Bash if Conditions: A Comprehensive Guide

This document explains how to use if conditions in Bash scripting, covering common scenarios with examples.

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1. Basic if, else, else if (elif)

Syntax:

if [ condition ]; then
    # Code if condition is true
elif [ another_condition ]; then
    # Code if another_condition is true
else
    # Code if all conditions are false
fi

Example:

read -p "Enter a number: " num

if [ "$num" -gt 10 ]; then
    echo "Greater than 10"
elif [ "$num" -eq 10 ]; then
    echo "Exactly 10"
else
    echo "Less than 10"
fi

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2. if with AND (&&) / OR (||) Conditions

Syntax:

• AND (&&) → Both conditions must be true.
• OR (||) → At least one condition must be true.

Example:

age=25
name="Alice"

if [ "$age" -gt 18 ] && [ "$name" == "Alice" ]; then
    echo "Welcome, Alice! You are an adult."
fi

if [ "$age" -lt 18 ] || [ "$name" == "Bob" ]; then
    echo "You are either under 18 or named Bob."
fi

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3. if with Command Result Condition

Check if a command succeeds (exit 0) or fails (exit non-zero).

Example:

if grep -q "error" /var/log/syslog; then
    echo "Error found in logs!"
else
    echo "No errors found."
fi

# Check if a file exists
if [ -f "/etc/passwd" ]; then
    echo "File exists."
fi

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4. if String Exists (or Not)

Check if a string is empty or non-empty.

Example:

str="Hello"

if [ -z "$str" ]; then
    echo "String is empty."
elif [ -n "$str" ]; then
    echo "String is not empty."
fi

# Check if a substring exists
if [[ "$str" == *"ell"* ]]; then
    echo "Substring 'ell' found."
fi

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5. if Equal to Number (Numeric Comparison)

Use -eq, -ne, -gt, -lt, -ge, -le.

Example:

num=15

if [ "$num" -eq 15 ]; then
    echo "Number is 15."
elif [ "$num" -lt 10 ]; then
    echo "Number is less than 10."
fi

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6. if Not Exist (Negative Conditions)

Check if a file/directory does not exist or a condition is false.

Example:

file="/tmp/test.txt"

if [ ! -f "$file" ]; then
    echo "File does not exist."
fi

if [ "$num" -ne 20 ]; then
    echo "Number is not 20."
fi

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7. Other Frequently Used if Conditions

Condition	Example	Description
Check if directory exists	if [ -d "/path" ]; then	True if directory exists
Check if file is readable	if [ -r "$file" ]; then	True if file has read permissions
Check if variable is set	if [ -v "$var" ]; then	True if variable exists
Check if file is executable	if [ -x "/bin/bash" ]; then	True if file is executable
Check if string matches regex	if [[ "$str" =~ ^[A-Z] ]]; then	True if string starts with uppercase

Example:

dir="/etc"

if [ -d "$dir" ]; then
    echo "$dir is a directory."
fi

if [[ "$USER" =~ ^root$ ]]; then
    echo "You are root!"
fi

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Summary Cheat Sheet

Condition Type	Syntax
Basic if-else	if [ cond ]; then ... fi
AND (&&)	if [ cond1 ] && [ cond2 ]; then
OR (`
Command Success	if cmd; then
String Exists	if [ -n "$str" ]; then
String Empty	if [ -z "$str" ]; then
Number Comparison	if [ "$num" -eq 10 ]; then
File Exists	if [ -f "$file" ]; then
Directory Exists	if [ -d "$dir" ]; then
Negative Condition	if [ ! cond ]; then

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Final Tips

✔ Always quote variables ("$var") to avoid word splitting.
✔ Use [[ ]] for advanced string matching (supports ==, =~).
✔ Prefer -eq for numbers, == for strings.