Linux Operating System Fundamentals

Table of Contents

Introduction to Linux

Linux is an open-source, Unix-like operating system kernel first released by Linus Torvalds in 1991. It forms the foundation for numerous distributions (distros) that provide complete operating systems for servers, desktops, and embedded devices.

Key Characteristics

| Feature | Description | |---------|-------------| | Open Source | Source code is freely available and modifiable | | Multi-user | Supports multiple users simultaneously | | Multi-tasking | Can run multiple processes concurrently | | Portable | Runs on various hardware architectures | | Secure | Built-in security features and permission system | | Stable | Known for reliability and uptime |

Popular Linux Distributions

| Distribution | Base | Target Use Case | Package Manager | |--------------|------|-----------------|-----------------| | Ubuntu | Debian | Desktop/Server | APT | | CentOS/RHEL | Red Hat | Enterprise Server | YUM/DNF | | Fedora | Red Hat | Desktop/Development | DNF | | Debian | Independent | Server/Desktop | APT | | Arch Linux | Independent | Advanced Users | Pacman | | SUSE | Independent | Enterprise | Zypper |

Linux Architecture

Linux follows a layered architecture model consisting of several components:

Architecture Layers

` +---------------------------+ | User Applications | +---------------------------+ | System Call | +---------------------------+ | Linux Kernel | +---------------------------+ | Hardware Layer | +---------------------------+ `

Kernel Components

| Component | Function | |-----------|----------| | Process Scheduler | Manages CPU time allocation | | Memory Manager | Handles RAM and virtual memory | | File System | Manages data storage and retrieval | | Device Drivers | Interface with hardware components | | Network Stack | Handles network communications |

File System Hierarchy

Linux uses a hierarchical file system structure starting from the root directory (/).

Standard Directory Structure

| Directory | Purpose | Contents | |-----------|---------|----------| | / | Root directory | Top-level directory containing all others | | /bin | Essential binaries | Basic system commands (ls, cp, mv) | | /boot | Boot files | Kernel and boot loader files | | /dev | Device files | Hardware device representations | | /etc | Configuration files | System-wide configuration files | | /home | User directories | Individual user home directories | | /lib | Shared libraries | Essential shared libraries | | /media | Removable media | Mount points for removable devices | | /mnt | Mount points | Temporary mount points | | /opt | Optional software | Third-party software packages | | /proc | Process information | Virtual filesystem for process data | | /root | Root user home | Home directory for root user | | /sbin | System binaries | System administration commands | | /tmp | Temporary files | Temporary storage space | | /usr | User programs | User applications and utilities | | /var | Variable data | Logs, databases, cache files |

Essential Commands

Navigation Commands

#### pwd (Print Working Directory) Displays the current directory path.

`bash pwd

Output: /home/username

Options: - -L : Display logical path (default) - -P : Display physical path without symbolic links

#### cd (Change Directory) Changes the current working directory.

`bash cd /home/username # Absolute path cd Documents # Relative path cd .. # Parent directory cd ~ # Home directory cd - # Previous directory `

Special Directory References: - . : Current directory - .. : Parent directory - ~ : Home directory - - : Previous directory

#### ls (List Directory Contents) Lists files and directories in the current or specified directory.

`bash ls # Basic listing ls -l # Long format ls -la # Long format including hidden files ls -lh # Long format with human-readable sizes ls -R # Recursive listing `

Common Options:

| Option | Description | |--------|-------------| | -l | Long format with detailed information | | -a | Show all files including hidden ones | | -h | Human-readable file sizes | | -R | Recursive listing of subdirectories | | -t | Sort by modification time | | -S | Sort by file size | | -r | Reverse order |

File Operations

Creating Files and Directories

#### touch Creates empty files or updates timestamps.

`bash touch filename.txt # Create single file touch file1.txt file2.txt file3.txt # Create multiple files touch -t 202301011200 oldfile.txt # Set specific timestamp `

#### mkdir (Make Directory) Creates directories.

`bash mkdir directory_name # Create single directory mkdir -p path/to/nested/directory # Create nested directories mkdir dir1 dir2 dir3 # Create multiple directories `

Options: - -p : Create parent directories as needed - -m : Set directory permissions

Copying and Moving Files

#### cp (Copy) Copies files and directories.

`bash cp source.txt destination.txt # Copy file cp -r source_dir destination_dir # Copy directory recursively cp *.txt backup/ # Copy all .txt files cp -u source.txt dest.txt # Copy only if source is newer `

Important Options:

| Option | Description | |--------|-------------| | -r or -R | Copy directories recursively | | -i | Interactive mode (prompt before overwrite) | | -u | Copy only when source is newer | | -v | Verbose output | | -p | Preserve file attributes |

