Use Double Brackets Instead Of Single Bracket For Tests In Bash

The aim of this page📝is to explain and list the advantages of using [[ conditional expressions instead of [ aka test builtin in bash. Some explanation first, then the three benefits from an SO thread.

• historically, before any conditional expressions, there was an only command called test
• test is still there for compatibility with other sh shells
• test is also known as [ — single square bracket instead of two square bracket
• you can encounter [ in the wild also in bash scripts a lot
• [ is a regular shell command
• therefore there are lots of tiny pitfalls for the old test command or [
• beware that [ exists but don't use it
• → rule: use [ only if you need a script that has to run on non-bash shell machines
• Make sure you have the #!/bin/bash shebang line for your script if you use double brackets.
• [[ originates from bash and is generally treated as bash-specific (don't know about other shells there may be adoption)
• [[ is easier
• [[ is not a command
• [[ is a bash extension
• it has a dedicated syntax and it is parsed in a special way
• e.g. no need to quote variables
• write help test to get info on essentials
• write help [[ for bash specific operations
• from my experience, however, I keep seeing [ also in bash-only scripts so the the knowledge of differences is still relevant

1. logical operators

• can use && and || operators for boolean tests
• can use < and > for string comparisons.
• [ cannot do this because it is a regular command
• &&, ||, <, and > are not passed to regular commands as command-line arguments

2. regex

• can use =~ operator for regex matches
• capture groups are stored in BASH_REMATCH var

if [[ $answer =~ ^y(es)?$ ]]

3. globbing

• can use pattern matching with wildcards

if [[ $ANSWER = y* ]]

links

• What’s the difference between single and double square brackets in Bash? — Stack Overflow