If/Else Statements

Conditional statements allow your scripts to make decisions and execute different code paths based on conditions. The if statement is the foundation of flow control in bash.

Basic If Syntax

The simplest form of an if statement:

if [ condition ]; then
    commands
fi

The spaces inside [ ] are required!

#!/bin/bash
AGE=18
if [ "$AGE" -ge 18 ]; then
    echo "You are an adult"
fi

If-Else

Execute alternative code when the condition is false:

if [ condition ]; then
    # runs when true
    commands
else
    # runs when false
    other_commands
fi

Example:

#!/bin/bash
FILE="test.txt"
if [ -f "$FILE" ]; then
    echo "File exists"
else
    echo "File not found"
fi

If-Elif-Else

Handle multiple conditions:

if [ condition1 ]; then
    commands1
elif [ condition2 ]; then
    commands2
elif [ condition3 ]; then
    commands3
else
    default_commands
fi

Example:

#!/bin/bash
SCORE=85

if [ "$SCORE" -ge 90 ]; then
    echo "Grade: A"
elif [ "$SCORE" -ge 80 ]; then
    echo "Grade: B"
elif [ "$SCORE" -ge 70 ]; then
    echo "Grade: C"
else
    echo "Grade: F"
fi

Exercise: If-Else Statement

Write a simple conditional:

The [ ] Test Command

[ ] is actually a command called test. These are equivalent:

if [ "$A" -eq 5 ]; then echo "Yes"; fi
if test "$A" -eq 5; then echo "Yes"; fi

Important rules:

• Spaces after [ and before ] are required
• Always quote variables: "$VAR"

The [[ ]] Extended Test

[[ ]] is a bash improvement over [ ]:

# [[ ]] advantages:
# - No word splitting on variables
# - Pattern matching with ==
# - Regex matching with =~
# - Logical && and || inside

if [[ $NAME == "admin" ]]; then
    echo "Welcome admin"
fi

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

# Regex matching
if [[ $EMAIL =~ ^[a-z]+@[a-z]+\.[a-z]+$ ]]; then
    echo "Valid email"
fi

[ ] vs [[ ]]

Feature	[ ]	[[ ]]
POSIX compatible	Yes	No (bash only)
Word splitting	Yes	No
Pattern matching	No	== with patterns
Regex matching	No	=~
Logical operators	-a, -o	&&, ||

Recommendation: Use [[ ]] in bash scripts for safety and features.

Combining Conditions

With [ ]

# AND
if [ "$A" -gt 5 ] && [ "$B" -lt 10 ]; then
    echo "Both conditions true"
fi

# OR
if [ "$A" -gt 5 ] || [ "$B" -lt 10 ]; then
    echo "At least one true"
fi

# NOT
if [ ! -f "$FILE" ]; then
    echo "File does not exist"
fi

With [[ ]]

# AND (cleaner syntax)
if [[ $A -gt 5 && $B -lt 10 ]]; then
    echo "Both conditions true"
fi

# OR
if [[ $A -gt 5 || $B -lt 10 ]]; then
    echo "At least one true"
fi

Negating Conditions

Use ! to negate:

# File does NOT exist
if [ ! -f "$FILE" ]; then
    echo "File missing"
fi

# String is NOT empty
if [ ! -z "$VAR" ]; then
    echo "Variable is set"
fi

# NOT equal
if [ "$A" != "$B" ]; then
    echo "Values differ"
fi

Nested If Statements

If statements can contain other if statements:

#!/bin/bash
AGE=25
HAS_LICENSE=true

if [ "$AGE" -ge 18 ]; then
    if [ "$HAS_LICENSE" = true ]; then
        echo "You can drive"
    else
        echo "You need a license"
    fi
else
    echo "Too young to drive"
fi

Exercise: Combined Conditions

Use multiple conditions:

One-Line Conditionals

For simple cases, use && and ||:

# If true, do this
[ -f "$FILE" ] && echo "File exists"

# If false, do this
[ -f "$FILE" ] || echo "File missing"

# Combined
[ -f "$FILE" ] && echo "Found" || echo "Missing"

Arithmetic Conditionals

Use (( )) for arithmetic:

#!/bin/bash
X=10

if (( X > 5 )); then
    echo "X is greater than 5"
fi

if (( X >= 5 && X <= 15 )); then
    echo "X is between 5 and 15"
fi

# Increment and check
if (( ++X == 11 )); then
    echo "X is now 11"
fi

Common Patterns

Check if variable is set

if [ -n "${VAR+x}" ]; then
    echo "VAR is set"
fi

# Or simpler
if [ -n "$VAR" ]; then
    echo "VAR is set and non-empty"
fi

Check command success

if command; then
    echo "Command succeeded"
else
    echo "Command failed"
fi

# Real example
if grep -q "pattern" file.txt; then
    echo "Pattern found"
fi

Default values with conditionals

if [ -z "$NAME" ]; then
    NAME="default"
fi

# Or shorter
[ -z "$NAME" ] && NAME="default"

# Or even shorter
NAME="${NAME:-default}"

Key Takeaways

• Use if [ condition ]; then ... fi for basic conditionals
• Always quote variables inside [ ]
• Use [[ ]] in bash for advanced features and safety
• Combine conditions with && (AND) and || (OR)
• Negate conditions with !
• Use elif for multiple exclusive conditions
• Use (( )) for arithmetic comparisons
• One-liner: [ condition ] && true_command || false_command