Operating System Shells and Permissions

Moore's Law

Processing power continues to increase with the density of transistors on an Integrated Circuit (IC) doubling every two years based on a principle called Moore's Law. Since postulated by Intel co-founder Gordon Moore in 1965, this prediction has held steady through innovations that continue to push physics beyond what could be imagined to this day. What was once two-dimensional, 90 nanometer (nm) strained silicon wafers that landed Apollo XI on the moon has morphed into the ability to cram over 100 billion transistors using extreme ultraviolet (EUV) lithography to fabricate a die that combines three-dimensional stacking and other advanced manufacturing processes to deliver processors such as the Apple's M1 Ultra Chip and Nvidia's RTX 4090 Graphics Processing Unit (GPU). By 2030, processors are estimated to pack 1 trillion transistors per chip (Kelleher, 2023).

These hardware improvements are directly seen with the Operating Systems (OSs) ability to also exponentially improve. As you progress through this class, keep in mind the direct relationship the OS has with advancements in hardware capabilities.

System Services

Because the OS manages the computer and all the programs that run on the computer, it is of critical importance to security. It can restrict what Programs and users do to make sure they can't cause too much trouble on the system. Conversely though, when OSs have security flaws, it can be a significant problem for businesses and Information Technology (IT) organizations.

NASA's Curiosity Mars explorer
You might wonder what OS gets used for something like NASA's Curiosity robot, which is exploring Mars as we speak. It's not one of the major PC or mobile OSs we talk about in ! Check out this article which describes the hardware and OS on the Curiosity. It includes a 10+ year-old CPU, and an OS that was initially released in 1985, but it has to be able to handle extremes in temperature, radiation, navigate an alien world on its own, and communicate with engineers so far away light itself takes 14 minutes to make the trip ... one-way!

The OS provides services to Users and programs — it does things they need done that they cannot or are not allowed to do for themselves. Some important kinds of OS-provided system services are:

• File Operations — the OS manages all the different storage-related peripherals, like hard drives, flash drives, DVDs, etc. The OS can create, modify, read and destroy files on behalf of other programs.
• Network Connections — the OS can actually send or receive data to/from a computer's Ethernet port (wired) or WiFi radio (wireless). A program that wishes to send data over a network asks the OS to do it on the program's behalf.
• User Accounts, Permissions, and Logins — the OS ensures that users log in properly, and that they can only access the things they're supposed to.
Knowledge Check: What part of the CIA-triad is applicable to users logging into a system?
Check Answer

• Processes — more about this below. The important thing here is that the OS manages the programs as they execute, scheduling when each program gets to use the CPU to progress in its instructions.

The OS generally provides three ways for programs and users to access its services:

• The GUI — GUI stands for Graphical User Interface and is the graphical or image-based users interact with through inputs for associated peripherals, such as using your fingers on a touch screen or clicking on a mouse to open an application. For example, in Windows you can right-click on a file name in the file browser, choose the "rename" option in the resulting menu, type in a new name, and then press enter. This asks the OS to change the file's name on the hard drive for you. The GUI is an OS interface designed for users and not as much for programs.
• The Shell — The shell allows commands for the OS to be entered in a Command Line Interface (CLI). Shell exercises are provided in this class for you to begin learning how it's used. The shell is an OS interface for both programs and users.
• The API — API stands for Application Programming Interface. It is a very direct way for an executing program to ask the OS to do something on its behalf. The API is an interface exclusively for programs and may also be accessed through a web interface. An opportunity to work with APIs will be provided later in the course.

Although you are probably only familiar with using the GUI to access OS services, understanding and using shell will be important for this course. That importance stems in no small measure from the fact that the shell is an interface to the OS for both users and programs alike. The OSs we'll use for this course are Windows and Linux, which will be the next item covered in this class.

The advancements in computer hardware also allowed for improvements in software. With command-line operating systems, like Microsoft Desktop Operating System (MS-DOS), released in the early 1980's, the Windows GUI wasn't launched until 1990 using an 8086 16-bit processor. Today's 64-bit processors allow for faster, more efficient, multi-thread capabilities but many of the underlying technologies within the OS haven't changed since the 2000's (i.e. the system registry for the fellow nerds that may be reading this).

