A builtin part of the shell, just like `if` and `then` and `fi`. Not all shells have `[[`, the one that doesn't which people unknowingly run into most is probab

A builtin part of the shell, just like `if` and `then` and `fi`. Not all shells have `[[`, the one that doesn't which people unknowingly run into most is probably `dash` as used by Alpine Linux. POSIX doesn't require `[[` in a shell, BASH & Zsh support it but others often don't.