If Statements

If statements allow your scripts to make decisions based on conditions. They're the foundation of conditional logic in Bash, enabling your scripts to respond differently based on values, file states, command results, and more. Understanding if statements is essential for writing dynamic, responsive shell scripts!

Basic If Statement

The simplest form of an if statement executes code only when a condition is true. The syntax uses if, then, and fi (if backwards).

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# Basic syntax
if [ condition ]; then
    # code to execute if condition is true
fi

# Example
age=18
if [ $age -ge 18 ]; then
    echo "You are an adult"
fi

If-Else Statement

When you want to handle both true and false cases, use else to specify what happens when the condition is false.

if [ condition ]; then
    # code if true
else
    # code if false
fi

# Example
age=15
if [ $age -ge 18 ]; then
    echo "You are an adult"
else
    echo "You are a minor"
fi

If-Elif-Else Statement

For multiple conditions, use elif (else if) to chain conditions. Bash checks conditions in order and executes the first one that's true.

if [ condition1 ]; then
    # code if condition1 is true
elif [ condition2 ]; then
    # code if condition2 is true
elif [ condition3 ]; then
    # code if condition3 is true
else
    # code if all conditions are false
fi

# Example: Grade calculation
score=85
if [ $score -ge 90 ]; then
    grade="A"
elif [ $score -ge 80 ]; then
    grade="B"
elif [ $score -ge 70 ]; then
    grade="C"
else
    grade="F"
fi

Single-Line If Statement

For simple conditions, you can write if statements on a single line. This is useful for quick checks and is common in shell scripts.

# Single-line if
if [ condition ]; then command; fi

# Using && and || operators
[ condition ] && command_if_true || command_if_false

# Examples
[ -f file.txt ] && echo "File exists"
[ $age -ge 18 ] && echo "Adult" || echo "Minor"

# Common pattern
[ -d "/tmp" ] && rm -rf /tmp/* || echo "Directory not found"

Nested If Statements

You can place if statements inside other if statements to create complex conditional logic. Each nested if needs its own fi.

# Nested if example
age=25
if [ $age -ge 18 ]; then
    echo "Adult"
    if [ $age -ge 65 ]; then
        echo "Senior citizen"
    else
        echo "Regular adult"
    fi
else
    echo "Minor"
fi

Advanced: Using [[ ]] Instead of [ ]

Bash provides two ways to test conditions: [ ] (POSIX compatible) and [[ ]] (Bash-specific, more powerful).

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# [[ ]] advantages
filename="test.txt"

# Pattern matching
if [[ $filename == *.txt ]]; then
    echo "Text file"
fi

# Regex matching
if [[ $email =~ ^[a-zA-Z0-9._%+-]+@ ]]; then
    echo "Valid email"
fi

# Logical operators
if [[ -f "$file" && -r "$file" ]]; then
    echo "File exists and is readable"
fi

Common Mistakes

# Wrong - no spaces
if [$age -ge 18]; then  # Syntax error!

# Correct - spaces required
if [ $age -ge 18 ]; then
    echo "Adult"
fi

# Wrong - = compares strings
if [ $count = 5 ]; then  # Works but string comparison

# Correct - -eq for numeric comparison
if [ $count -eq 5 ]; then  # Proper numeric comparison

# For strings, = is correct
if [ "$name" = "Alice" ]; then  # String comparison

# Wrong - missing semicolon
if [ condition ]
then  # Error if then is on next line without proper syntax

# Correct - semicolon or then on same line
if [ condition ]; then
    command
fi

# Or then on next line (needs proper formatting)
if [ condition ]
then
    command
fi

# Wrong - assignment, not comparison
if [ $count = 5 ]; then  # Actually assigns (in some contexts)

# Correct - use == or -eq
if [ $count -eq 5 ]; then  # Comparison
if [[ $count == 5 ]]; then  # Comparison

# For string comparison
if [ "$name" = "Alice" ]; then  # String comparison (single =)
if [[ "$name" == "Alice" ]]; then  # String comparison (==)

Exercise: Create a Decision Script

#!/bin/bash
# Decision script

age=25
status="active"

# Age categorization
if [ $age -le 12 ]; then
    category="child"
elif [ $age -le 17 ]; then
    category="teenager"
elif [ $age -le 64 ]; then
    category="adult"
else
    category="senior"
fi
echo "Age $age: $category"

# File check
[ -f /etc/passwd ] && echo "File exists" || echo "File not found"

# Nested if
if [ $age -ge 18 ]; then
    if [ "$status" = "active" ]; then
        echo "Active adult"
    else
        echo "Inactive adult"
    fi
fi

Summary

What's Next?

Great job mastering if statements! Next, we'll learn about Case Statements - a cleaner alternative to long if-elif chains when checking a single variable against multiple values. Case statements are perfect for menu systems and command parsing!