For the upcoming Lab 2.1, Windows Operating Systems, you'll familiarize yourself with some of the GUI components and navigation but primarily for comparing output provided by shell commands. A shell simply allows a user to interact with the OS by running input commands that may result in outputs sent to the screen. By default, the Windows OS boots into its GUI, which is the color images you see and interact with on a regular basis. The Windows Start menu is a graphical element used to access additional programs and is in the lower left corner of the taskbar. Throughout the different versions of Windows, the Start menu may look different but its purpose and function are all the same.

Activity: Command Prompt

The Windows OS has two different programs that can be used to access the shell (1) command prompt and (2) PowerShell. This course has been updated to use PowerShell but you may take a quick peek at command prompt and its functions by executing the following instructions:

1. Click on Windows Start and type cmd followed by the ↩ Enter key.
2. Select the Command Prompt app and a new window should appear.
   
    Note: Command Prompt is the name of the application but a prompt will be where commands are submitted. The illustration of a console window below shows a prompt containing the current working directory and ending in the > symbol.
   
   
   Microsoft Windows [Version 10.0.22621.1992]
   (c) Microsoft Corporation. All rights reserved.
   
   C:\Users\m9999>


3. Type echo "Hello World!" followed by the ↩ Enter key and you'll see the message appear on the screen.
4. Type color 02 followed by the ↩ Enter key and see what happens to the console!
5. Now type title Hacker Console followed by the ↩ Enter key and look at the upper left corner of the window.
6. What command do you think you would type to 'exit' the program?

Windows Ownership and Permissions

Local Users and Groups

Windows local user accounts are stored in the Security Account Manager (SAM) database located in C:\Windows\System32\config and password hashes stored in the system registry HKEY_LOCAL_MACHINE\SAM. The default administrator account for the Windows OS is called Administrator and comes standard when installing any Windows OS for the first time. Many systems come with default administrator accounts and it is vital that any default usernames and passwords are changed! The Windows Registry is a hierarchical database that contains configuration settings for the Windows OS, kernel, drivers, services, and programs that may store values that restrict access to kernel mode.

Why is your account not in the local users group on your computer? Because it's a Windows Active Directory (AD) account under the academy domain. In this case, your user account is an object located in AD and not the local system. All of the permissions and objects are stored on the AD server.

1. Click on Windows Start and type computer management followed by the ↩ Enter key.
2. Click on Windows Start and type file explorer followed by the ↩ Enter key. Right click on This PC and select Show More Options, then Manage.

Directories, Files, and Programs

There are several access control methods used in cyberspace to determine if a user has permissions to conduct certain actions on a system. Microsoft describes access control for Windows as the process of authorizing users, groups, and computers to access objects on the network or computer. Objects can be files, directories, user accounts, and more. To keep things simple, stick to files and directories in a file system. Some of the common permissions seen in Windows include:

• F: Full Access
• M: Modify Access
• RX: Read & Execute Access
• R: Read-Only Access
• W: Write-Only Access

PS C:\Users\m9999\Documents> icacls .\Nimitz.txt
.\Nimitz.txt NT AUTHORITY\SYSTEM:(I)(F)
             BUILTIN\Administrators:(I)(F)
             ACADEMY\m9999>:(I)(F)

Successfully processed 1 files; Failed processing 0 files
C:\Users\m999\Documents>

Linux Shells

Linux has a complicated history since its initial deviation from Unix, initially discussed in the Intro to Cyberspace Domain class. There have been hundreds of flavors, or distributions (distros), of Linux since its initial release in 1991. The common distributions today include Debian, RedHat, and FreeBSD. Ubuntu and Kali are additional distros of Debian, or Debian-based, while CentOS and Fedora are RedHat-based distros. These are just some of the maintained distros, with many more that are no longer supported and ended up fading away into history (i.e. not supported with updates to fix bugs or patches to fix vulnerabilities).

