[ ] vs [[ ]] in Bash: Single vs Double Brackets

Single Brackets [ ] vs Double Brackets [[ ]]

The choice between [ ] and [[ ]] is one of the most confusing aspects of shell scripting. They look similar but behave quite differently.

[ ] — The test Command

[ ] is actually the test command. It is a regular command, not shell syntax:

# These are equivalent:
[ "$var" = "hello" ]
test "$var" = "hello"

Rules for [ ]:

• Variables MUST be quoted: [ "$var" = "value" ]
• Use = for string comparison (not ==)
• Use -eq, -lt, -gt for numeric comparison
• No pattern matching or regex
• && and || must be outside: [ cond1 ] && [ cond2 ]

[[ ]] — Bash Keyword

[[ ]] is a Bash/Zsh/Ksh built-in keyword with additional features:

# No word splitting — quoting optional (but still recommended)
[[ $var = "hello" ]]

# Pattern matching with ==
[[ $file == *.txt ]]

# Regex matching with =~
[[ $email =~ ^[a-zA-Z]+@[a-zA-Z]+\.[a-zA-Z]+$ ]]

# Logical operators inside
[[ $a -gt 0 && $b -lt 100 ]]

Common Mistake: == in [ ]

# WRONG: == is not POSIX in [ ]
if [ "$var" == "value" ]; then  # Works in bash but not sh

# CORRECT for [ ]:
if [ "$var" = "value" ]; then

# CORRECT for [[ ]]:
if [[ "$var" == "value" ]]; then

Comparison Table

Feature	[ ]	[[ ]]
POSIX compatible	Yes	No (bash/zsh/ksh)
Word splitting	Yes (must quote)	No
Pattern matching	No	== *.txt
Regex	No	=~ regex
Logical operators	External &&/`
String comparison	=	= or ==

Recommendation

• Use [[ ]] in Bash scripts (with #!/bin/bash shebang)
• Use [ ] only when POSIX compatibility is required (#!/bin/sh)
• Always quote variables in [ ] to prevent word splitting