#### mv (Move/Rename) Moves or renames files and directories.

`bash mv oldname.txt newname.txt # Rename file mv file.txt /path/to/destination/ # Move file mv *.log logs/ # Move all .log files mv -i source.txt destination.txt # Interactive move `

Removing Files and Directories

#### rm (Remove) Deletes files and directories.

`bash rm filename.txt # Remove file rm -r directory_name # Remove directory recursively rm -f filename.txt # Force removal without prompt rm -rf directory_name # Force recursive removal `

Critical Options:

| Option | Description | Warning Level | |--------|-------------|---------------| | -r | Recursive removal | High | | -f | Force removal without prompts | Very High | | -i | Interactive confirmation | Safe | | -v | Verbose output | Safe |

Note: Be extremely careful with rm -rf as it permanently deletes files without confirmation.

Process Management

Viewing Processes

#### ps (Process Status) Displays information about running processes.

`bash ps # Show processes for current user ps aux # Show all processes with detailed info ps -ef # Full format listing ps -u username # Show processes for specific user `

Process Information Fields:

| Field | Description | |-------|-------------| | PID | Process ID | | PPID | Parent Process ID | | USER | Process owner | | CPU | CPU usage percentage | | MEM | Memory usage percentage | | VSZ | Virtual memory size | | RSS | Resident set size (physical memory) | | TTY | Terminal type | | STAT | Process state | | START | Start time | | TIME | CPU time used | | COMMAND | Command that started the process |

#### top Real-time process viewer.

`bash top # Interactive process viewer top -u username # Show processes for specific user top -p PID # Monitor specific process `

Interactive Commands in top: - q : Quit - k : Kill process - r : Renice process - M : Sort by memory usage - P : Sort by CPU usage - h : Help

Process Control

#### kill Terminates processes by PID.

`bash kill PID # Send TERM signal kill -9 PID # Send KILL signal (force) kill -15 PID # Send TERM signal (graceful) kill -HUP PID # Send HUP signal (restart) `

Common Signals:

| Signal | Number | Description | |--------|--------|-------------| | TERM | 15 | Graceful termination (default) | | KILL | 9 | Force kill (cannot be caught) | | HUP | 1 | Hang up (restart) | | INT | 2 | Interrupt (Ctrl+C) | | STOP | 19 | Stop process | | CONT | 18 | Continue stopped process |

#### killall Terminates processes by name.

`bash killall firefox # Kill all Firefox processes killall -9 unresponsive_app # Force kill by name killall -u username # Kill all processes for user `

Background and Foreground Jobs

#### Running Jobs in Background

`bash command & # Run command in background nohup command & # Run command immune to hangups `

#### Job Control Commands

`bash jobs # List active jobs fg %1 # Bring job 1 to foreground bg %1 # Send job 1 to background Ctrl+Z # Suspend current job Ctrl+C # Terminate current job `

User and Group Management

User Information Commands

#### whoami Displays current username.

`bash whoami

Output: username

#### id Shows user and group IDs.

`bash id # Current user info id username # Specific user info id -u username # User ID only id -g username # Primary group ID only `

#### who and w Show logged-in users.

`bash who # Show logged-in users w # Show detailed user activity last # Show login history `

User Management (Administrative)

#### useradd Creates new user accounts.

`bash sudo useradd newuser # Create basic user sudo useradd -m -s /bin/bash newuser # Create with home directory and shell sudo useradd -g primarygroup -G secondarygroups newuser `

Common Options:

| Option | Description | |--------|-------------| | -m | Create home directory | | -s | Specify shell | | -g | Primary group | | -G | Secondary groups | | -d | Home directory path | | -c | Comment/full name |

#### usermod Modifies user accounts.

`bash sudo usermod -aG groupname username # Add user to group sudo usermod -s /bin/zsh username # Change user shell sudo usermod -l newname oldname # Change username `

#### userdel Deletes user accounts.

`bash sudo userdel username # Delete user (keep home directory) sudo userdel -r username # Delete user and home directory `

Group Management

#### groups Shows group membership.

`bash groups # Current user's groups groups username # Specific user's groups `

#### groupadd Creates new groups.

`bash sudo groupadd newgroup # Create new group sudo groupadd -g 1001 newgroup # Create with specific GID `

#### groupdel Deletes groups.

`bash sudo groupdel groupname # Delete group `

File Permissions

Understanding Permission System

Linux uses a permission system based on three types of access for three categories of users.

