CIS241

System-Level Programming and Utilities

SSH

Erik Fredericks, frederer@gvsu.edu
Fall 2025

Based on material provided by Erin Carrier, Austin Ferguson, and Katherine Bowers

First, Linux

What is it?

• Often referred to as an operating system (OS)

  • Competing with Mac/Windows

• Technically, it is a kernel

Many OSs use the Linux kernel

• Ubuntu (what is on EOS)
• Arch, Kali, Gentoo, Fedora, etc.

Isn't Mac Linux too?

• Nope, Darwin! (Unix-based)

Protection Rings

A bit of history

1970s: UNIX source code typically stripped from distribution

• Exorbitant amounts of money to acquire source

Linux first developed by Linus Torvalds to be a copy of UNIX

• Released it to Internet for suggestions on how to improve it

• Kernel only updated by core developers and from suggestions

• Short history of Linux:

  • https://cvw.cac.cornell.edu/linux/intro/history

Unix?

Unix was an earlier operating system

• Linux and other systems are heavily influenced by Unix

We now use “Unix-like” to describe systems

• They follow the same general ideas as Unix
• This means most things (commands, properties) carry over!

"Unix-like" systems:

• Linux
• macOS
• FreeBSD
• Solaris

Unix Timeline

Advantages of a Linux system

• Risk reduction
• Business needs
• Stability and security
• Flexibility
• Customization
• Support
• Cost reduction

Advantages - Risk reduction

• Changes in the market or customer needs may cause companies to change software frequently
• Can be costly and time-consuming
• Support for closed source software may end
• Vendor may go out of business
• Software version may be retired
• OSS products offer the opportunity to maintain and change the source code

Advantages - Business needs

• Common software available for Linux includes:
• Scientific and engineering software
• Software emulators
• Web servers, web browsers, and e-commerce suites
• Desktop productivity software
• Graphics manipulation software
• Database software
• Security software

Advantages - Stability and security

• Customers using a closed source OS must rely on the OS vendor to fix any bugs
  • Waiting for a hot fix may take weeks or months
• The collaborative open source approach to testing and fixing bugs increases the stability of Linux
• Bugs and security loopholes in OSS programs can be identified and fixed quickly
  • Code is freely available and scrutinized by many developers

Advantages - Flexibility

• Partial list of hardware platforms on which Linux can run:
  • Linux can be (and is) customized to work on mobile and embedded devices

Advantage - Customization

Ability to control the inner workings of the OS

• To use Linux as an Internet Web server, recompile the kernel to include only the support needed to be an Internet Web server
  • Results in a much smaller and faster kernel
• Can choose to install only software packages needed to perform required tasks
• Linux supports several programming languages, such as shell and PERL scripts to customize or automate tasks

Advantage - Support

Linux documentation can be easily found on the Internet

• Frequently asked questions (FAQs)
• HOWTO documents

HOWTO documents are maintained by their authors but are centrally collected by the Linux Documentation Project (LDP)

Linux newsgroups

Linux User Groups (LUGs): Open forum of Linux users who discuss and assist each other in using and modifying the Linux OS

Advantage - Cost reduction

Linux is less expensive than other OSs

• There is no cost associated with acquiring the software
• A wealth of OSS can run on a variety of different hardware platforms running Linux

The largest costs associated with Linux:

• Costs associated with hiring people to maintain the Linux system
• Potential support from vendors (i.e., Red Hat)

Total cost of ownership (TCO): overall cost of using a particular OS

Disadvantages of a Linux system

No standard edition of Linux

Support means that you’ll be searching for very specific fixes

• Browsing SuperUser/ServerFault, forums, etc.

Not as easy to use as closed-source (Windows)

Program support

Common uses of Linux

• Internet servers
• File/print server
• Application server
• Cloud servers
• Supercomputing
• Workstations (Office, personal, scientific, etc.)
• Mobile

Windows

Windows (today), is not Unix-like

• Can you do your homework on a Windows machine?
• Not by default

However, we have WSL! (◕‿◕✿)

• Windows Subsystem for Linux
• If you are on Windows, you must install WSL
  https://learn.microsoft.com/en-us/windows/wsl/install
  • Be very happy about this - I dreamed about just having the ability to SSH from the terminal when in industry

Terminal

Shells (local)

See the little dollar sign?

Shell!

What shell are we using?

echo $0

Shells (local)

Where is the shell here?

• Ring 3, typically!

How do we run in other rings?

Shells (remote)

Or, connecting to another machine

• Or or, doing your homework ಠ_ಠ

ssh --> Secure Shell

Usage: ssh username@address

For example, connecting to EOS:

ssh username@eos1.cis.gvsu.edu

Lab fun

DO NOT

SHUT DOWN

THESE MACHINES

Last - the web interface

https://computing.gvsu.edu

This will provide a web-based point of access for you!

• Allows for desktop or terminal

• Note that the user experience is not great!

  • Can't copy/paste files from your computer to the desktop
  • Random connection breaks common

• However - don't need a VPN!

Ok now really the last

To exit an SSH session (or to kill your terminal)

exit