Fill section "Merge or rebase?"

[git-wiki.git] / pep-git.txt

PEP: XXX
Title: Collecting information about git
Version: $Revision$
Last-Modified: $Date$
Author: Oleg Broytman <phd@phdru.name>
Status: Active
Type: Informational
Content-Type: text/x-rst
Created: 01-Jun-2015
Post-History: 

Abstract
========

This Informational PEP collects information about git. There is, of
course, a lot of documentation for git, so the PEP concentrates on
more complex issues, scenarios and topics.

The plan is to extend the PEP in the future collecting information
about equivalence of Mercurial and git scenarios to help migrating
Python development from Mercurial to git.

The author of the PEP doesn't currently plan to write a Process PEP on
migration from Mercurial to git.


Documentation
=============

Git is accompanied with a lot of documentation, both online and
offline.

Documentation for starters
--------------------------

Git Tutorial: `part 1
<https://www.kernel.org/pub/software/scm/git/docs/gittutorial.html>`_,
`part 2
<https://www.kernel.org/pub/software/scm/git/docs/gittutorial-2.html>`_.

`Git User's manual
<https://www.kernel.org/pub/software/scm/git/docs/user-manual.html>`_.
`Everyday GIT With 20 Commands Or So
<https://www.kernel.org/pub/software/scm/git/docs/everyday.html>`_.
`Git workflows
<https://www.kernel.org/pub/software/scm/git/docs/gitworkflows.html>`_.

`Git Magic
<http://www-cs-students.stanford.edu/~blynn/gitmagic/index.html>`_,
also with a number of translations.

Advanced documentation
----------------------

`Pro Git <https://git-scm.com/book>`_. The Book about git. Buy it at
Amazon or download in PDF, mobi, or ePub form. Has translations to
many different languages. Download Russian translation from `GArik
<https://github.com/GArik/progit/wiki>`_.

`Git Wiki <https://git.wiki.kernel.org/index.php/Main_Page>`_.

Offline documentation
---------------------

Git has builtin help: run ``git help TOPIC``. For example, run
``git help git`` or ``git help help``.


Quick start
===========

Download and installation
-------------------------

Unix users: download and install using your package manager.

Microsoft Windows: download `git-for-windows
<https://github.com/git-for-windows/git/releases>`_ or `msysGit
<https://github.com/msysgit/msysgit/releases>`_.

MacOS X: use git installed with `XCode
<https://developer.apple.com/xcode/downloads/>`_ or download from
`MacPorts <https://www.macports.org/ports.php?by=name&substr=git>`_ or
`git-osx-installer
<http://sourceforge.net/projects/git-osx-installer/files/>`_ or
install git with `Homebrew <http://brew.sh/>`_: ``brew install git``.

`git-cola <https://git-cola.github.io/index.html>`_ is a sleek and
powerful Git GUI written in Python and GPL licensed. Linux, Windows,
MacOS X.

Initial configuration
---------------------

This simple code is often appears in documentation, but it is
important so let repeat it here. Git stores author and committer
names/emails in every commit, so configure your real name and
preferred email::

    $ git config --global user.name "User Name"
    $ git config --global user.email user.name@example.org


Examples in this PEP
====================

Examples of git commands in this PEP use the following approach. It is
supposed that you, the user, works with a local repository named
``python`` that has an upstream remote repo named ``origin``. Your
local repo has two branches ``v1`` and ``v2``. For most examples the
currently checked out branch is ``v2``. That is, it's assumed you have
done something like that::

    $ git clone -b v2 http://git.python.org/python.git
    $ cd python
    $ git branch v1 origin/v1

The last command creates a new local branch v1 and sets
remotes/origin/v1 as its upstream remote branch.

The same result can achieved with commands::

    $ git clone -b v1 http://git.python.org/python.git
    $ cd python
    $ git checkout --track origin/v2

The last command creates a new local branch v2, sets
remotes/origin/v2 as its upstream remote branch and checks it out into
the working directory.


Branches and branches
=====================

Git terminology can be a bit misleading. Take, for example, the term
"branch". In git it has two meanings. A branch is a directed line of
commits (possibly with merges). And a branch is a label or a pointer
assigned to a line of commits. It is important to differentiate when
you talk about commits and when about their labels. Lines of commits
are by itself unnamed and are usually only lengthening and merging.
Labels, on the other hand, can be created, moved, renamed and deleted
freely.


Remote repository and remote branches
=====================================

Another example of slightly misleading terminology. Remote
repositories are really remote, you access them via network (well, a
remote repository can be on your local disk, but it's still remote
because it's not the current repo).

