Bash `if` Conditions: A Comprehensive Guide

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

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

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

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

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

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

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

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

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`

Final Tips

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