What does “!eol” in gitattributes do?

Git has 2 attributes that deal with end-of-lines:

1. text

Documentation says:

This attribute enables and controls end-of-line normalization. When a text file is normalized, its line endings are converted to LF in the repository

This effectively means that when you commit to the repo, it will convert line-endings to LF

2. eol

Documentation says:

This attribute sets a specific line-ending style to be used in the working directory. It enables end-of-line normalization without any content checks, effectively setting the text attribute.

So while the text attribute affects how the file will look like IN THE REPO, eol affects how the file looks like in the working directory.

Now, an attribute can have 4 states:

set with no value
example: * text

unset
example: * -text

set with specific value
example: * text=auto

unspecified
example: * !text

So, * text=auto !eol means this:

All files have the attribute text set to auto and the eol attribute unspecified. Reading the documentation we find out that text=auto means that you let Git decide if a file is text and if it is it will normalize it (set line-endings in the repo to LF).

!eol means that the attribute eol is set to unspecified explicitly. In this case it is the same as not specifying it at all, instructing Git to look at the core.autocrlf and core.eol configuration settings to see how to deal with line-endings in the working directory. Note this:

The core.eol configuration variable controls which line endings Git will use for normalized files in your working directory; the default is to use the native line ending for your platform, or CRLF if core.autocrlf is set.

But you would use !eol in a situation like the following:

* text=auto eol=crlf
test.txt !eol

basically overriding the eol attribute from CRLF to unspecified for test.txt. This means that for all files except test.txt, Git will convert line-endings to CRLF on checkout. For test.txt Git will defer to the core.autocrlf and core.eol configuration settings so on any given system the line-ending may be either LF or CRLF.