[git-wiki.git] / pep-103.txt

PEP: 103
Title: Collecting information about git
Version: $Revision$
Last-Modified: $Date$
Author: Oleg Broytman <phd@phdru.name>
Status: Withdrawn
Type: Informational
Content-Type: text/x-rst
Created: 01-Jun-2015
Post-History: 12-Sep-2015

Withdrawal
==========

This PEP was withdrawn as it's too generic and doesn't really deals
with Python development. It is no longer updated.

The content was moved to `Python Wiki`_. Make further updates in the
wiki.

.. _`Python Wiki`: https://wiki.python.org/moin/Git

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 (and more related to Python development) issues,
scenarios and examples.

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 Python development 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/giteveryday.html>`_.
`Git workflows
<https://www.kernel.org/pub/software/scm/git/docs/gitworkflows.html>`_.


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

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

`Pro Git <https://git-scm.com/book>`_. The Book about git. Buy it at
Amazon or download in PDF, mobi, or ePub form. It 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>`_.

`Git Buch <http://gitbu.ch/index.html>`_ (German).


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
<https://git-scm.com/download/linux>`_.

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

MacOS X: use git installed with `XCode
<https://developer.apple.com/xcode/>`_ 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>`_ (`repository
<https://github.com/git-cola/git-cola>`__) is a Git GUI written in
Python and GPL licensed. Linux, Windows, MacOS X.

`TortoiseGit <https://tortoisegit.org/>`_ is a Windows Shell Interface
to Git based on TortoiseSVN; open source.


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 ``master``. For most examples
the currently checked out branch is ``master``. That is, it's assumed
you have done something like that::

    $ git clone https://git.python.org/python.git
    $ cd python
    $ git branch v1 origin/v1

The first command clones remote repository into local directory
`python``, creates a new local branch master, sets
remotes/origin/master as its upstream remote-tracking branch and
checks it out into the working directory.

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

The same result can be achieved with commands::

    $ git clone -b v1 https://git.python.org/python.git
    $ cd python
    $ git checkout --track origin/master

The last command creates a new local branch master, sets
remotes/origin/master as its upstream remote-tracking 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 distinguish 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 repositories and remote branches
=======================================

Remote-tracking branches are branches (pointers to commits) in your
local repository. They are there for git (and 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-tracking branches live under ``remotes/$REMOTE`` namespaces,
e.g. ``remotes/origin/master``.

To see the status of remote-tracking branches run::

    $ git branch -rv

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

    $ git log --decorate

You never do your own development on remote-tracking branches. You
create a local branch that has a remote branch as upstream and do
development on that local branch. On push git pushes commits to the
remote repo and updates remote-tracking branches, on pull git fetches
commits from the remote repo, updates remote-tracking branches and
fast-forwards, merges or rebases local branches.

When you do an initial clone like this::

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

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

Some commands, like ``git status --branch`` and ``git branch --verbose``,
report the difference between local and remote branches.
Please remember they only do comparison with remote-tracking branches
in your local repository, and the state of those remote-tracking
branches can be outdated. To update remote-tracking branches you
either fetch and merge (or rebase) commits from the remote repository
or update remote-tracking branches without updating local branches.


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

To update remote-tracking branches without updating local branches run
``git remote update [$REMOTE...]``. For example::

    $ git remote update
    $ git remote update origin


Fetch and pull
''''''''''''''

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, updates
remote-tracking branch and leaves the id (the hash) of the head commit
in file .git/FETCH_HEAD.

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-tracking branch. But
it refuses to update branches in case of non-fast-forward. And it
refuses to update the current branch (currently checked out branch,
where HEAD is pointing to).

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

::

    $ git pull $REMOTE $BRANCH

is equivalent to

::

    $ git fetch $REMOTE $BRANCH
    $ git merge FETCH_HEAD

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 master  # Update the current branch master
                                       # 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 master
    $ git pull --rebase origin master
    $ 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

Default remote repository for fetching/pulling is ``origin``. Default
set of references to fetch is calculated using matching algorithm: git
fetches all branches having the same name on both ends.


Push
''''

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

::

    $ git push origin v1 master

git pushes local v1 to remote v1 and local master to remote master.
The same as::

    $ git push origin v1:v1 master:master

Git pushes commits to the remote repo and updates remote-tracking
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

Default remote repository for pushing is ``origin``. Default set of
references to push in git before 2.0 is calculated using matching
algorithm: git pushes all branches having the same name on both ends.
Default set of references to push in git 2.0+ is calculated using
simple algorithm: git pushes the current branch back to its
@{upstream}.

To configure git before 2.0 to the new behaviour run::

$ git config push.default simple

To configure git 2.0+ to the old behaviour run::

$ git config push.default matching

Git doesn't allow 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.