Remote branches, on the other hand, are branches (pointers to commits)
in your local repository. They are there for you to remember what
branches and commits have been pulled from and pushed to what remote
repos (you can pull from and push to many remotes). Remote branches
live under ``remotes/REMOTE`` namespaces, e.g. ``remotes/origin/v2``.

To see the status of remote branches run::

    $ git branch -rv

To see local and remote branches (and tags) pointing to commits::

    $ git log --decorate

You never do your own development on remote branches. You create a
local branch that has a remote branch as upstream and do development
on that local branch. On push git updates remote branches, and on pull
git updates remote branches and fast-forwards, merges or rebases local
branches.

When you do an initial clone like this::

    $ git clone -b v1 http://git.python.org/python.git

git clones remote repository ``http://git.python.org/python.git`` to
directory ``python``, creates remote branches, creates a local branch
``v1``, configure it to track upstream remotes/origin/v1 branch and
checks out ``v1`` into the working directory.

Updating local and remote branches
----------------------------------

There is a major difference between

::

    $ git fetch REMOTE BRANCH

and

::

    $ git fetch REMOTE BRANCH:BRANCH

The first command fetches commits from the named BRANCH in the REMOTE
repository that are not in your repository and leaves the id (the
hash) of the head commit in file .git/FETCH_HEAD. But it doesn't
update any branch (doesn't move any pointer).

The second command fetches commits from the named BRANCH in the REMOTE
repository that are not in your repository and updates both the local
branch BRANCH and its upstream remote branch. But it refuses to update
branches in case of non-fast-forward. And it refuses to update the
current branch.

The first command is used internally by ``git pull``.

::

    $ git pull REMOTE BRANCH

is equivalent to

::

    $ git fetch REMOTE BRANCH
    $ git merge FETCH_HEAD # FETCH_HEAD is a literal here

Certainly, BRANCH in that case should be your current branch. If you
want to merge a different branch into your current branch first update
that non-current branch and then merge::

    $ git fetch origin v1:v1 # Update v1
    $ git pull --rebase origin v2 # Update the current branch v2 using
                                  # rebase instead of merge
    $ git merge v1

If you have not yet pushed commits on ``v1``, though, the scenario has
to become a bit more complex. Git refuses to update
non-fast-forwardable branch, and you don't want to do force-pull
because that would remove your non-pushed commits and you would need
to recover. So you want to rebase ``v1`` but you cannot rebase
non-current branch. Hence, checkout ``v1`` and rebase it before
merging::

    $ git checkout v1
    $ git pull --rebase origin v1
    $ git checkout v2
    $ git pull --rebase origin v2
    $ git merge v1

It is possible to configure git to make it fetch/pull a few branches
or all branches at once, so you can simply run

::

    $ git pull origin

or even

::

    $ git pull

Push
''''

Pushing is a bit simpler. There is only one command ``push``. When you
run

::

    $ git push origin v1 v2

git guesses (knowing upstream remote branches) that you really want

::

    $ git push origin v1:v1 v2:v2

Git pushes commits to the remote repo and updates remote branches. Git
refuses to push commits that aren't fast-forwardable. You can
force-push anyway, but please remember - you can force-push to your
own repositories but don't force-push to public or shared repos. If
you find git refuses to push commits that aren't fast-forwardable,
better fetch and merge commits from the remote repo (or rebase your
commits on top of the fetched commits), then push. Only force-push if
you know what you do and why you do it. See the section `Commit
editing and caveats`_ below.

It is possible to configure git to make it push a few branches or all
branches at once, so you can simply run

::

    $ git push origin

or even

::

    $ git push

Git refuses to push a branch if it's the current branch in the remote
non-bare repository: git refuses to update remote working directory.
You really should push only to bare repositories. For non-bare
repositories git prefers pull-based workflow.

When you want to deploy code on a remote host and can only use push
(because your workstation is behind a firewall and you cannot pull
from it) you do that in two steps using two repositories: you push
from the workstation to a bare repo on the remote host, ssh to the
remote host and pull from the bare repo to a non-bare deployment repo.

Tags
''''

Git automatically fetches tags that point to commits being fetched
during fetch/pull. To fetch all tags (and commits they point to) run
``git fetch --tags origin``. To fetch some specific tags fetch them
explicitly::

    $ git fetch origin tag TAG1 tag TAG2...

For example::

    $ git fetch origin tag 1.4.2 tag 2.1.7

Git doesn't automatically pushes tags. That allows you to have private
tags (lightweight tags are also private for a repo, they cannot be
pushed). To push tags list them explicitly::

    $ git push origin tag 1.4.2
    $ git push origin v1 v2 tag 2.1.7

Don't move tags with ``git tag -f`` after they have been published.