There are currently 10 official Ubuntu 'flavors' (distros) and 3 unofficial flavors as of Aug 2024. So distros can have distros that branch out into further specialized OSs. For example, Edubuntu was modified and designed to support preschool, primary, secondary and tertiary educational institutions; Lubuntu and Xubuntu are intended for use on less-powerful computers or those who seek a highly efficient desktop environment on faster systems; and Ubuntu Studio focuses on open-source applications for multimedia creation aimed at audio, video and graphic editors. The Linux workstations in our labs on campus all run Ubuntu Mate (pronounced "mah-tay").

As far as what's used here in , you'll become familiar with Ubuntu Server, which will run Apache to host your websites, and Kali, which will be leveraged to exploit internal systems to conduct cyber attacks, both of which are Ubuntu-based distros!

Is your smartphone a computer? Did you know that it requires an OS? Since we're on the topic of Linux, an iPhone is similar to OSX in that it's based on Darwin(BSD). When AndroidOS was developed in 2008, it deviated into its own distro to establish an Android Open Source Project (AOSP) platform based on a modified Linux kernel.

Activity: Remote Linux Shell

In order to access the Linux server, you'll need to remotely connect to the shell using an application called secure shell (ssh). Follow the steps below to use PowerShell to remotely connect to the server:

1. Click on Windows Start and type powershell followed by the ↩ Enter key.
2. Select the Windows PowerShell app and a new window should appear.
3. Type ssh m9999@ followed by the ↩ Enter key. Replace '9999' with your own alpha!
 Note: If this is the first time you are connecting to the server, the message below will appear:
 
Are you sure you want to continue connecting (yes/no/[fingerprint])?


Syntax errors and invalid commands.
The shell will often NOT respond with any output when entering valid commands but remember that computers are not able to understand the intent of a user and will only do what it is told. If there is a problem with how a command is formatted, the shell will return a syntax error or if it is formatted correctly but the system is not able to complete an action, it will return an error. Be aware when the shell returns errors as that is the only way for the system to communicate to the user when a problem occurs with a command.
 Type yes followed by the ↩ Enter key.


4. Enter your password but note, ***you will not see anything as you type.***
Note: Use the domain authentication password. If you violate the password policy, unlock your account using the Password Portal.


5. Once authenticated to the server, a prompt that ends in '$' will be displayed. Welcome to the Linux command line!
6. If that is not the case, return to step 3 and review what went wrong. If you are good, enter hostname to see the name of the computer you are connected to. If it returns with a value beginning with ward-rweb then you are connected to the remote system!
7. Type uname -v and it should indicate that the OS is Ubuntu.
More than just one shell.
Just as Windows has command prompt and PowerShell, Linux has more than one shell available. The default shell accessed is the Bourne-Again SHell, or BASH. Other shells available on the system include KornShell (ksh) and C Shell (csh). Building out the Linux extra credit used bash scripting to create the thousands of files and folders without having to enter commands over and over again to create the capture-the-flag scenario.
8. Now close the connection by entering exit.
9. To make sure the connection has been closed, verify the hostname by typing hostname and it should return your computer name similar to WK0XX0000SY0MID format, indicating that you are now back on the local shell.

When connecting to a remote system, it's important to know what the logical connections look like when accessing physical networks. Below is a basic graphical depiction of the client (laptop) establishing a connection to the server for the remote shell session that was just conducted.

+--------+          +--------+
| Windows|---SSH--->| Linux  |
|(Laptop)|          |(Server)|
+--------+          +--------+

Linux Ownership and Permissions

Local Users and Groups

The local Linux account database is stored in /etc/passwd, with passwords stored in /etc/shadow. The default administrator account on Unix-based systems, which includes Linux, is root. Remember, there is also the root directory in a file hierarchy but "root" is also a username on a Linux system. In a cybersecurity concept referred to as least privilege, never log into a system as root as there's a program called sudo, which stands for "superuser do," that enables users to run commands with escalated privileges.

