accomplished mostly by the GNU project as they collected and wrote the core programs that most operating systems at the time used. They then released that collection of programs under a GNU General Public License or GPL which allowed for sharing, modification and commercial use of the software. At the time the project was only missing an operating system kernel to tie everything together. This is where Linus Torvalds comes in. Linus was a Swedish software developer who first wrote what came to be known as the Linux kernel for a personal computer. He later published it for people to share, but not under the GNU license. Eventually after attending a speech by Richard Stallman, the founder of the GNU organization, he changed his kernel's license to the GNU GPL. From there it was only a matter of time that the kernel became widely adopted.

The heavy involvement of GNU in the creation of what became widely known as Linux is why you'll sometimes see people correct you and say it should be called "GNU-Linux".

Fun note, the name Linux only came about as Linus' friend who hosted the FTP server that held the kernel named his directory "linux".

Minix

A different OS named Minix actually came out before Linux that was free and open source. It was created by professor Andrew S. Tanenbaum who wrote the program as a way for students to learn Unix. It came packaged with a textbook but was also distributed for free to universities. While this OS predated the Linux kernel it never gained popularity as it was not designed for commercial use, while the GNU project had commercial use in mind.

It's a very cool project, only 12,000 lines of C code for a fully functional OS and uses a different kernel architecture as compared to Linux (microkernel v.s. monolithic kernel). It still has some use today and Intel has even been found to use it in their CPUs.

POSIX

With the prevalence of UNIX-like operating systems the IEEE decided to standardize how OSes and applications interact to make porting applications and OSes much easier. POSIX (Portable Operating System Interface) is also a trademark owned by the IEEE so operating systems can be POSIX certified. While many OSes follow the POSIX standard, the only commercially used OS that is POSIX certified is MacOS (ironically).

Commands

The following are commands that should appear in any UNIX-like OS. Most of them are basic commands but there are some useful ones I've also listed out.

The simpler and common commands are grouped by similarity. The more complex commands have their own sections.

I/O

Processes

File System

User

Extras

Programs

SSH

SSH is a program that allows us to easily access Linux machines form the command line remotely. Almost anything that can be done on the machine locally can be done from SSH.

SSH-Client

An SSH connection can be invoked something similar to the following:

ssh <user>@<host>

<user> being the username of the user you would like to connect to on the remote server and <host> being the IP address or domain name of the remote server.

If the SSH server is running on a different port e.g. port 8100 the -p flag can be appended:

ssh <user>@<host> -p 8100

If you are using an key that is not named id_rsa.pub you can specify the public key with the -i flag:

ssh <user>@<host> -i /path/to/pubkey

SSH Config

SSH has a configuration file that can be found at ~/.ssh/config. The formatting of a config file would be similar to the following:

Host hostname1
  HostName www.example.com
  User user
  Port 8100
  IdentityFile /path/to/pubkey

With that file set the command ssh hostname1 will initiate a connection with all the parameters set for hostname1.

This file can be expanded to as many hosts as you would like and can even be configured with basic regex to match certain parameters.

Read more about SSH config here

SSH Keys

SSH Has a more secure method of authentication than user and password. This is called SSH keys. One can be generated by using the following command and following the instructions:

ssh-keygen

I usually add the flags -t rsa -b 4090 to specify the RSA encryption algorithm and generate a longer key.

SSH-Server

The SSH server configuration can be found at /etc/ssh/sshd_config.

I usually set the following parameters:

PermitRootLogin no
PasswordAuthentication no
Port 10023

Read more about the ssh_config file here

crontab

Crontab is a program that is useful to run commands on a schedule. It uses a simple config file that can be accessed by running:

crontab -e

When the config file is open, you can add a scheduled job by using the following syntax:

MIN HR DOM MONTH DOW /path/to/command/or/script

e.g. Run a backup script at every 1st minute. (Runs the first minute of every hour of the day 24/7, 356 days a year.):

1 * * * * /home/user/backup.sh

It can get a little complex but I suggest looking at Crontab Guru to get some practice and check your scheduling syntax.

chmod

If you run ls -l you will see each file prepended with something similar to the following:

-rw-r--r-- 

These are the read, write and execute permissions for the file to the owner of the file, group owner of the file, and everyone else. This can be modified using the chmod command.

After flags are given to the command the second to last argument of the chmod command is an octal number representation of the permissions you would like to give the file.

Each of the 3 digits corresponds to the file owner permissions, group owner permissions, and any user permissions respectively. This is also reflected in the ls -l command with each grouping of three characters minus the very first character which indicates if the file is a directory or not.

Refer to the #Octal Chart to find the correct octal number representation of file permissions for the user, group and everyone else.

e.g. the following command:

chmod 754 /path/to/myfile

Gives the owner of myfile read, write and execute permissions, the group owner of myfile read and execute permissions, and all other users read only permissions.

Octal Chart

Shells

Common Shells:

• #Bash
• Zsh
• Fish
• PowerShell

Less Common:

• Sh
• Ksh
• Csh

Common Functionality

The following is functionality that most modern shells have.

• History
• Settings
• Autocomplete/Suggestion
• I/O redirection
• Scripting

Bash

Bash, or the Bourne-Again Shell is the most prevalent shell in today's world. It is partly due to the fact that UNIX-like and Linux operating systems are often shipped with it.

Configuration

Bash has a fair bit of configuration you can do. This is done by either creating or modifying one of the following files in your $HOME directory:

• .bash_profile
• .bash_login
• .profile
• .bashrc*

Aliases

Aliases can be defined in one of the configuration files and are defined as following:

alias ls="ls -lh"

This is useful for setting common options on frequently used commands or making long commands shorter.

Environment Variables

These can be set to use globally in bash:

export filepath="/path/to/file"

I use this for setting common file paths but it can also be used to clean up your home directory.

History Expansion

A few bash history commands I find useful.

Refer to this guide for more detailed info.

Linux Distributions