Question

I know that Git somehow automatically detects if a file is binary or text and that .gitattributes can be used to set this manually if needed. But is there also a way to ask Git how it treats a file?

So let’s say I have a Git repository with two files in it: An ascii.dat file containing plain-text and a binary.dat file containing random binary stuff. Git handles the first .dat file as text and the secondary file as binary. Now I want to write a Git web front end which has a viewer for text files and a special viewer for binary files (displaying a hex dump for example). Sure, I could implement my own text/binary check but it would be more useful if the viewer relies on the information how Git handles these files.

So how can I ask Git if it treats a file as text or binary?

Answer 1

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.

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