CompTIA Linux+ XK0-006 (V8) Practice Question

The [[ … ]] compound command is parsed by Bash itself, so it can use the =~ binary operator to apply a POSIX-extended regular expression to the string on its left side. The pattern ^([0-9A-Fa-f]{2}:){5}[0-9A-Fa-f]{2}$ anchors the match at the start (^) and end ($) of the string and requires exactly six pairs of hexadecimal digits separated by colons. When the match succeeds, [[ … ]] returns 0; when it fails, it returns 1, so the script continues. All other choices either use the single-bracket test (which does not understand =~), quote the right-hand side so that Bash treats it as a literal string instead of a regular expression or rely on an unportable operator. Because no external utilities are invoked, the solution is efficient and safe on systems with a minimal userland.