Operating Systems Cheat Sheet

1. Basics

• Operating System (OS): Software that manages computer hardware and software resources, providing common services for computer programs.
• Kernel: Core part of the OS, managing system resources and communication between hardware and software.

2. Types of Operating Systems

• Batch OS: Processes batches of jobs without user interaction.
• Time-Sharing OS: Multiple users can access the system simultaneously.
• Distributed OS: Manages a group of distinct computers and makes them appear as a single computer.
• Embedded OS: Designed for embedded systems.
• Real-Time OS (RTOS): Processes data as it comes in, typically used in systems that require immediate processing.
• Network OS: Provides services to computers connected in a network.
• Mobile OS: Designed specifically for mobile devices.

3. Processes and Threads

• Process: A program in execution, with its own address space.
• Thread: The smallest unit of processing, sharing the process’s resources.
• Process States: New, Ready, Running, Waiting, Terminated.
• Multithreading: Running multiple threads within a single process.

4. Process Scheduling

• Scheduling Algorithms:
  • FCFS (First-Come, First-Served): Processes are attended in the order they arrive.
  • SJF (Shortest Job First): Processes with the shortest burst time are scheduled first.
  • Priority Scheduling: Processes are scheduled based on priority.
  • Round Robin (RR): Each process gets a small unit of CPU time in a cyclic order.
  • Multilevel Queue: Processes are divided into queues based on priority.

5. Memory Management

• RAM (Random Access Memory): Volatile memory used by processes.
• Virtual Memory: Extends physical memory onto the disk.
• Paging: Divides memory into fixed-size pages.
• Segmentation: Divides memory into segments of varying sizes.
• Page Replacement Algorithms:
  • FIFO (First-In, First-Out): Replaces the oldest page.
  • LRU (Least Recently Used): Replaces the least recently used page.
  • Optimal Page Replacement: Replaces the page that will not be used for the longest time.

6. File Systems

• File: A collection of data stored in a storage device.
• Directory: A container that holds files and other directories.
• File System Types: NTFS, FAT32, ext3, ext4, HFS+, APFS.
• File Operations: Create, Open, Read, Write, Delete, Close.
• File Attributes: Metadata about the file, such as name, size, type, permissions.

7. I/O Management

• I/O Devices: Hardware used for input and output operations.
• Device Drivers: Software that controls a hardware device.
• I/O Scheduling: Manages the order in which I/O requests are processed.
• Buffering: Storing data temporarily while it is being moved from one place to another.

8. Concurrency

• Concurrency: Multiple processes or threads executing simultaneously.
• Race Condition: When multiple processes or threads access shared data concurrently, leading to unpredictable results.
• Mutex (Mutual Exclusion): Prevents multiple processes from accessing a critical section simultaneously.
• Semaphore: A signaling mechanism to control access to shared resources.
• Deadlock: A situation where two or more processes are unable to proceed because each is waiting for the other to release resources.
  • Deadlock Prevention: Ensuring at least one of the necessary conditions for deadlock cannot occur.
  • Deadlock Avoidance: Using algorithms like Banker's algorithm to avoid unsafe states.
  • Deadlock Detection and Recovery: Allowing deadlock to occur, then detecting and recovering from it.

9. Security and Protection

• Authentication: Verifying the identity of a user or process.
• Authorization: Granting or denying access to resources based on permissions.
• Encryption: Protecting data by transforming it into an unreadable format.
• Firewall: Controls incoming and outgoing network traffic.
• Antivirus: Software that detects and removes malware.
• User and Group Management: Managing permissions and access rights for users and groups.

10. System Calls

• Definition: Interface between user programs and the OS.
• Types:
  • Process Control: fork(), exit(), wait()
  • File Management: open(), read(), write(), close()
  • Device Management: ioctl(), read(), write()
  • Information Maintenance: getpid(), alarm(), sleep()
  • Communication: pipe(), shmget(), mmap()

11. Common Commands (Unix/Linux)

• File Operations:
  • ls: List directory contents.
  • cd: Change directory.
  • cp: Copy files.
  • mv: Move or rename files.
  • rm: Remove files.
  • cat: Concatenate and display files.
• Process Management:
  • ps: Report a snapshot of current processes.
  • top: Display and update sorted information about processes.
  • kill: Terminate processes.
  • bg: Resume suspended jobs in the background.
  • fg: Resume suspended jobs in the foreground.
• System Information:
  • uname: Print system information.
  • df: Report filesystem disk space usage.
  • du: Estimate file space usage.
  • uptime: Tell how long the system has been running.
  • free: Display amount of free and used memory.

12. Virtualization

• Virtual Machine (VM): Emulates a physical computer system.
• Hypervisor: Software that creates and manages VMs.
  • Type 1: Runs directly on hardware (e.g., VMware ESXi).
  • Type 2: Runs on a host operating system (e.g., VirtualBox, VMware Workstation).
• Containerization: Lightweight virtualization using containers (e.g., Docker).

13. Backup and Recovery

• Backup: Copying data to protect against data loss.
  • Full Backup: Complete copy of all data.
  • Incremental Backup: Copies only data that has changed since the last backup.
  • Differential Backup: Copies all data changed since the last full backup.
• Recovery: Restoring data from a backup after data loss.

14. Common OS Examples

• Windows: Microsoft's OS with a graphical user interface.
• Linux: Open-source Unix-like OS, commonly used for servers.
• macOS: Apple's Unix-based OS for Mac computers.
• Unix: Multiuser, multitasking OS, known for stability and security.