How to move function declaration to another file yet retain Git history?

Question

I am refactoring a single-file PHP script to declare functions and classes in separate files. How does one move a block of code from one file to another file yet preserve the Git history for each individual line? I would like the ability to use at the very least git blame on the code. Comparing to older versions would also be helpful.

Note that I am not interested in 'workaround' solutions that would require additional commands or flags (such as --follow) on the code to view its 'old' history, for which the person viewing the history would have to know that the file needs special treatment. I am looking for a solution which writes the necessary metadata such that normal git commands such as blame, log, diff, and such 'just work' without presenting to them additional flags or arguments.

I have read quite a few posts looking for solutions to similar problems, but none that address the issue of git blame on individual lines. One solution that I think would work would be to copy the file and its entire history, then work off of that. However, that question remains without a satisfactory answer.

Answer 1

Warning: This assumes that rewriting history is acceptable. Do not use this unless you know the implications of rewriting history.

To split a file into multiple files while maintaining history use git filter-branch:

git filter-branch --tree-filter 'if [ -f original.php ]; then 
    echo part1.php part2.php part3.php | xargs -n 1 cp original.php; fi' HEAD

Then delete the lines that you do not want from each file (leaving only the desired functions and classes) and commit the result!