Operating Systems

File System Structure; Types of Operating Systems; Linux/Unix Commands Overview

C-CAT

File System Structure

Directory Structure

Directories organize files in a hierarchical structure.

Unix/Linux Filesystem Hierarchy:

/ (root)
├── bin/     ← Essential binaries (ls, cp, mv)
├── sbin/    ← System binaries (fsck, init)
├── etc/     ← Configuration files
├── home/    ← User home directories
│   ├── user1/
│   └── user2/
├── var/     ← Variable data (logs, spool)
├── tmp/     ← Temporary files
├── usr/     ← User programs and data
│   ├── local/
│   └── share/
├── lib/     ← Libraries
├── proc/    ← Virtual filesystem (process info)
├── dev/     ← Device files
└── mnt/     ← Mount points for external drives

File Permissions (Unix)

-rwxr-xr--  1  owner  group  size  date  filename
│└──┘└──┘└─┘
│ Owner Group Others
│  rwx  r-x   r--
│  7    5     4 (Octal representation)
│
└── Type: - (file), d (directory), l (symlink), c (char device), b (block device)

Permission bits:

BitSymbolValueMeaning
Readr4View file contents
Writew2Modify file contents
Executex1Execute as program

chmod examples:

chmod 755 file.sh   # rwxr-xr-x  (owner all; group/others read+execute)
chmod 644 file.txt  # rw-r--r--  (owner read+write; others read only)
chmod 700 secret/   # rwx------  (owner only)

Types of Operating Systems

29.1 Batch Operating System

  • Processes jobs in batches with no user interaction
  • Similar jobs grouped together

Examples: IBM mainframes (JCL — Job Control Language)

  • Advantages: Very efficient for large repetitive tasks
  • Disadvantages: High turnaround time; hard to debug

29.2 Time-Sharing / Multitasking OS

  • Multiple users share CPU time via time slicing
  • Each user gets the illusion of dedicated access
  • Examples: Unix, Linux, mainframe TSO
  • Advantages: Fast response; multiple users
  • Disadvantages: Complex synchronization

29.3 Distributed OS

  • Processes distributed across multiple computers
  • Appears as single system to user

Examples: Google's systems, Amoeba

  • Advantages: High performance, fault tolerance

29.4 Real-Time OS (RTOS)

  • Processes have strict timing deadlines
  • Must respond within defined time limits

Hard RTOS: Missing deadline is catastrophic (aircraft, nuclear reactor)

  • Soft RTOS: Degraded service if deadline missed (video streaming)
  • Examples: VxWorks, FreeRTOS, RTEMS, QNX
  • Use: Embedded systems, robotics, industrial control

29.5 Embedded OS

  • Designed for embedded systems (microcontrollers, IoT)
  • Small footprint, low power consumption
  • Examples: Android (mobile), iOS, FreeRTOS, Embedded Linux

29.6 Network OS

  • OS designed to support networks
  • Examples: Windows Server, Ubuntu Server, CentOS

29.7 Mobile OS

  • Designed for mobile devices
  • Examples: Android (Linux kernel), iOS (BSD/Darwin kernel)

Linux/Unix Commands Overview

Essential Linux Commands

# File and Directory Commands
ls      # List directory contents
ls -la  # Long format including hidden files
pwd     # Print working directory
cd dir  # Change directory
mkdir dir   # Make directory
rm file     # Remove file
rm -rf dir  # Remove directory recursively
cp src dst  # Copy file
mv src dst  # Move/rename file
cat file    # Display file contents
less file   # Page through file
head -n 20 file   # First 20 lines
tail -n 20 file   # Last 20 lines
grep pattern file # Search for pattern
find / -name "*.log"  # Find files by name
chmod 755 file    # Change file permissions
chown user:group file  # Change ownership

# Process Commands
ps aux   # List all processes
top      # Real-time process monitor
htop     # Interactive process monitor
kill PID # Kill process by PID
kill -9 PID  # Force kill
jobs     # List background jobs
fg 1     # Bring job 1 to foreground
bg 1     # Send job 1 to background
sleep 10 &   # Run in background

# System Information
uname -a   # Kernel info
hostname   # Show hostname
whoami     # Current user
id         # Current user/group IDs
uptime     # System uptime and load
free -h    # Memory usage
df -h      # Disk space usage
du -sh dir # Directory disk usage
lscpu      # CPU info

# Network Commands
ifconfig   # Network interface info
ip addr    # Modern network interface info
ping host  # Test connectivity
netstat -an # Network connections
ss -tulpn  # Socket statistics
curl url   # Download from URL
wget url   # Download file from URL
ssh user@host  # Remote login
scp file user@host:path  # Secure copy

# Package Management
# Debian/Ubuntu:
apt update
apt install package
apt remove package

# RedHat/CentOS:
yum install package
yum remove package
dnf install package

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