Depends on what you mean by "works". Yes, all three shells recognize [[ ]] construct, as it was specifically added to SUS (or POSIX?) for implementation-specifi

Depends on what you mean by "works". Yes, all three shells recognize [[ ]] construct, as it was specifically added to SUS (or POSIX?) for implementation-specific things, but how they interpret it is up to the specific shell. Let's take people's favourite =~ operator. In bash, according to documentation, it uses ERE syntax. In zsh, it is ERE or PCRE, depending on an option being set. This is a subtle but important difference in how [[ ]] works. And then again, your environment can change the behaviour of a script you're running, which is (a) unexpected and (b) undesirable, and the change in the behaviour can range from "doesn't find thing it should" to "crashes" to "goes awry in an unexpected way".