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Operating System Basics

Understanding operating system fundamentals is crucial for efficient software development. This guide covers essential OS concepts that every developer should know.

Table of Contents

  1. Command-Line Interface (CLI)
  2. Process Management
  3. File System & Permissions
  4. System Monitoring
  5. Shell Scripting Basics

Command-Line Interface (CLI)

Why CLI Matters

  • Faster than GUI for many tasks
  • Essential for server management
  • Automation and scripting capabilities
  • Better for remote systems

Essential Commands

# Print working directory
pwd

# List files and directories
ls                    # Basic listing
ls -l                 # Long format with details
ls -la                # Include hidden files
ls -lh                # Human-readable file sizes

# Change directory
cd /path/to/directory # Absolute path
cd ../                # Up one level
cd ~                  # Home directory
cd -                  # Previous directory

File Operations

# Create files
touch filename.txt
echo "content" > file.txt

# Create directories
mkdir new-folder
mkdir -p parent/child/grandchild

# Copy files/directories
cp source.txt destination.txt
cp -r source-dir/ destination-dir/

# Move/rename
mv old-name.txt new-name.txt
mv file.txt /new/location/

# Delete
rm file.txt
rm -r directory/      # Recursive delete
rm -rf directory/     # Force recursive delete (use carefully!)

Viewing Files

# View entire file
cat file.txt

# View with pagination
less file.txt         # Navigate with arrows, q to quit
more file.txt

# View first/last lines
head file.txt         # First 10 lines
head -n 20 file.txt   # First 20 lines
tail file.txt         # Last 10 lines
tail -f log.txt       # Follow/watch file (great for logs)

Search & Filter

# Search within files
grep "search-term" file.txt
grep -r "term" directory/       # Recursive search
grep -i "term" file.txt         # Case-insensitive
grep -n "term" file.txt         # Show line numbers

# Find files
find . -name "*.php"            # Find by name
find . -type f                  # Find files only
find . -type d                  # Find directories only
find . -mtime -7                # Modified in last 7 days
find . -size +100M              # Files larger than 100MB

# Which/whereis
which php                       # Show command location
whereis php                     # Show binary, source, manual locations

Pipes & Redirection

# Pipe output to another command
ls -l | grep ".php"
cat file.txt | grep "error" | wc -l

# Redirect output
command > output.txt            # Overwrite file
command >> output.txt           # Append to file
command 2> error.txt            # Redirect errors
command &> all.txt              # Redirect output and errors

# Input redirection
command < input.txt

Practical Examples

# Count PHP files in current directory
find . -name "*.php" | wc -l

# Find and delete all .log files
find . -name "*.log" -delete

# Search for TODO comments in PHP files
grep -rn "TODO" . --include="*.php"

# Find large files
find . -type f -size +50M -exec ls -lh {} \;

# Monitor real-time logs
tail -f /var/log/apache2/error.log | grep "PHP"

Process Management

Understanding Processes

A process is a running instance of a program. Every command you run creates a process.

Process Information

# List all processes
ps aux

# Interactive process viewer
top
htop                  # Enhanced version (may need to install)

# Process tree
pstree
ps auxf

# Find specific processes
ps aux | grep php
pgrep -l php         # Process ID and name
pidof php-fpm        # Get PID of process

Process Control

# Run process in background
command &

# Bring to foreground
fg

# Send to background
bg

# List background jobs
jobs

# Suspend current process
Ctrl + Z

# Kill processes
kill PID                # Graceful termination (SIGTERM)
kill -9 PID            # Force kill (SIGKILL)
killall process-name   # Kill by name
pkill process-name     # Kill by name (pattern)

Common Signals

kill -l                # List all signals

# Common signals
SIGTERM (15)          # Graceful shutdown (default)
SIGKILL (9)           # Force kill (cannot be caught)
SIGINT (2)            # Interrupt (Ctrl+C)
SIGHUP (1)            # Hangup (reload config)
SIGSTOP (19)          # Pause process
SIGCONT (18)          # Continue paused process

Process Priority

# Run with different priority
nice -n 10 command     # Lower priority (nice to others)
nice -n -10 command    # Higher priority (requires sudo)

