6. Operators and Expressions

In Chapter 5, you learned how to create variables and use expansion. Now it’s time to do something with those values: compare them, calculate with them, and make decisions based on them.

Operators are the tools that let you:

• Compare: Is this value greater than that?

• Calculate: Add, subtract, multiply, divide

• Combine: AND, OR logic for complex conditions

• Test: Does this file exist? Is this variable set?

Master these, and you’ll be able to write scripts that make intelligent decisions.

What You'll Learn

By the end of this chapter, you will be able to:

✓ Understand different types of operators (arithmetic, comparison, logical) ✓ Perform calculations using arithmetic operators and expressions ✓ Compare values with comparison operators (<, >, ==, !=, etc.) ✓ Combine conditions using logical operators (&&, ||, !) ✓ Test files and variables with test operators ([], [[ ]]) ✓ Use operators effectively in conditional statements ✓ Evaluate complex expressions for decision-making ✓ Avoid common operator mistakes and pitfalls

Chapter Map

Section	Topic	Key Concepts
0602	Arithmetic Operators	+, -, *, /, %, $(( )), let
0603	Comparison & Logical	<, >, ==, -lt, -gt, &&, ||, !
0604	String & File Tests	-z, -n, -f, -d, -r, -w, -x
0605	Using Expressions	Conditionals, brackets, evaluation
0606	Lab: Operator Practice	Hands-on exercises, real scenarios

Why This Matters

Operators are the building blocks of decision-making in scripts:

• Automation: Scripts make decisions based on data (if load > 80%, restart service)

• Validation: Check if inputs are valid before using them

• Conditionals: Execute different code based on conditions

• Data manipulation: Transform and calculate values

• System admin: Monitor thresholds and trigger alerts

Real-world example: A monitoring script uses comparison operators to check if CPU usage is critical, then uses logical operators to determine if an alert should be sent.

Prerequisites

You should already understand:

• Variables and variable expansion (Chapter 5)

• Basic command syntax and quoting

• How to run bash scripts

• Basic file operations

If Chapter 5 feels unfamiliar, review it first—this chapter builds directly on those concepts.

Real-World Scenario

Imagine you’re building a backup script that needs to validate conditions:

# Check if backup succeeded
if [ $backup_size -gt 1000000 ]; then
    echo "Backup created successfully"
else
    echo "ERROR: Backup too small!"
    exit 1
fi

# Run backup only if disk has space AND directory is writable
if [ $available_space -gt $needed_space ] && [ -w "$backup_dir" ]; then
    tar -czf backup.tar.gz important/
else
    echo "Cannot create backup: insufficient space or permission denied"
fi

# Validate user input
if [ -z "$username" ] || [ -z "$password" ]; then
    echo "ERROR: Username and password required"
    exit 1
fi

Each line uses operators to make intelligent decisions. This chapter teaches you how to write similar logic.

Sample Script: System Health Check

Here’s a complete script that uses operators to evaluate system health:

#!/bin/bash

# Get system metrics
cpu_usage=$(top -bn1 | grep "Cpu(s)" | awk '{print int($2)}')
memory_usage=$(free | grep Mem | awk '{print int($3/$2 * 100)}')
disk_usage=$(df / | tail -1 | awk '{print int($5)}')

# Evaluate conditions using comparison operators
if [ $cpu_usage -gt 80 ]; then
    echo "⚠️  WARNING: High CPU usage ($cpu_usage%)"
elif [ $cpu_usage -gt 60 ]; then
    echo "⚡ CAUTION: Elevated CPU usage ($cpu_usage%)"
else
    echo "✓ CPU healthy ($cpu_usage%)"
fi

# Check memory with comparison operators
if [ $memory_usage -gt 90 ]; then
    echo "🔴 CRITICAL: Memory nearly full ($memory_usage%)"
elif [ $memory_usage -gt 75 ]; then
    echo "🟡 WARNING: Memory usage high ($memory_usage%)"
else
    echo "✓ Memory healthy ($memory_usage%)"
fi

# Check disk space
if [ $disk_usage -gt 95 ]; then
    echo "🔴 CRITICAL: Disk almost full ($disk_usage%)"
elif [ $disk_usage -gt 85 ]; then
    echo "🟡 WARNING: Disk usage high ($disk_usage%)"
else
    echo "✓ Disk healthy ($disk_usage%)"
fi

# Complex condition: alert only if BOTH CPU and Memory are critical
if [ $cpu_usage -gt 80 ] && [ $memory_usage -gt 80 ]; then
    echo "🚨 CRITICAL ALERT: CPU and Memory both extremely high!"
fi

# Different behavior based on time of day
hour=$(date +%H)
if [ $hour -ge 22 ] || [ $hour -lt 6 ]; then
    if [ $cpu_usage -gt 50 ]; then
        echo "⚠️  Unexpected activity during night: CPU at $cpu_usage%"
    fi
fi

What this script demonstrates:

• Arithmetic operators: Getting numeric values with math

• Comparison operators: -gt (greater than), -lt (less than), -eq (equal)

• Logical operators: && (AND), || (OR) for combining conditions

• Test operators: [ ] for evaluating conditions

• Practical application: Real system monitoring

Progression

This chapter is intermediate-level:

Level	Chapters	Focus
Beginner	1-5	Fundamentals: shell, files, variables
Intermediate	6-7	Operators and control flow
Advanced	8-15	Functions, automation, real systems

You’ve built the foundation. Now you’re learning to make decisions—the key to intelligent scripts.