Git tricks

My own git notes.

See also the more universally acclaimed classic git tips.

There are better learning resources than this. Some are noted here, in fact.

Learning git

See the fastai masterclass for many more helpful tips/links/scripts/recommendations. Learn Git Branching explains the mechanics in a friendly fashion.

Handy git commands

merging

During a merge, git checkout --theirs filename (or --ours) will checkout respectively their, or my, version. The following sweet hack will resolve all files accordingly:

grep -lr '<<<<<<<' . | xargs git checkout --theirs

TODO: Surely I can find conflicted files using git natively without grep. Should look that up.

Searching

…for a matching file

git grep

…for a matching commit

Easy, except for the abstruse naming; It is called “pickaxe” and spelled -S.

git log -Sword

Clone a single branch

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

Remove file from versioning without deleting my copy

git rm --cached blah.tmp

Ignore macOS `.DS_Store` files

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

delete remote branch

git push <remote_name> --delete <branch_name>

Push to a non-obvious branch

git push origin HEAD:refs/heads/backdoor

This is almost obvious except the git naming of things seem arbitrary. Why refs/heads/SOMETHING? Well…

What git calls things

By which I mean that which is formally referred to as git references. git references is the canonical description of the mechanics here. 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. The uses are (at least partly) convention and other references can be used too. For example gerrit uses refs/for/ for code review purposes.

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.

Commit hooks

For doing stuff before you put it in cold storage. For me this means, 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

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 save time overall for a solo developer, since the kind of person who remembers to install a pre-commit hock 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.

Subtrees/submodules/subprojects/subdirs/subterranean mole people

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

Submodule

Include external projects as separate repositories within a repository is possible, but I won’t document it here, since it’s well documented elsewhere, and I use it less often NB: some discipline required to make it go; you need to remember to git submodule init etc.

Subtrac

Have not yet tried.

subtrac is a helper tool that makes it easier to keep track of your git submodule contents. It collects the entire contents of the entire history of all your submodules (recursively) into a separate git branch, which can be pushed, pulled, forked, and merged however you want.

Subtree

Subtree subsumes one git tree into another is a usually-transparent way (no separate checkout as with submodules.) It can be used for temporary merging or for splicing and dicing projects.

Splicing a subtree onto a project

Creatin’:

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

Updatin’:

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

Con: Rebasin’ with a subtree in your repo is slow and involved.

Pruning off a sub-project

Use subtree split to prise out one chunk. It has some wrinkles but is fast and easy.

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

Alternatively, to comprehensively rewrite history to exclude everything outside a subdir:

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

Download a sub-directory from a git tree

This works for github at least. I think anything running git-svn?

Heinous hack

replace tree/master => trunk
svn co the new url

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

Deleting all tags

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

Helpers

git-branchless

arxanas/git-branchless: Branchless workflow for Git

The branchless workflow is designed for use in a repository with a single main branch that all commits are rebased onto. It improves developer velocity by encouraging fast and frequent commits, and helps developers operate on these commits fearlessly.

In the branchless workflow, the commits you're working on are inferred based on your activity, so you no longer need branches to keep track of them. Nonetheless, branches are sometimes convenient, and git-branchless fully supports them. If you prefer, you can continue to use your normal workflow and benefit from features like git sl or git undo without going entirely branchless.

Also : git undo: We can do better

How is it so easy to “lose” your data in a system that’s supposed to never lose your data?

Well, it’s not that it’s too easy to lose your data — but rather, that it’s too difficult to recover it. For each operation you want to recover from, there’s a different “magic” incantation to undo it. All the data is still there in principle, but it’s not accessible to many in practice.

…To address this problem, I offer git undo

gerrit

Gerrit is a code review system for git.

`legit`

legit simplifies feature branch workflows.

`rerere`

Not repeating yourself during merges? git rerere automates this:

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

Importing some files across a branch

git checkout my_branch -- my_file/

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.

Editing history

Cleaning out all big files

Every time I find a good picture of an octopus on the internet I put in on my git blog pages

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

Deleting specific things

I think bfg also does this. There is also native support:

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

Making it 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

Detecting if there are changes to commit

Thomas Nyman:

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

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 the 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

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.

SSH credentials

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

Jupyter

For sanity in git+jupyter, see jupyter.

Decent GUIs

See Git GUIs.

Which repo am I in?

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

Data versioning

See data versioning.