Permissions, Ownership and Access Control Lists

user and group identity

Numeric user and group IDs identify principals and are used with ownership and permission bits during access checks.

read write execute permissions

Linux records read, write and execute bits for the owning user, owning group and all other users.

directory permissions

Read permits listing directory entries, write permits adding or removing entries and execute permits pathname traversal and access to known entries.

chmod

chmod changes mode bits using symbolic expressions such as g+w or octal values such as 640; it does not change file ownership.

chown and chgrp

chown changes a file’s owning user and optionally group, while chgrp changes only its group ownership, subject to privilege and filesystem rules.

umask

The process umask removes permission bits from the mode requested when a new file or directory is created

setuid setgid sticky bit

Setuid or setgid can change effective identity on execution; on directories setgid inherits the group and the sticky bit restricts deletion.

access control list

A POSIX access control list adds permissions for named users and groups beyond the file owner's traditional user-group-other mode bits.

ACL mask

The ACL mask is the maximum effective permission granted to named users, named groups and the owning group.

least privilege

Granting each identity only the permissions and duration required for its current task.

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:

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)

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

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)