Commit editing and caveats
==========================

A warning not to edit published (pushed) commits also appears in
documentation but it's repeated here anyway as it's very important.

It is possible to recover from forced push but it's PITA for the
entire team. Please avoid it.

To see what commits have not been published yet compare the head of the
branch with its upstream remote branch::

    $ git log origin/v2..
    $ git log origin/v1..v1

For every branch that has an upstream remote branch git maintains an
alias @{upstream} (short version @{u}), so the commands above can be
given as::

    $ git log @{u}..
    $ git log v1@{u}..v1

To see the status of all branches::

    $ git branch -avv

To compare the status of local branches with a remote repo::

    $ git remote show origin

Read `how to recover from upstream rebase
<https://git-scm.com/docs/git-rebase#_recovering_from_upstream_rebase>`_.
It is in ``git help rebase``.

On the other hand don't be too afraid about commit editing. You can
safely edit, remove, reorder, combine and split commits that hasn't
been pushed yet. You can even push commits to your own (backup) repo,
edit them later and force-push edited commits to replace what has
already been pushed. Not a problem until commits are in a public
or shared repository.


Undo
====

TODO: describe undo strategies: git reset, git revert, git checkout,
git reflog. "Commit early, commit often".

How to undo a merge
https://kernel.org/pub/software/scm/git/docs/howto/revert-a-faulty-merge.html


Merge or rebase?
================

Internet is full of heated discussions on the topic: "merge or
rebase?" Most of them are meaningless. When a DVCS is being used in a
big team with a big and complex project with many branches there is
simply no way to avoid merges. So the question diminished to "whether
to use rebase, and if yes - when to use rebase?" Considering that it
is very much recommended not to rebase published commits the question
diminished even further: "whether to use rebase on non-pushed
commits?"

That small question is for the team to decide. The author of the PEP
recommends to use rebase when pulling, i.e. always do ``git pull
--rebase`` or even configure automatic setup of rebase for every new
branch::

    $ git config branch.autosetuprebase true

and configure rebase for existing branches::

    $ git config branch.NAME.rebase true

After that ``git pull origin v2`` will be equivalent to ``git pull
--rebase origin v2``.

In case when merge is preferred it is recommended to create new
commits in a separate feature or topic branch while using rebase to
update the mainline branch. When the topic branch is ready merge it
into mainline. To avoid a tedious task of resolving conflicts you can
merge the topic branch to the mainline from time to time and switch
back to the topic branch to continue working on it. The entire
workflow would be something like::

    $ git checkout -b issue-42 # create and switch to a new branch
        ...edit/test/commit...
    $ git checkout v2
    $ git pull --rebase origin v2 # update v2 from the upstream
    $ git merge issue-42
    $ git branch -d issue-42 # delete the topic branch
    $ git push origin v2

When the topic branch is deleted only the label is removed, commits
are stayed in the database, they are now merged into v2::

    --o--o--o--o--o--o-M-<v2 - it is the mainline branch
            \         /
             --*--*--*       - it is the topic branch, now unnamed

The topic branch is deleted to avoid cluttering branch namespace with
small topic branches. Information on what issue was fixed or what
feature was implemented should be in the commit messages.


Null-merges
===========

Git has a builtin strategy for what Python core developers call
"null-merge"::

    $ git merge -s ours v1 # null-merge v1 into v2


ReReRe
======

https://git-scm.com/book/en/Git-Tools-Rerere


Advanced topics
===============

Staging area
------------

Staging area aka index is a distinguishing feature of git. See
`WhatIsTheIndex
<https://git.wiki.kernel.org/index.php/WhatIsTheIndex>`_ and
`IndexCommandQuickref
<https://git.wiki.kernel.org/index.php/IndexCommandQuickref>`_ in Git
Wiki.


Advanced configuration
======================

Line endings
------------

Git has builtin mechanisms to handle line endings.

TODO: describe crlf configuration and .gitattributes.


Database maintenance
====================

TODO: dangling objects, git gc, git repack.


Tips and tricks
===============

TODO: sticky options; example: git grep -O.

TODO: bash/zsh completion, bash/zsh prompt.


git on server
=============

TODO: anonymous access; git over ssh; gitolite; gitweb; cgit; gitlab.


From Mercurial to git
=====================

Mercurial for Git users https://mercurial.selenic.com/wiki/GitConcepts

https://github.com/felipec/git-remote-hg

https://hg-git.github.io/


References
==========

.. [] 


Copyright
=========

This document has been placed in the public domain.


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