That changed in git 2.3, but see `the blog post
<https://github.com/blog/1957-git-2-3-has-been-released#push-to-deploy>`_
for caveats; in 2.4 the push-to-deploy feature was `further improved
<https://github.com/blog/1994-git-2-4-atomic-pushes-push-to-deploy-and-more#push-to-deploy-improvements>`_.


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
    $ git fetch origin v1:v1 tag 2.1.7

Git doesn't automatically pushes tags. That allows you to have private
tags. To push tags list them explicitly::

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

Or push all tags at once::

    $ git push --tags origin

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


Private information
'''''''''''''''''''

When cloning/fetching/pulling/pushing git copies only database objects
(commits, trees, files and tags) and symbolic references (branches and
lightweight tags). Everything else is private to the repository and
never cloned, updated or pushed. It's your config, your hooks, your
private exclude file.

If you want to distribute hooks, copy them to the working tree, add,
commit, push and instruct the team to update and install the hooks
manually.


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 a 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-tracking branch::

    $ git log origin/master..  # from origin/master to HEAD (of master)
    $ git log origin/v1..v1  # from origin/v1 to the head of v1

For every branch that has an upstream remote-tracking 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, reorder, remove, combine and split commits that haven'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 have
already been pushed. Not a problem until commits are in a public
or shared repository.


Undo
====

Whatever you do, don't panic. Almost anything in git can be undone.


git checkout: restore file's content
------------------------------------

``git checkout``, for example, can be used to restore the content of
file(s) to that one of a commit. Like this::

    git checkout HEAD~ README

The commands restores the contents of README file to the last but one
commit in the current branch. By default the commit ID is simply HEAD;
i.e. ``git checkout README`` restores README to the latest commit.

(Do not use ``git checkout`` to view a content of a file in a commit,
use ``git cat-file -p``; e.g. ``git cat-file -p HEAD~:path/to/README``).


git reset: remove (non-pushed) commits
--------------------------------------

``git reset`` moves the head of the current branch. The head can be
moved to point to any commit but it's often used to remove a commit or
a few (preferably, non-pushed ones) from the top of the branch - that
is, to move the branch backward in order to undo a few (non-pushed)
commits.

``git reset`` has three modes of operation - soft, hard and mixed.
Default is mixed. ProGit `explains
<https://git-scm.com/book/en/Git-Tools-Reset-Demystified>`_ the
difference very clearly. Bare repositories don't have indices or
working trees so in a bare repo only soft reset is possible.


Unstaging
'''''''''

Mixed mode reset with a path or paths can be used to unstage changes -
that is, to remove from index changes added with ``git add`` for
committing. See `The Book
<https://git-scm.com/book/en/Git-Basics-Undoing-Things>`_ for details
about unstaging and other undo tricks.


git reflog: reference log
-------------------------

Removing commits with ``git reset`` or moving the head of a branch
sounds dangerous and it is. But there is a way to undo: another
reset back to the original commit. Git doesn't remove commits
immediately; unreferenced commits (in git terminology they are called
"dangling commits") stay in the database for some time (default is two
weeks) so you can reset back to it or create a new branch pointing to
the original commit.

For every move of a branch's head - with ``git commit``, ``git
checkout``, ``git fetch``, ``git pull``, ``git rebase``, ``git reset``
and so on - git stores a reference log (reflog for short). For every
move git stores where the head was. Command ``git reflog`` can be used
to view (and manipulate) the log.

In addition to the moves of the head of every branch git stores the
moves of the HEAD - a symbolic reference that (usually) names the
current branch. HEAD is changed with ``git checkout $BRANCH``.

By default ``git reflog`` shows the moves of the HEAD, i.e. the
command is equivalent to ``git reflog HEAD``. To show the moves of the
head of a branch use the command ``git reflog $BRANCH``.

So to undo a ``git reset`` lookup the original commit in ``git
reflog``, verify it with ``git show`` or ``git log`` and run ``git
reset $COMMIT_ID``. Git stores the move of the branch's head in
reflog, so you can undo that undo later again.

In a more complex situation you'd want to move some commits along with
resetting the head of the branch. Cherry-pick them to the new branch.
For example, if you want to reset the branch ``master`` back to the
original commit but preserve two commits created in the current branch
do something like::

    $ git branch save-master  # create a new branch saving master
    $ git reflog  # find the original place of master
    $ git reset $COMMIT_ID
    $ git cherry-pick save-master~ save-master
    $ git branch -D save-master  # remove temporary branch


git revert: revert a commit
---------------------------

``git revert`` reverts a commit or commits, that is, it creates a new
commit or commits that revert(s) the effects of the given commits.
It's the only way to undo published commits (``git commit --amend``,
``git rebase`` and ``git reset`` change the branch in
non-fast-forwardable ways so they should only be used for non-pushed
commits.)

