• UNIX is an operating system which was first developed in the 1960s, and has been under constant development ever since. By operating system, we mean the suite of programs which make the computer work. It is a stable, multi-user, multi-tasking system for servers, desktops and laptops.

• UNIX systems also have a graphical user interface (GUI) similar to Microsoft Windows which provides an easy to use environment. 

• However, knowledge of UNIX is required for operations which aren't covered by a graphical program, or for when there is no windows interface available, for example, in a telnet session.

• The UNIX operating system is made up of three parts; the kernel, the shell and the programs.

• The kernel

  1. The kernel of UNIX is the hub of the operating system: it allocates time and memory to programs and handles the filestore and communications in response to system calls.

  2. As an illustration of the way that the shell and the kernel work together, suppose a user types rm myfile (which has the effect of removing the file myfile). The shell searches the filestore for the file containing the program rm, and then requests the kernel, through system calls, to execute the program rm on myfile. When the process rm myfile has finished running, the shell then returns the UNIX prompt % to the user, indicating that it is waiting for further commands.

• The shell

  1. The shell acts as an interface between the user and the kernel. When a user logs in, the login program checks the username and password, and then starts another program called the shell. The shell is a command line interpreter (CLI). It interprets the commands the user types in and arranges for them to be carried out. The commands are themselves programs: when they terminate, the shell gives the user another prompt (% on our systems).

  2. The adept user can customise his/her own shell, and users can use different shells on the same machine. Staff and students in the school have the tcsh shell by default.

  3. The tcsh shell has certain features to help the user inputting commands.

  4. Filename Completion - By typing part of the name of a command, filename or directory and pressing the [Tab] key, the tcsh shell will complete the rest of the name automatically. If the shell finds more than one name beginning with those letters you have typed, it will beep, prompting you to type a few more letters before pressing the tab key again.

  5. History - The shell keeps a list of the commands you have typed in. If you need to repeat a command, use the cursor keys to scroll up and down the list or type history for a list of previous commands.

• Files and processes

• Everything in UNIX is either a file or a process.

• A process is an executing program identified by a unique PID (process identifier).

• A file is a collection of data. They are created by users using text editors, running compilers etc.

• Examples of files:

  1. a document (report, essay etc.)

  2. the text of a program written in some high-level programming language

  3. instructions comprehensible directly to the machine and incomprehensible to a casual user, for example, a collection of binary digits (an executable or binary file);

  4. a directory, containing information about its contents, which may be a mixture of other directories (subdirectories) and ordinary files.

macOS is the current operating system developed and marketed by Apple for its Mac computers. It is a Unix-based, proprietary graphical operating system first introduced in 2001, succeeding the classic Mac OS that ran from 1984 to 2001. macOS is now the second most widely used desktop OS after Microsoft Windows and ahead of Linux distributions.

Key Features and History

• Origins and Development: macOS (originally Mac OS X, then OS X) was developed after Apple’s acquisition of NeXT, which brought Steve Jobs back to Apple and introduced the NeXTSTEP architecture that forms the basis of macOS.

• Processor Support: macOS has supported multiple processor architectures over the years, including PowerPC (1999–2005), Intel (2006–2020), and Apple’s own M-series ARM chips (since 2020).

• Release Cycle: Apple releases a new major version of macOS approximately every year, typically in the fall.

• Naming Conventions: Early versions were named after big cats (e.g., Cheetah, Panther, Tiger), while later versions reference locations in California (e.g., Yosemite, Mojave, Sequoia). The naming was streamlined to "OS X" in 2011 and then to "macOS" in 2016 to align with other Apple operating systems.

Recent Versions and Features

• macOS 15 Sequoia: Released in September 2024, it introduced features such as effortless window tiling, enhanced web browsing, iPhone Mirroring, and Apple Intelligence—a suite of AI-driven tools for writing, image creation, and smarter Siri integration.

