How to determine if Git handles a file as binary or as text?

builtin_diff()¹ calls diff_filespec_is_binary() which calls buffer_is_binary() which checks for any occurrence of a zero byte (NUL “character”) in the first 8000 bytes (or the entire length if shorter).

I do not see that this “is it binary?” test is explicitly exposed in any command though.

git merge-file directly uses buffer_is_binary(), so you may be able to make use of it:

git merge-file /dev/null /dev/null file-to-test

It seems to produce the error message like error: Cannot merge binary files: file-to-test and yields an exit status of 255 when given a binary file. I am not sure I would want to rely on this behavior though.

Maybe git diff --numstat would be more reliable:

isBinary() {
    p=$(printf '%s\t-\t' -)
    t=$(git diff --no-index --numstat /dev/null "$1")
    case "$t" in "$p"*) return 0 ;; esac
    return 1
}
isBinary file-to-test && echo binary || echo not binary

For binary files, the --numstat output should start with - TAB - TAB, so we just test for that.

¹
builtin_diff() has strings like Binary files %s and %s differ that should be familiar.