There is a problem with reverting a merge commit. ``git revert`` can
undo the code created by the merge commit but it cannot undo the fact
of merge. See the discussion `How to revert a faulty merge
<https://www.kernel.org/pub/software/scm/git/docs/howto/revert-a-faulty-merge.html>`_.


One thing that cannot be undone
-------------------------------

Whatever you undo, there is one thing that cannot be undone -
overwritten uncommitted changes. Uncommitted changes don't belong to
git so git cannot help preserving them.

Most of the time git warns you when you're going to execute a command
that overwrites uncommitted changes. Git doesn't allow you to switch
branches with ``git checkout``. It stops you when you're going to
rebase with non-clean working tree. It refuses to pull new commits
over non-committed files.

But there are commands that do exactly that - overwrite files in the
working tree. Commands like ``git checkout $PATHs`` or ``git reset
--hard`` silently overwrite files including your uncommitted changes.

With that in mind you can understand the stance "commit early, commit
often". Commit as often as possible. Commit on every save in your
editor or IDE. You can edit your commits before pushing - edit commit
messages, change commits, reorder, combine, split, remove. But save
your changes in git database, either commit changes or at least stash
them with ``git stash``.


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's 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's diminished even further: "whether to use rebase on
non-pushed commits?"

That small question is for the team to decide. To preserve the beauty
of linear history it's recommended to use rebase when pulling, i.e. do
``git pull --rebase`` or even configure automatic setup of rebase for
every new branch::

    $ git config branch.autosetuprebase always

and configure rebase for existing branches::

    $ git config branch.$NAME.rebase true

For example::

    $ git config branch.v1.rebase true
    $ git config branch.master.rebase true

After that ``git pull origin master`` becomes equivalent to ``git pull
--rebase origin master``.

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 large number of conflicts at once 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 a new issue branch and switch to it
        ...edit/test/commit...
    $ git checkout master
    $ git pull --rebase origin master  # update master from the upstream
    $ git merge issue-42
    $ git branch -d issue-42  # delete the topic branch
    $ git push origin master

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

    o--o--o--o--o--M--< master - the mainline branch
        \         /
         --*--*--*             - 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.

But even that small amount of rebasing could be too big in case of
long-lived merged branches. Imagine you're doing work in both ``v1``
and ``master`` branches, regularly merging ``v1`` into ``master``.
After some time you will have a lot of merge and non-merge commits in
``master``. Then you want to push your finished work to a shared
repository and find someone has pushed a few commits to ``v1``. Now
you have a choice of two equally bad alternatives: either you fetch
and rebase ``v1`` and then have to recreate all you work in ``master``
(reset ``master`` to the origin, merge ``v1`` and cherry-pick all
non-merge commits from the old master); or merge the new ``v1`` and
loose the beauty of linear history.


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

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

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


Branching models
================

Git doesn't assume any particular development model regarding
branching and merging. Some projects prefer to graduate patches from
the oldest branch to the newest, some prefer to cherry-pick commits
backwards, some use squashing (combining a number of commits into
one). Anything is possible.

There are a few examples to start with. `git help workflows
<https://www.kernel.org/pub/software/scm/git/docs/gitworkflows.html>`_
describes how the very git authors develop git.

ProGit book has a few chapters devoted to branch management in
different projects: `Git Branching - Branching Workflows
<https://git-scm.com/book/en/Git-Branching-Branching-Workflows>`_ and
`Distributed Git - Contributing to a Project
<https://git-scm.com/book/en/Distributed-Git-Contributing-to-a-Project>`_.

There is also a well-known article `A successful Git branching model
<http://nvie.com/posts/a-successful-git-branching-model/>`_ by Vincent
Driessen. It recommends a set of very detailed rules on creating and
managing mainline, topic and bugfix branches. To support the model the
author implemented `git flow <https://github.com/nvie/gitflow>`_
extension.


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

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

Git has builtin mechanisms to handle line endings between platforms
with different end-of-line styles. To allow git to do CRLF conversion
assign ``text`` attribute to files using `.gitattributes
<https://www.kernel.org/pub/software/scm/git/docs/gitattributes.html>`_.
For files that have to have specific line endings assign ``eol``
attribute. For binary files the attribute is, naturally, ``binary``.

For example::

    $ cat .gitattributes
    *.py text
    *.txt text
    *.png binary
    /readme.txt eol=CRLF

To check what attributes git uses for files use ``git check-attr``
command. For example::

$ git check-attr -a -- \*.py


Useful assets
-------------

`GitAlias <http://gitalias.com/>`_ (`repository
<https://github.com/GitAlias/gitalias>`_) is a big collection of
aliases. A careful selection of aliases for frequently used commands
could save you a lot of keystrokes!

