Where niubash differs from bash

One-sentence positioning: the compatibility is tested, but the machinery is different. Against GNU Bash's own 83 upstream test suites, the niubash engine (rubash) scores 83/83 passing, with 82 suites byte-identical in output; and underneath that verified compatibility sits an entirely different machine — a zero-fork, three-tier execution model, path folding with no MSYS layer, byte-for-byte verbatim argv, all four path dialects accepted, 62 real builtins plus 12 inlined coreutils, and a deliberately maintained uname = MSYS_NT compatibility persona.

This section takes the two sides of that coin apart: which side is "behaves like bash" (with suite and byte-level evidence), and which side is "built differently from bash" (with architecture and contract documents). Whether you are migrating scripts, evaluating performance, or chasing down a behavioral difference — start from the matching article.