Git tricks

1 Learning git

See the fastai masterclass for many more helpful tips/links/scripts/recommendations. Learn Git Branching explains the mechanics in a friendly fashion. Steve Bennett’s 10 things I hate about Git is also useful.

Mark Dominus’ master class:

• The Universe of Discourse : Things I wish everyone knew about Git (Part I)
• The Universe of Discourse : Things I wish everyone knew about Git (Part II)

2 Handy git config

git config --global core.editor "code-insiders --wait"  # insiders

echo .DS_Store >> .gitignore_global
git config --global core.excludesfile $HOME/.gitignore_global

3 Handy git commands

git diff --name-only --diff-filter=U | xargs git checkout --theirs --

echo "alias git_theirs 'git diff --name-only --diff-filter=U | xargs git checkout --theirs --'" >  ~/.config/fish/conf.d/git_theirs.fish
echo "alias git_ours   'git diff --name-only --diff-filter=U | xargs git checkout --ours   --'" >> ~/.config/fish/conf.d/git_theirs.fish
chmod a+x ~/.config/fish/conf.d/git_theirs.fish
source ~/.config/fish/conf.d/git_theirs.fish

git log -Sword

git log --oneline --find-renames --stat --follow -- src/somefile.ts

git clone --single-branch --branch <branchname> <remote-repo>

git rm --cached blah.tmp

git push <remote_name> --delete <branch_name>

git push origin HEAD:refs/heads/backdoor

4 What git calls things

By which I mean that which is formally denoted as git references. git references is the canonical description of the mechanics. tl;dr the most common names are refs/heads/SOMETHING for branch SOMETHING, refs/tags/SOMETHING and remotes/SOMEREMOTE/SOMETHING for (last known state of) a remote branch.

As alexwlchan explains, these references are friendly names for commits, and should be thought of as pointers to commits.

And yet there is something a little magical going on. How come if I pull a branch, I get the latest version of that branch, not the earliest to use that name? Other stuff is happening.

The uses are (at least partly) convention and other references can be used too. For example gerrit uses refs/for/ for code review purposes.

5 Filters

Commands applied to your files on the way in and out of the repository. Keywords, smudge, clean, .gitattr These are a long story, but not so complicated in practice. A useful one is stripping crap from jupyter notebooks.

6 Commit hooks

For doing stuff before you put it in cold storage. e.g asking DID YOU REALLY WANT TO INCLUDE THAT GIANT FILE?

Here is a commit hook that does exactly that. I made a slightly modernized version:

curl -L https://gist.github.com/danmackinlay/6e4a0e5c38a43972a0de2938e6ddadba/raw/install.sh | bash

UPDATE: I decided this was a waste of time and removed it.

After that installation you can retrofit the hook to an existing repository thusly

p -R ~/.git_template/hooks .git/

There are various frameworks for managing hooks, if you have lots. For example, pre-commit is a mini-system for managing git hooks, based on python. Husky is a node.js-based one.

I am not sure whether hook management system actually saves time overall for a solo developer, since the kind of person who remembers to install a pre-commit hook is also the kind of person who is relatively less likely to need one. Also it is remarkably labour-intensive to install the dependencies for all these systems, so if you are using heterogeneous systems this becomes tedious.

To skip the pre-commit hook,

git commit --no-verify

7 Subtrees/submodules/subprojects/subdirs/subterranean mole people

Sub-projects inside other projects? External projects? The simplest way of integrating external projects seems to be as subtrees. Once this is set up we can mostly ignore them and things work mostly as expected. Alternatively there are submodules, which have various complications. More recently, there is the subtrac system, which I have not yet used.

git fetch remote branch
git subtree add --prefix=subdir remote branch --squash

git fetch remote branch
git subtree pull --prefix=subdir remote branch --squash
git subtree push --prefix=subdir remote branch --squash

pushd superproject
git subtree split -P project_subdir -b project_branch
popd
mkdir project
pushd project
git init
git pull ../superproject project_branch

pushd superproject
cd ..
git clone superproject subproject
pushd subproject
git filter-branch \
    --subdirectory-filter project_subdir \
    --prune-empty -- \
    --all

svn co https://github.com/buckyroberts/Source-Code-from-Tutorials/trunk/Python

8 Deleting all tags

git tag -l | xargs -I %% git push -v origin :refs/tags/%%
git tag -l | xargs git tag -d && git fetch -t

9 Conventions

Many possible workflows are feasible with git. Large teams often have elaborate conventions for what to name branches and who gets to merge what with whom. Here are some I have seen in the wild:

• Conventional Commits
• Trunk-based Development
• GitHub flow (which is not the same as gitflow)

Some of these systems have associated helpers, see next.

