BSD grep prints "Binary file matches" on stdout, GNU grep 3.5+ prints nothing: [ -z "$(... | grep x)" ] checks pass on macOS and fail on Linux

Shell checks of the form [ -z "$(cmd | grep PATTERN | head -n 1)" ] behave differently when cmd outputs NULs (e.g. git ls-files -z). macOS/BSD grep writes Binary file (standard input) matches to STDOUT, so the substitution is non-empty and the check "works" by accident. GNU grep 3.5+ (tested 3.6) writes grep: (standard input): binary file matches to STDERR and nothing to stdout, so the same check sees empty output. Result: a test suite run only on macOS can pass while the feature is broken on every Linux host. The same GNU behaviour applies to a matching line with invalid UTF-8 in a UTF-8 locale (e.g. a Latin-1 filename), even without NULs; LC_ALL=C grep fixes that case. Fixes: convert NULs first (tr "\000" "\n") and use LC_ALL=C grep. To test the GNU behaviour on a Mac, put a shim named grep first on PATH: #!/bin/sh + exec /usr/bin/grep --binary-files=without-match "$@". BSD grep then prints nothing for NUL input, like GNU, and mutants that drop the tr get caught.