Files, Directories, and Programs

Permissions in Linux are displayed using the shell when listing directory contents across ten characters in the format:
  -rwxr-xr--

Directory. The first value will identify directories with d or files with a null value -.

Owner. The next three values represent owner permissions as r:read, w:write, and x:execute. In this example, the owner has all three permissions.

Group. Similar to owner, the middle three values represent group permissions, with r-x identified in the example. So the group has read and execute permissions.

Other. Everyone else that doesn't apply to the identified owner and group will fall into this category, with r-- defined, or only read permissions.

For no permissions, null values across the three read, write, execute will be displayed as ---.

So what do these permissions mean? For a file, read and write permissions are relatively straightforward, allowing you to read data from, or write data to, a file. Execute permission allows a user to execute a file, e.g., run it as a program.

For directories, read permission lets you read directory contents - e.g., list out files and subdirectories. Write permission allows you to create, delete, or rename files and subdirectoreis. Execute permissions allow you to access a directory, e.g., cd into the directory. However, there's actually a slight wrinkle to these last two - you need write and execute permission to modify directory contents, not just write permission. Interestingly, with write permission on a directory, you can actually delete a file within that directory, even if you have no permissions on the file itself.

Let's evaluate the console output example that was first shown in the File Systems and Hierarchies class:

m9999@ubuntu:~$ ls -l
total 16901988
drwxr-xr-x  2 m9999  mids        4096 Feb 15  2023 Desktop
drwxr-xr-x  6 m9999  mids        4096 May 12 18:07 Documents
drwxr-xr-x  2 m9999  mids        4096 May  5 14:41 Downloads
-rw-r--r--  1 m9999  mids         276 Aug 16  2023 Class01.txt
drwxr-xr-x  2 m9999  mids        4096 Feb 15  2023 Music
drwxr-xr-x  2 m9999  mids        4096 Feb 15  2023 Pictures
drwxr--r--  4 m9999  mids        4096 Oct  4  2023 projects
drwxr-xr-x  2 m9999  mids        4096 Feb 15  2023 Public
drwxr-xr-x 12 m9999  mids        4096 Jul 23 14:43 public_html
drwxr-xr-x  2 m9999  mids        4096 Feb 15  2023 Templates
drwxr-xr-x  2 m9999  mids        4096 Feb 15  2023 Videos

The commands chown, chgrp, and chmod modifies file ownership for the owner and group and file permissions. Given the rwx format for file permissions, they can be specified using the letter or decimal format. For example, the command chmod 754 ./filename would change filename permissions for the owner to rwx, the group to r-x, and other to r--.

  The long listing of the filename would look like: -rwxr-xr--

  Breaking it down further:       

Supplemental Media:

You're The OS

Operating System Security

Review Questions:

1. What is Moore's Law and how does it apply to Computer Architecture?
2. What is the purpose of an OS?
3. What are the FOUR system services an OS provides?
4. What are the THREE methods users can access OS services?
5. How do you access a Windows Shell, both command prompt and PowerShell, using the GUI?
6. Where is the primary database for the Windows configuration settings located?
7. How do you remotely access the Linux shell on ?
8. What are the default administrator accounts for Windows and Linux systems?
9. Where is the primary location for user accounts and credentials stored on Windows and Linux systems?
10. What are the differences between OS Kernel and User modes?
11. How are the terms shell, console, terminal, command line, and CLI used?

References

1. Intel Press Kit, “Moore’s Law,” Intel Newsroom, 2023.
2. A. B. Kelleher, “Moore’s Law – Now and in the Future,” Intel, Feb. 2022.
3. “Intel Research Fuels Moore’s Law … to a Trillion Transistors by 2030,” HPCwire, Dec. 3, 2022.
4. Microsoft, “PowerShell SSH Remoting,” Microsoft Docs, 2025.
5. ScienceDirect Topics, “PowerShell Shell,” ScienceDirect, Accessed July 2025.
6. Z3pH7, “Introduction to Linux Shells,” Medium, Oct. 2024.
7. S. McBrien, “Linux file permissions explained,” Red Hat Blog, Jan. 10, 2023.