Bash if else Statement Tutorial & Examples

This tutorial explains the Bash if else statement: what it does, its syntax, and how to use it in real shell scripts. You’ll learn the basic if, if else, elif, and nested variants, plus common test conditions and practical examples you can run right away.

What Is the Bash if else Statement?

The if else statement is a conditional construct in the Bash shell that lets your script make decisions. It evaluates a command or test expression, and if that expression returns a zero exit status (success), it runs one block of code; otherwise it runs another. This is the core of any non-trivial script, enabling logic like “if the file exists, back it up; else, create it.”

Bash’s if is not a function or a builtin comparison operator like in C; it’s a shell keyword that runs a command and checks its exit code. That’s why you’ll see [[ ... ]], [ ... ], or even plain commands like grep inside the condition.

Basic Syntax and Structure

The general form of an if else statement in Bash is:

if condition
then
    # commands when condition is true
else
    # commands when condition is false
fi

Key points:

• The if line ends with the condition, and the next line must be then (or ; then on the same line).
• Every if must be closed with fi (reverse of if).
• The else block is optional; if omitted and the condition is false, nothing happens.
• For multiple branches, use elif (else if) between if and else.

Example with elif:

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

Test Conditions: [ ] vs [[ ]] vs (( ))

Bash offers several ways to write the condition. The most common are:

• [ condition ] (single bracket) – POSIX test command. Works in all shells, but requires spaces around brackets and uses -eq, -lt, etc. for numeric comparisons.
• [[ condition ]] (double bracket) – Bash keyword. More powerful: supports &&, ||, regex matching with =~, and no word-splitting issues.
• (( expression )) – Arithmetic evaluation. Best for integer math; you can use >, <, == directly.

For numeric comparisons in [[ ]] or [ ], use these operators:

• -eq – equal
• -ne – not equal
• -gt – greater than
• -lt – less than
• -ge – greater than or equal
• -le – less than or equal

For string comparisons, use = (or == in [[ ]]) for equality and != for inequality. For file tests, common flags are -f (regular file), -d (directory), -e (exists), -r (readable), -w (writable), -x (executable).

Practical Examples You Can Run

Here are several ready-to-use examples that illustrate common scenarios.

1. Check if a file exists

#!/bin/bash
if [[ -f "/etc/passwd" ]]; then
    echo "The file exists."
else
    echo "File not found."
fi

2. Compare two numbers

#!/bin/bash
a=10
b=20
if (( a > b )); then
    echo "a is greater"
elif (( a < b )); then
    echo "b is greater"
else
    echo "They are equal"
fi

3. Check if a variable is empty

#!/bin/bash
name=""
if [[ -z "$name" ]]; then
    echo "Name is empty"
else
    echo "Name is $name"
fi

4. Multiple conditions with && and ||

#!/bin/bash
age=25
if [[ $age -gt 18 && $age -lt 65 ]]; then
    echo "Working age"
else
    echo "Not working age"
fi

5. Using a command as the condition

#!/bin/bash
if grep -q "error" /var/log/syslog; then
    echo "Errors found"
else
    echo "No errors"
fi

Note the -q flag in grep suppresses output; the exit status is what matters.

Common Mistakes and Pitfalls

Beginners often run into these issues:

• Missing spaces around brackets. if[$a -eq 1] is wrong; it must be if [ $a -eq 1 ]. The brackets are separate tokens.
• Using > inside single brackets. In [ ], > is a redirection operator, not a comparison. Use -gt or switch to (( )).
• Forgetting fi. Every if needs a closing fi; missing it causes a syntax error.
• Using = for numeric comparison. In [ ], = compares strings. For numbers, use -eq.
• Quoting variables. Always quote "$var" in [ ] to avoid word-splitting and globbing issues, especially if the variable might be empty.
• Using else if instead of elif. else if is not valid; you need elif.

Nested if Statements and Early Exit

You can nest if statements inside each other, but deep nesting hurts readability. Often you can flatten logic with elif or by combining conditions with && and ||.

Example of nesting:

#!/bin/bash
if [[ -d "$dir" ]]; then
    if [[ -w "$dir" ]]; then
        echo "Directory exists and is writable"
    else
        echo "Directory exists but not writable"
    fi
else
    echo "Directory does not exist"
fi

For early exit, you can use return (in a function) or exit (in a script) inside an if block to stop execution, which is often cleaner than a long if else chain.

if else in One Line (for quick commands)

You can write a simple if else on a single line using semicolons:

if [[ -f "$file" ]]; then echo "exists"; else echo "missing"; fi

This is handy for one-off commands in the terminal, but for scripts, multi-line is more readable. Also note that the then must be preceded by a semicolon or be on a new line.

Frequently Asked Questions

What is the difference between [ ] and [[ ]] in Bash if?

[ ] is the POSIX test command and works in any shell. [[ ]] is a Bash keyword that adds features like && and || inside the brackets, regex matching with =~, and does not require quoting variables as often because it does not perform word-splitting. For new Bash scripts, [[ ]] is generally recommended.

Can I use if else with strings?

Yes. Use [[ "$str1" == "$str2" ]] for equality, != for inequality, and -z or -n to check if a string is empty or non-empty. Remember to quote the variables to avoid issues with spaces.

How do I check if a command succeeded in an if statement?

Simply put the command directly after if. For example: if grep -q pattern file; then .... The condition is true if the command exits with status 0. You can also negate with ! before the command.

What is the correct syntax for multiple conditions?

Inside [[ ]], use && for AND and || for OR. Inside [ ], use -a and -o (but they are deprecated and can be unreliable). For arithmetic, use (( ... )) with && and || as well.

Why do I get "syntax error near unexpected token `fi'"?

This usually means a missing then or a missing space before the closing bracket. Check that every if has a then (either on the next line or after a semicolon) and that the condition is properly spaced. Also ensure the fi is on its own line or preceded by a semicolon.

Can I use if else to compare floating-point numbers?

Bash's built-in arithmetic only handles integers. For floating-point comparisons, use bc or awk. For example: if (( $(echo "$a > $b" | bc -l) )); then ... or use awk in a command substitution.

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