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Git Kata: Git attributes file

We'll work a bit with the .gitattributes file in this kata. In this file, you're able to specify how git handles files, e.g., if they are binary or text files. For instance, it's possible to describe what type of line ending that should be used, or if you have some binary files in your repository, you can specify what program that should be used for showing the diff. The following exercises are for GNU/Linux platforms and Mac.

Setup

  1. Run source setup.sh (or .\setup.ps1 in PowerShell)

The task

  1. Create a .gitattributes file with the following content:

    *.txt text=auto eol=lf working-tree-encoding=UTF-8

  2. Run git add file1.txt .gitattributes and you should see

    warning: CRLF will be replaced by LF in file1.txt.

  3. Try to experiment changing eol to either auto or crlf. Can you explain why only one of them changes the line endings of the file, i.e., gives a similar warning as above.

  4. Some files must have DOS (CRLF) line endings in order to work. One such example is bat-files, e.g., Jenkins requires the line endings in bat-files to be CRLF before being able to run them.

    *.bat text=auto eol=crlf

    A similar problem is present with Linux scripts using a shebang in the first line to point to an interpreter. GNU/Linux systems will have a problem running them, e.g., bash\r is not recognized as an interpreter.

    Note: The line endings can also be controlled through git config but that is a different topic.

  5. Git is mostly suitable for text files, but it can also handle binary files and you can set programs to show the diff. In this example, we are going to use the command line tool exiftool to show and compare meta-data for iamges. You will need to have this tool installed for the example to work. (There is also a similar tool called just exif which would also work, but only for jpg files). If you don't have exiftool already it is usually quite easy to install with your favorite package manager e.g. brew install exiftool or apt install exiftool

    Once we have exiftool installed, we will need to configure a new "diff driver" in the git config that uses exiftool to "convert" a binary file to a text representation that can be diff'ed.

    Add the following to your global ~/.gitconfig or to the local .git/config file.

    [diff "useexif"]
textconv = exiftool
cachetextconv = true

    Then we need to actually tell Git to use this new "driver" for any files of the type PNG. Add the following to the .gitattributes file you created at the start of the exercise:

    *.png diff=useexif

Advanced bonus task

  1. Now that everything is set up, let us test it out. You can use the image mario.png located next to this README file. ( Mario png borrowed from https://www.freepngimg.com/download/mario/20698-7-mario-transparent-background.png )

    Mario large

    Copy/move this image to the exercise folder (e.g. with cp ../mario.png .), then use git add to stage the image and put it under Git's control.

    Now we can run git diff --staged to view the change, and hopefully Git will print out the full exiftool output in green as it is comparing to the previous (non-existing) /dev/null version.

  2. To really see the value, replace the image with a different one, e.g., using the smaller version of the image also in this folder:

    Mario small

    If you copy this file to the exerise folder and rename it to mario.png Git will correctly see it as a changed file. (you can do this easily using cp ../mario-small.png ./mario.png to get the smaller one and overwrite the existing)

    Now, just run the regular git diff to have Git compare the new smaller one with the original that is currently staged. It should be easy to see which attributes of the image were changed in the resize.

Useful commands

  • file file1.txt (on GNU/Linux and Mac)
  • git add
  • git status
  • iconv is useful for converting between different character encodings.

Additional information

The .gitattributes has another alternative, if you are just interested in having the same line endings and character encodings across the files in your repository. editorconfig can handle indentation size (useful for Python), whether to use tabs or spaces, adding a final newline etc.