#### Permission Types

| Permission | Symbol | Numeric | File Effect | Directory Effect | |------------|--------|---------|-------------|------------------| | Read | r | 4 | View file contents | List directory contents | | Write | w | 2 | Modify file contents | Create/delete files in directory | | Execute | x | 1 | Run file as program | Enter directory |

#### User Categories

| Category | Description | |----------|-------------| | Owner (u) | User who owns the file | | Group (g) | Group that owns the file | | Other (o) | All other users |

Viewing Permissions

`bash ls -l filename

Output: -rw-r--r-- 1 user group 1024 Jan 1 12:00 filename

Permission String Breakdown: - First character: File type (- for file, d for directory, l for link) - Next 9 characters: Permissions in groups of 3 (owner, group, other)

Changing Permissions

#### chmod (Change Mode) Modifies file permissions.

Symbolic Mode: `bash chmod u+x filename # Add execute for owner chmod g-w filename # Remove write for group chmod o=r filename # Set other to read only chmod a+r filename # Add read for all chmod u+rwx,g+rx,o+r filename # Complex permissions `

Numeric Mode: `bash chmod 755 filename # rwxr-xr-x chmod 644 filename # rw-r--r-- chmod 600 filename # rw------- chmod 777 filename # rwxrwxrwx (full access) `

Common Permission Combinations:

| Numeric | Symbolic | Use Case | |---------|----------|----------| | 755 | rwxr-xr-x | Executable files | | 644 | rw-r--r-- | Regular files | | 600 | rw------- | Private files | | 666 | rw-rw-rw- | Shared writable files | | 777 | rwxrwxrwx | Full access (use carefully) |

Changing Ownership

#### chown (Change Owner) Changes file ownership.

`bash sudo chown newowner filename # Change owner only sudo chown newowner:newgroup filename # Change owner and group sudo chown :newgroup filename # Change group only sudo chown -R newowner directory # Recursive ownership change `

#### chgrp (Change Group) Changes group ownership.

`bash sudo chgrp newgroup filename # Change group sudo chgrp -R newgroup directory # Recursive group change `

Package Management

Different Linux distributions use different package management systems.

APT (Advanced Package Tool) - Debian/Ubuntu

#### Package Installation and Removal

`bash sudo apt update # Update package list sudo apt upgrade # Upgrade all packages sudo apt install package_name # Install package sudo apt remove package_name # Remove package sudo apt purge package_name # Remove package and config files sudo apt autoremove # Remove unused dependencies `

#### Package Information

`bash apt search keyword # Search for packages apt show package_name # Show package information apt list --installed # List installed packages apt list --upgradable # List upgradable packages `

YUM/DNF - Red Hat/CentOS/Fedora

#### Package Operations

`bash sudo dnf update # Update all packages sudo dnf install package_name # Install package sudo dnf remove package_name # Remove package sudo dnf search keyword # Search packages sudo dnf info package_name # Package information sudo dnf list installed # List installed packages `

Package Management Comparison

| Operation | APT (Debian/Ubuntu) | DNF (Fedora) | Pacman (Arch) | |-----------|-------------------|--------------|---------------| | Update package list | apt update | dnf check-update | pacman -Sy | | Install package | apt install pkg | dnf install pkg | pacman -S pkg | | Remove package | apt remove pkg | dnf remove pkg | pacman -R pkg | | Search package | apt search term | dnf search term | pacman -Ss term | | List installed | apt list --installed | dnf list installed | pacman -Q |

System Monitoring

System Information

#### uname System information.

`bash uname -a # All system information uname -r # Kernel version uname -m # Machine architecture `

#### df (Disk Free) Shows disk space usage.

`bash df # Basic disk usage df -h # Human-readable format df -T # Include filesystem type df /path/to/directory # Specific filesystem `

#### du (Disk Usage) Shows directory space usage.

`bash du # Current directory usage du -h # Human-readable format du -s # Summary only du -sh * # Size of each item in current directory `

#### free Shows memory usage.

`bash free # Basic memory info free -h # Human-readable format free -m # Show in MB free -g # Show in GB `

Memory Information Fields:

| Field | Description | |-------|-------------| | total | Total installed memory | | used | Used memory | | free | Unused memory | | shared | Memory used by tmpfs | | buff/cache | Buffers and cache | | available | Available memory for new applications |

Process Monitoring

#### htop Enhanced version of top (if installed).

`bash htop # Interactive process viewer `

htop Advantages: - Color-coded display - Mouse support - Tree view of processes - Easy process killing - System resource meters

#### iostat I/O statistics (part of sysstat package).

`bash iostat # Basic I/O stats iostat -x 1 # Extended stats every second iostat -h # Human-readable format `

Network Commands

Network Configuration

#### ip Modern network configuration tool.

`bash ip addr show # Show IP addresses ip route show # Show routing table ip link show # Show network interfaces ip addr add 192.168.1.100/24 dev eth0 # Add IP address `

#### ifconfig Traditional network interface configuration.

`bash ifconfig # Show all interfaces ifconfig eth0 # Show specific interface ifconfig eth0 up # Enable interface ifconfig eth0 down # Disable interface `