# Change priority of running process
renice 10 -p PID

# View process priority
ps -el | grep process-name

Background & Daemon Processes

# Run and detach from terminal
nohup command &

# Run in background, ignore hangup
nohup ./script.sh > output.log 2>&1 &

# Disown a job
command &
disown

File System & Permissions

File System Hierarchy

/                     Root directory
├── bin              Essential binaries
├── etc              Configuration files
├── home             User home directories
├── var              Variable files (logs, cache)
│   ├── log          Log files
│   └── www          Web files
├── usr              User programs
│   ├── bin          User binaries
│   └── local        Locally installed software
└── tmp              Temporary files

File Permissions

Understanding Permission Format

-rwxrw-r--  1 user group 4096 Jan 1 12:00 file.txt
│└┬┘└┬┘└┬┘
│ │  │  └── Others permissions (read)
│ │  └───── Group permissions (read, write)
│ └──────── Owner permissions (read, write, execute)
└────────── File type (- = file, d = directory, l = link)

Permission Values

  • r (read): 4
  • w (write): 2
  • x (execute): 1

Common Permission Combinations

  • 777: rwxrwxrwx (all permissions for everyone) - AVOID in production!
  • 755: rwxr-xr-x (owner can do anything, others can read/execute)
  • 644: rw-r--r-- (owner can read/write, others can only read)
  • 600: rw------- (only owner can read/write)

Managing Permissions

# Change file permissions
chmod 755 script.sh          # Numeric
chmod u+x script.sh          # Add execute for user
chmod go-w file.txt          # Remove write for group and others
chmod -R 755 directory/      # Recursive

# Change ownership
chown user:group file.txt
chown -R user:group directory/
chgrp group file.txt         # Change group only

# View permissions
ls -l file.txt
stat file.txt                # Detailed info

Special Permissions

# Setuid (SUID) - run as file owner
chmod u+s file
chmod 4755 file

# Setgid (SGID) - run as group owner
chmod g+s file
chmod 2755 file

# Sticky bit - only owner can delete
chmod +t directory
chmod 1777 directory

System Monitoring

Disk Usage

# Disk space
df -h                        # Human-readable
df -i                        # Inodes

# Directory sizes
du -sh directory/            # Summary
du -sh *                     # All items in current dir
du -h --max-depth=1          # One level deep
ncdu                         # Interactive disk usage (install separately)

Memory Usage

# Memory information
free -h                      # Human-readable
free -m                      # In megabytes

# Detailed memory info
cat /proc/meminfo
vmstat                       # Virtual memory statistics
vmstat 5                     # Update every 5 seconds

CPU & Load

# CPU information
lscpu
cat /proc/cpuinfo

# Load average
uptime                       # System uptime and load
w                           # Who's logged in and system load

# Monitor CPU usage
top
htop
mpstat                      # Multiprocessor statistics

Network

# Network interfaces
ifconfig                    # Show network interfaces
ip addr                     # Modern alternative
ip link                     # Network devices

# Network statistics
netstat -tuln              # Listening ports
ss -tuln                   # Modern alternative
lsof -i :80                # What's using port 80

# Test connectivity
ping example.com
curl -I https://example.com
wget https://example.com
traceroute example.com

System Information

# OS information
uname -a                   # All system info
cat /etc/os-release        # Distribution info
hostnamectl               # System hostname and info

# Hardware information
lshw                      # Detailed hardware
lsblk                     # Block devices
lsusb                     # USB devices
lspci                     # PCI devices

Shell Scripting Basics

Creating a Script

#!/bin/bash
# This is a comment

echo "Hello, World!"