• macOS 26 Tahoe: Announced at WWDC 2025, this upcoming version will feature a new design, improved cross-device integration, and productivity enhancements. New features include advanced accessibility tools, enhanced parental controls, a dedicated Journal app, updated Photos and FaceTime experiences, and a new Games app.

Ecosystem and Security

• Integration: macOS is tightly integrated with Apple’s ecosystem, enabling seamless workflows across Mac, iPhone, iPad, and other Apple devices.

• Security and Support: Apple typically provides security updates for the latest three major macOS releases, but support for older versions is not guaranteed.

• DOS stands for Disk Operating System. 

• It was developed for IBM by Microsoft Corporation, which also produced its own almost identical version called MS-DOS. Other computers at the time, such as the Commodore 64, Atari 800, and Apple II, all featured a disk operating system, CBM DOS, Atari DOS, and Apple DOS, respectively. (DOS/360 was an operating system for IBM mainframes which first appeared in 1966, but is unrelated to the 8086-based DOS of the 1980s.

• Disk operating system is also used to describe several very similar command line disk operating systems. PC-DOS (Personal Computer Disk Operating System) was the first widely-installed disk operating system used in personal computers running on Intel 8086 16-bit processors.

• All other compatible personal computers use MS-DOS. The commands used in most versions of DOS are identical and their purpose is to enable the otherwise useless electronics that go to make up a personal computer to operate as a computer.

• All personal computers require at least one disk drive on which are stored “files”. 

• A file can be either a set of computer instructions, a program, or files of data in either numerical or alphabetical form. The purpose of a disk operating system is to enable the computer to transfer the data stored on a magnetic disk to another part of the computer: its main memory, known as RAM (Random Access Memory), the video screen or the printer. 

• Linux began in 1991 as a personal project by Finnish student Linus Torvalds: to create a new free operating system kernel. 

• The resulting Linux kernel has been marked by constant growth throughout its history. 

• Since the initial release of its source code in 1991, it has grown from a small number of C files under a license prohibiting commercial distribution to the 4.15 version in 2018 with more than 23.3 million lines of source code, not counting comments, under the GNU General Public License v2.

• The Linux kernel is a free and open-source, monolithic, modular, multitasking, Unix-like operating system kernel. It was conceived and created in 1991 by Linus Torvalds  for his i386 based PC, and it was soon adopted as the kernel for the GNU Operating System, which was created as open source and free software, and based on UNIX as a by-product of the fallout of the Unix wars. Since then, it has spawned a plethora of operating system distributions, commonly also called Linux.

• Linux is deployed on a wide variety of computing systems, such as embedded devices, mobile devices (including its use in the Android operating system), personal computers, servers, mainframes, and supercomputers. 

• It can be tailored for specific architectures and for several usage scenarios using a family of simple commands (that is, without the need of manually editing its source code before compilation); privileged users can also fine-tune kernel parameters at runtime.

• Linux is a monolithic kernel with a modular design (e.g, it can insert and remove loadable kernel modules at runtime), supporting most features once only available in closed source kernels of non-free operating systems:

  1. concurrent computing and (with the availability of enough CPU cores for tasks that are ready to run) even true parallel execution of many processes at once (each of them having one or more threads of execution) on SMP and NUMA architectures;

  2. selection and configuration of hundreds of kernel features and drivers (using one of the "make *config" family of commands, before running compilation),modification of kernel parameters before booting (usually by inserting instructions into the lines of the GRUB2 menu), and fine tuning of kernel behavior at run-time (using the sysctl(8) interface to /proc/sys/);

  3. advanced memory management with paged virtual memory;

  4. inter-process communications and synchronization mechanism;

  5. a virtual filesystem on top of several concrete filesystems (ext4, Btrfs, XFS, JFS, FAT32, and many more);

  6. OS-level virtualization (with Linux-VServer), paravirtualization and hardware-assisted virtualization (with KVM or Xen, and using QEMU for hardware emulation);

  7. security mechanisms for discretionary and mandatory access control (SELinux, AppArmor, POSIX ACLs, and others);

  8. several types of layered communication protocols (including the Internet protocol suite).