Network Connectivity

#### ping Test network connectivity.

`bash ping google.com # Continuous ping ping -c 4 google.com # Ping 4 times ping -i 2 google.com # Ping every 2 seconds `

#### wget and curl Download files from web.

`bash wget https://example.com/file.txt # Download file wget -O newname.txt https://example.com/file.txt # Download with new name curl https://api.example.com # Make HTTP request curl -o file.txt https://example.com/file.txt # Download with curl `

#### netstat Network statistics.

`bash netstat -tuln # Show listening ports netstat -r # Show routing table netstat -i # Show interface statistics `

SSH (Secure Shell)

#### Basic SSH Usage

`bash ssh username@hostname # Connect to remote host ssh -p 2222 username@hostname # Connect using specific port ssh-keygen -t rsa # Generate SSH key pair ssh-copy-id username@hostname # Copy public key to remote host `

#### SCP (Secure Copy)

`bash scp file.txt username@hostname:/path/ # Copy file to remote host scp username@hostname:/path/file.txt . # Copy file from remote host scp -r directory username@hostname:/path/ # Copy directory recursively `

Text Processing

File Viewing Commands

#### cat Display file contents.

`bash cat filename.txt # Display entire file cat file1.txt file2.txt # Display multiple files cat -n filename.txt # Display with line numbers `

#### less and more Page through file contents.

`bash less filename.txt # View file with navigation more filename.txt # View file page by page `

less Navigation: - Space: Next page - b: Previous page - /: Search forward - ?: Search backward - q: Quit

#### head and tail View beginning or end of files.

`bash head filename.txt # First 10 lines head -n 20 filename.txt # First 20 lines tail filename.txt # Last 10 lines tail -n 5 filename.txt # Last 5 lines tail -f logfile.txt # Follow file changes (live) `

Text Search and Manipulation

#### grep Search text patterns.

`bash grep "pattern" filename.txt # Search for pattern grep -i "pattern" filename.txt # Case-insensitive search grep -r "pattern" directory/ # Recursive search grep -n "pattern" filename.txt # Show line numbers grep -v "pattern" filename.txt # Invert match (exclude pattern) `

Common grep Options:

| Option | Description | |--------|-------------| | -i | Case-insensitive search | | -r | Recursive search in directories | | -n | Show line numbers | | -v | Invert match | | -c | Count matching lines | | -l | Show only filenames with matches | | -w | Match whole words only |

#### sed (Stream Editor) Text transformation tool.

`bash sed 's/old/new/' filename.txt # Replace first occurrence per line sed 's/old/new/g' filename.txt # Replace all occurrences sed '5d' filename.txt # Delete line 5 sed '/pattern/d' filename.txt # Delete lines matching pattern `

#### awk Pattern scanning and processing.

`bash awk '{print $1}' filename.txt # Print first column awk '/pattern/ {print}' filename.txt # Print lines matching pattern awk 'NR==5' filename.txt # Print line 5 awk '{print NF}' filename.txt # Print number of fields per line `

File Comparison

#### diff Compare files line by line.

`bash diff file1.txt file2.txt # Basic comparison diff -u file1.txt file2.txt # Unified format diff -r dir1/ dir2/ # Compare directories recursively `

#### sort Sort file contents.

`bash sort filename.txt # Alphabetical sort sort -n filename.txt # Numerical sort sort -r filename.txt # Reverse sort sort -k2 filename.txt # Sort by second column `

#### uniq Remove duplicate lines.

`bash uniq filename.txt # Remove consecutive duplicates sort filename.txt | uniq # Remove all duplicates uniq -c filename.txt # Count occurrences uniq -d filename.txt # Show only duplicates `

Input/Output Redirection

#### Redirection Operators

| Operator | Description | Example | |----------|-------------|---------| | > | Redirect output (overwrite) | command > file.txt | | >> | Redirect output (append) | command >> file.txt | | < | Redirect input | command < input.txt | | 2> | Redirect stderr | command 2> error.log | | 2>&1 | Redirect stderr to stdout | command > file.txt 2>&1 | | | | Pipe output to another command | command1 | command2 |

#### Practical Examples

`bash ls -l > directory_listing.txt # Save directory listing to file echo "New line" >> file.txt # Append text to file grep "error" /var/log/syslog > errors.txt # Save error lines to file ps aux | grep firefox # Find Firefox processes cat file.txt | sort | uniq # Sort and remove duplicates find /home -name "*.txt" 2>/dev/null # Suppress error messages `

This comprehensive guide covers the fundamental aspects of Linux operating system usage, from basic navigation to advanced text processing and system administration tasks. Each command includes practical examples and important options to help users understand and effectively utilize Linux systems.