Making Script Executable

chmod +x script.sh
./script.sh

Variables

#!/bin/bash

# Variable assignment (no spaces around =)
NAME="John"
AGE=30

# Using variables
echo "Name: $NAME"
echo "Age: ${AGE}"

# Command output
CURRENT_DIR=$(pwd)
DATE=`date +%Y-%m-%d`

# Read user input
read -p "Enter your name: " USERNAME
echo "Hello, $USERNAME"

Conditionals

#!/bin/bash

if [ "$1" = "start" ]; then
    echo "Starting..."
elif [ "$1" = "stop" ]; then
    echo "Stopping..."
else
    echo "Unknown command"
fi

# File tests
if [ -f "file.txt" ]; then
    echo "File exists"
fi

if [ -d "directory" ]; then
    echo "Directory exists"
fi

Loops

#!/bin/bash

# For loop
for i in 1 2 3 4 5; do
    echo "Number: $i"
done

# For loop with range
for i in {1..5}; do
    echo "Count: $i"
done

# For loop with files
for file in *.txt; do
    echo "Processing: $file"
done

# While loop
COUNT=0
while [ $COUNT -lt 5 ]; do
    echo "Count: $COUNT"
    COUNT=$((COUNT + 1))
done

Functions

#!/bin/bash

# Define function
greet() {
    echo "Hello, $1!"
}

# Call function
greet "World"
greet "Developer"

# Function with return
add() {
    return $(($1 + $2))
}

add 5 3
result=$?
echo "Result: $result"

Practical Script Examples

Backup Script

#!/bin/bash

# Backup directory script
SOURCE="/var/www/html"
DEST="/backup"
DATE=$(date +%Y%m%d_%H%M%S)
BACKUP_NAME="backup_$DATE.tar.gz"

echo "Starting backup..."
tar -czf "$DEST/$BACKUP_NAME" "$SOURCE"

if [ $? -eq 0 ]; then
    echo "Backup completed: $BACKUP_NAME"
else
    echo "Backup failed!"
    exit 1
fi

Log Monitor

#!/bin/bash

# Monitor log file for errors
LOG_FILE="/var/log/app.log"
ERROR_COUNT=$(grep -c "ERROR" "$LOG_FILE")

echo "Found $ERROR_COUNT errors in log"

if [ $ERROR_COUNT -gt 0 ]; then
    echo "Recent errors:"
    grep "ERROR" "$LOG_FILE" | tail -5
fi

Scheduled Tasks (Cron Jobs)

Crontab Format

* * * * * command
│ │ │ │ │
│ │ │ │ └─── Day of week (0-7, Sunday = 0 or 7)
│ │ │ └───── Month (1-12)
│ │ └─────── Day of month (1-31)
│ └───────── Hour (0-23)
└─────────── Minute (0-59)

Crontab Examples

# Every minute
* * * * * /path/to/script.sh

# Every hour
0 * * * * /path/to/script.sh

# Every day at 2:30 AM
30 2 * * * /path/to/script.sh

# Every Monday at 5 PM
0 17 * * 1 /path/to/script.sh

# First day of month at midnight
0 0 1 * * /path/to/script.sh

Managing Cron Jobs

# Edit crontab
crontab -e

# List cron jobs
crontab -l

# Remove all cron jobs
crontab -r

# Edit cron for specific user (requires sudo)
sudo crontab -u username -e

Learning Resources

Documentation

Practice

Cheat Sheets

Quiz Questions

Test your knowledge:

  1. What is the difference between rm and rm -rf?
  2. How do you find all files modified in the last 24 hours?
  3. What signal does kill send by default?
  4. What does the permission 644 mean?
  5. How do you run a process in the background?
  6. What's the difference between ps aux and top?
  7. How do you schedule a script to run every day at midnight?
  8. What does the | (pipe) operator do?

Best Practices

  1. Always test destructive commands first (especially rm -rf)
  2. Use tab completion to avoid typos
  3. Read man pages: man command-name
  4. Use --help flag: command --help
  5. Be careful with sudo - understand what you're doing
  6. Regular backups before system changes
  7. Log important operations for audit trail
  8. Use version control for scripts and configs

Common Pitfalls to Avoid

❌ Don't: sudo rm -rf /
✅ Do: Double-check paths before deleting

❌ Don't: Run untrusted scripts with sudo
✅ Do: Review scripts before executing

❌ Don't: Store sensitive data in plain text
✅ Do: Use proper encryption and permissions

❌ Don't: Ignore error messages
✅ Do: Read and understand errors

Next Step: Web Fundamentals