What’s the Difference Between [ ] and [[ ]] in Shell Scripts?

Introduction

Recently, I had an opportunity to write some shell scripts at work.

Since I didn’t have much experience with shell scripting, I looked things up online while writing the code. That’s when I ran into this question:

What’s the difference between writing if [ ]; then and if [[ ]]; then ?

In this article, I’ll share what I learned about the differences between the two and how to choose which one to use.

The Basic Difference Between [ ] and [[ ]]

The POSIX-standard syntax is defined so that it behaves the same across all UNIX-like systems.

It works on Linux, macOS, BSD, and any POSIX-compliant shell.

On the other hand, the Bash extension syntax works on Bash and Zsh, but does not work on /bin/sh or other strictly POSIX-compliant shells.

[ ] internally calls the test command.

So this:

if [ -f file ]; then

    echo "Exists"

fi

is actually equivalent to:

if test -f file; then

    echo "Exists"

fi

Meanwhile, [[ ]] is parsed directly by the shell, which allows safer handling of complex expressions and pattern matching.

Common Conditional Expressions Compared

Here are common examples written using both [ ] and [[ ]].

1. Checking File or Directory Existence

Both work the same, but with [[ ]] you can omit quotes safely.

2. String Comparison

• 
  [ ] → Always quote variables


• 
  [[ ]] → Quotes optional and safer

3. Numeric Comparison

Same syntax in both cases.

4. AND / OR Conditions

[ ] syntax

[ -f "$a" ] && [ -r "$a" ]    # AND（both true）

[ -f "$a" ] || [ -f "$b" ]    # OR（one true）

[[ ]] syntax

[[ -f $a && -r $a ]]    # AND（both true）

[[ -f $a || -f $b ]]    # OR（one true）

[[ ]] allows combining conditions inside a single expression.

5. Pattern Matching

file="report.txt"



# Works in [[ ]]

if [[ $file == *.txt ]]; then

    echo "Text file"

fi



# Does NOT work in [ ] (always false)

if [ $file == *.txt ]; then

    echo "Will not run"

fi

Wildcards (*, ?) are supported only in [[ ]].

How to Choose

Pick One Style and Stick to It

You can use either style, but it’s best to choose one and stay consistent.

Here’s why:

1. Better readability and maintainability

Mixing [ ] and [[ ]] in one script makes readers wonder:

“Why is this one using [ ]? Is there a special reason?”

Unintended inconsistencies increase mental load and confusion.

2. Prevent incompatibility across shells

[[ ]] works only in Bash.

It fails in /bin/sh:

#!/bin/sh

if [[ -f file ]]; then

    echo "Exists"

fi

• Works in bash
  


• Errors in sh
  

Therefore:

• Writing a Bash script? → Use [[ ]]
  


• Want POSIX portability? → Use [ ]
  

3. Avoid subtle bugs

Since [ ] and [[ ]] evaluate expressions differently, mixing them can cause unexpected behavior.

Example:

var="*.txt"



# [ ] → filename expansion occurs

if [ $var = "*.txt" ]; then

    echo "match"

fi



# [[ ]] → treated as a pattern

if [[ $var == *.txt ]]; then

    echo "match"

fi

Even though the lines look similar, their behavior differs.

Consistency prevents surprises.

Summary

• 
  [ ] is the POSIX-standard test command


• 
  [[ ]] is a Bash extension: safer and more flexible


• Both support -n / -z, but [[ ]] is safer without quotes


• 
  [[ ]] supports combining conditions and pattern matching


• Either is fine—but don’t mix them

Closing Thoughts

Before writing this article, I didn’t even know there were two different syntaxes.

I was copying conditional expressions from the internet and ended up mixing [ ] and [[ ]] in the same script without realizing it.

If this article helps even one person avoid that mistake, I’ll be happy.

Be mindful when choosing which syntax to use!

💬 How about you?

• What syntax do you usually use in your shell scripts — [ ] or [[ ]]? And why?


• Have you ever run into unexpected behavior because of differences between [ ] and [[ ]]?