10 Helpers

Git has various layers of abstraction, from a very basic infrastructure of plumbing, through notionally use-friendly, higher-level porcelain commands which are supposed by authors of git to be user friendly but universally regarded as “a passable first attempt at best”, through to various helper commands that ease various workflows and pain points.

git config --global rerere.enabled true
git config --global rerere.autoupdate true

11 Importing some files across a branch

git checkout my_branch -- my_file/

12 Garbage collecting

In brief, this will purge a lot of stuff from a constipated repo in emergencies:

git reflog expire --expire=now --all && git gc --prune=now

In-depth explanation here.

13 Editing history

13.1 Cleaning out all big files

bfg does that:

git clone --mirror git://example.com/some-big-repo.git
cd some-big-repo.git
git repack
bfg --strip-blobs-bigger-than 10M .
git reflog expire --expire=now --all && git gc --prune=now --aggressive
git push -f

git filter-branch -f \
    --index-filter
    'git rm -r --cached --ignore-unmatch unwanted_files'

git archive HEAD --format=zip > archive.zip

git checkout -b temp_branch
git rebase --root -i

git reset $(git commit-tree HEAD^{tree} -m "A new start")

git checkout --orphan temp_branch
git add -A
git commit -am "Initial commit"
git branch -D master
git branch -m master
git push -f origin master

TREE=`git cat-file -p master |sed '1,/^$/s/^tree //p;d;'`
COMMIT=`echo Truncated tree | git commit-tree $TREE`
git branch truncated-master $COMMIT
git branch backup-master-just-in-case-i-regret-it-later master
git push -f origin truncated-master:master

14 Making git work with a broken-permission FS

e.g. you are editing a git repo on NTFS via Linux and things are silly.

git config core.filemode false

15 Detecting if there are changes to commit

Thomas Nyman:

if output=$(git status --porcelain) && [ -z "$output" ]; then
  # Working directory clean
else
  # Uncommitted changes
fi

16 Emergency commit

Oh crap I’m leaving the office in a hurry and I just need to get my work into git ASAP for continuing on another computer. I don’t care about sensible commit messages because I am on my own private branch and no-one else will see them when I squash the pull request.

I put this little script in a file called gitbang to automate this case.

#!/usr/bin/env bash
# I’m leaving the office. Capture all changes in my private branch and push to server.
if output=$(git status --porcelain) && [ -z "$output" ]; then
  echo "nothing to commit"
else
  git add --all && git commit -m bang
fi
git pull && git submodule update --init --recursive  && git push

Or: this could be a fish function why not

function gitbang --argument-names remoteref --description "push to remote"
  set pushrel ""
  set -q remoteref; or set pushrel HEAD:remoteref
  set gitstatus (git status --porcelain)
  if (test -z "$gitstatus" )
    echo "nothing to commit"
  else
    git add --all ; and git commit -m bang
  end
  git pull ; and git submodule update --init --recursive ; and git push
end

Pro tip: if you use VS code there is a feature called Cloud Changes that synchronises your changes to the cloud, so you can pick up where you left off on another computer without arsing about with git.

17 Git hosting

• Codeberg Documentation is a gitea host
• Gitlab
• github

Or maybe avoid the need for hosting by using…

18 git email workflows

Learn to use email with git.

19 Content-specific diffing

Tools such as git-latexdiff provide custom diffing for, in this case, LaTeX code. These need to be found on a case-by-case basis.

20 SSH credentials

Managing SSH credentials in git is non-obvious. See SSH.

21 Jupyter

For sanity in git+jupyter, see jupyter.

22 Decent GUIs

See Git GUIs.

23 Which repo am I in?

For fish and bash shell, see bash-git-prompt.

24 Data versioning

See data versioning.

25 Incoming

• Trying to convince academics to use git - Juulia Suvilehto
• The Universe of Discourse : I wish people would stop insisting that Git branches are nothing but refs