Runs both binaries over 154 cases and compares stdout, stderr and exit
code: encoding at every wrap width, decode round-trips, canonical-tail
rules, padding placement, concatenated streams, every prefix of a known
encoding, usage errors and file operands.
Locates a genuine GNU base64 rather than trusting /usr/bin/base64, since
distributions increasingly ship uutils there and it disagrees with GNU on
most malformed input — running against it fails 51 of the 154 cases, which
doubles as a negative control for the harness itself.
The three known differences are asserted as expected rather than ignored.
Windows has no base64 command; certutil is file-only, adds PEM markers
and cannot be piped. This is a real base64.exe matching GNU coreutils:
same flags (-d, -i, -w), stdin/stdout streaming, exit codes and error
messages, so existing muscle memory and scripts carry over.
Verified against coreutils 9.7 with a 153-case differential test. The
subtle behaviours are reproduced: bytes are emitted incrementally so a
stream that goes bad still writes its valid prefix, unpadded tails are
accepted only when canonically encoded, concatenated streams decode as
one, and -w0 suppresses the trailing newline.
Deliberate deviation: '\r' is skipped when decoding alongside '\n', as
CRLF base64 files are routine on Windows and GNU rejects them.
Zero dependencies, so the build works offline.
LyAhn