Which statement best describes [[ expression ]] compared to [ expression ]?

• A. [[ expression ]] is a newer form of the test command that supports regex matching and is generally preferable.
• B. [[ expression ]] is the older form and lacks regex support.
• C. Both forms are identical in every shell and have no differences.
• D. [[ expression ]] cannot be used in scripts.

Answer: (A)

The key idea here is that the double-bracket conditional is a newer, Bash-specific way to test strings and patterns, and it includes native support for regex matching via the =~ operator. This lets you write concise checks like [[ $var =~ ^[0-9]+$ ]] to ensure a value matches a regex, without needing external tools. Because it’s designed for Bash, it also tends to handle quotes and pattern comparisons more predictably and avoids some common pitfalls with word splitting and pathname expansion that can occur with the traditional single-bracket test form. While the single-bracket form is older and POSIX-compatible, it doesn’t provide native regex matching, so for pattern checks in Bash scripts the dual-bracket form is generally preferable. The other statements are not accurate: the old form does have limitations (no built-in regex) and the two forms are not identical in every shell, nor is the double-bracket form unusable in scripts.