How to do git tricks

Find last commit where line existed

git blame --reverse START

Comparing files

From https://stackoverflow.com/questions/8131135/git-how-to-diff-two-different-files-in-different-branches

git diff branch1:full/path/to/foo.txt branch2:full/path/to/foo-another.txt

How to link to such comparison view on github?

Compare and copy files from other branches

• https://stackoverflow.com/questions/2364147/how-to-get-just-one-file-from-another-branch
• https://stosb.com/blog/retaining-history-when-moving-files-across-repositories-in-git/

Include branches from an unrelated repository

git remote add rep2-git url_to_unrelated_repository
git fetch rep2-git
git checkout remotes/rep2-git/branch2
git checkout -b rep2/branch2
git push -u origin rep2/branch2:rep2/branch2

The resulting repository has another unrelated history. There is no need for the two repositories to have a common initial commit or anything. The commit hashes are preserved. You can even git cherry-pick commit from the other repostory and git will often do a pretty good job finding files to apply the commits to, even if the files have somewhat different names and are in somewhat different locations in the two file trees.

Using git cherry pick

Across unrelated histories

After importing branches from another unrelated repository as an unrelated history, git cherry pick can apply a commit from the unrelated history, but sometimes git cherry-pick will work across files having different paths and sometimes it just creates copies on new paths, which is not helpful. Not sure why. The same cherry-pick on different computers can do different things on the same repository and same commits for some reason.

Review changes before commit

Be sure to do

git diff HEAD

or

git diff <your base>

for multiple commits piled on top of <your base> to review what changes are being made and to minimize them. git diff seems to produce misleading results.

Undesirable changes from commits

• Often git cherry pick will make more changes including those from other commits. Here is why: Git goes back in the history until the cherry pick source matches the target and creates the patch based on this revision. That's why more changes might appear.
• In a series of cherry-picked commit, do not make additional changes on the modified lines. The following cherry-picked commit will put them back! You have to make them all at once after you are done.

One more reason why it is important to review changes before commit and edit as needed, not just resolve the conflict.

Using patch files

Instead of cherry-pick

• Generate and use patch

git log -p -1 <sha1> | git apply --3way -

But this seems to make much the same changes and can pull more changes too. The --3way flag will mark conflicts, without --3way aplying the patch just failed for me.

• Create a patch file

git log -p -1 <sha1-of-your-commit> > patch.file

edit it manually as desired and then apply the patch:

it apply --3way patch.file

To modify only one file

1. Create a patch file for that individual file:

git show [commit hash] -- path/to/old/file > patchfile

1. Manually edit the newly created patchfile and replace all occurrences of the old path with the new path.
2. Apply the patch via

git apply --3way patchfile

Merging unrelated histories

• https://git-scm.com/book/en/v1/Git-Tools-Subtree-Merging
• https://help.github.com/en/articles/about-git-subtree-merges
• https://medium.com/@spences10/git-allow-unrelated-histories-a39a3814b981
• https://stackoverflow.com/questions/37937984/git-refusing-to-merge-unrelated-histories-on-rebase
• https://stackoverflow.com/questions/13040958/merge-two-git-repositories-without-breaking-file-history
• https://stosb.com/blog/retaining-history-when-moving-files-across-repositories-in-git/
• https://stackoverflow.com/questions/16937359/git-copy-file-preserving-history
• https://stackoverflow.com/questions/6434995/how-to-re-merge-a-file-in-the-git

Filter

• https://manishearth.github.io/blog/2017/03/05/understanding-git-filter-branch/
  • to extract a subdirectory: git filter-branch --subdirectory-filter name_of_subdir @
• https://stackoverflow.com/questions/7430717/git-filter-branch-on-all-branches
• https://git-scm.com/docs/git-filter-branch
• https://github.com/newren/git-filter-repo/

Filter subdirectory

mkdir wrf-fire-filtered
cd wrf-fire-filtered
git clone --bare github:/openwfm/wrf-fire.git .git
git config --bool core.bare false
git reset --hard
git filter-branch --subdirectory-filter wrfv2_fire --prune-empty --tag-name-filter cat -- --all
git remote add filtered git@github.com:janmandel/wrf-fire-filtered.git
git push filtered --mirror

Submodules

• https://git-scm.com/book/en/v2/Git-Tools-Submodules

See also

• git cherry pick man page
• git diff man page
• https://randyfay.com/content/avoiding-git-disasters-gory-story
• https://www.git-scm.com/docs/git-mergetool
• git mergetool tutorial
• https://git-blame.blogspot.com/2013/09/fun-with-first-parent-history.html