I used to think
[ would spawn
[[ was a builtin, but apparently, it doesn't:
$ strace bash -c " [ -z "$FOO" ] && echo yes " 2>&1 | grep test
$ type [ [ is a shell builtin
[ is a shorter and seems more portable too.
GNU bash, version 4.3.11(1)-release (x86_64-pc-linux-gnu).