`GitIgnore <https://www.gitignore.io/>`_ and
https://github.com/github/gitignore are collections of ``.gitignore``
files for all kinds of IDEs and programming languages. Python
included!

`pre-commit <http://pre-commit.com/>`_ (`repositories
<https://github.com/pre-commit>`_) is a framework for managing and
maintaining multi-language pre-commit hooks. The framework is written
in Python and has a lot of plugins for many programming languages.


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

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

Staging area aka index aka cache is a distinguishing feature of git.
Staging area is where git collects patches before committing them.
Separation between collecting patches and commit phases provides a
very useful feature of git: you can review collected patches before
commit and even edit them - remove some hunks, add new hunks and
review again.

To add files to the index use ``git add``. Collecting patches before
committing means you need to do that for every change, not only to add
new (untracked) files. To simplify committing in case you just want to
commit everything without reviewing run ``git commit --all`` (or just
``-a``) - the command adds every changed tracked file to the index and
then commit. To commit a file or files regardless of patches collected
in the index run ``git commit [--only|-o] -- $FILE...``.

To add hunks of patches to the index use ``git add --patch`` (or just
``-p``). To remove collected files from the index use ``git reset HEAD
-- $FILE...`` To add/inspect/remove collected hunks use ``git add
--interactive`` (``-i``).

To see the diff between the index and the last commit (i.e., collected
patches) use ``git diff --cached``. To see the diff between the
working tree and the index (i.e., uncollected patches) use just ``git
diff``. To see the diff between the working tree and the last commit
(i.e., both collected and uncollected patches) run ``git diff HEAD``.

See `WhatIsTheIndex
<https://git.wiki.kernel.org/index.php/WhatIsTheIndex>`_ and
`IndexCommandQuickref
<https://git.wiki.kernel.org/index.php/IndexCommandQuickref>`_ in Git
Wiki.


Root
----

Git switches to the root (top-level directory of the project where
``.git`` subdirectory exists) before running any command. Git
remembers though the directory that was current before the switch.
Some programs take into account the current directory. E.g., ``git
status`` shows file paths of changed and unknown files relative to the
current directory; ``git grep`` searches below the current directory;
``git apply`` applies only those hunks from the patch that touch files
below the current directory.

But most commands run from the root and ignore the current directory.
Imagine, for example, that you have two work trees, one for the branch
``v1`` and the other for ``master``. If you want to merge ``v1`` from
a subdirectory inside the second work tree you must write commands as
if you're in the top-level dir. Let take two work trees,
``project-v1`` and ``project``, for example::

    $ cd project/subdirectory
    $ git fetch ../project-v1 v1:v1
    $ git merge v1

Please note the path in ``git fetch ../project-v1 v1:v1`` is
``../project-v1`` and not ``../../project-v1`` despite the fact that
we run the commands from a subdirectory, not from the root.


ReReRe
------

Rerere is a mechanism that helps to resolve repeated merge conflicts.
The most frequent source of recurring merge conflicts are topic
branches that are merged into mainline and then the merge commits are
removed; that's often performed to test the topic branches and train
rerere; merge commits are removed to have clean linear history and
finish the topic branch with only one last merge commit.

Rerere works by remembering the states of tree before and after a
successful commit. That way rerere can automatically resolve conflicts
if they appear in the same files.

Rerere can be used manually with ``git rerere`` command but most often
it's used automatically. Enable rerere with these commands in a
working tree::

    $ git config rerere.enabled true
    $ git config rerere.autoupdate true

You don't need to turn rerere on globally - you don't want rerere in
bare repositories or single-branch repositories; you only need rerere
in repos where you often perform merges and resolve merge conflicts.

See `Rerere <https://git-scm.com/book/en/Git-Tools-Rerere>`_ in The
Book.


Database maintenance
--------------------

Git object database and other files/directories under ``.git`` require
periodic maintenance and cleanup. For example, commit editing left
unreferenced objects (dangling objects, in git terminology) and these
objects should be pruned to avoid collecting cruft in the DB. The
command ``git gc`` is used for maintenance. Git automatically runs
``git gc --auto`` as a part of some commands to do quick maintenance.
Users are recommended to run ``git gc --aggressive`` from time to
time; ``git help gc`` recommends to run it  every few hundred
changesets; for more intensive projects it should be something like
once a week and less frequently (biweekly or monthly) for lesser
active projects.

``git gc --aggressive`` not only removes dangling objects, it also
repacks object database into indexed and better optimized pack(s); it
also packs symbolic references (branches and tags). Another way to do
it is to run ``git repack``.

There is a well-known `message
<https://gcc.gnu.org/ml/gcc/2007-12/msg00165.html>`_ from Linus
Torvalds regarding "stupidity" of ``git gc --aggressive``. The message
can safely be ignored now. It is old and outdated, ``git gc
--aggressive`` became much better since that time.

For those who still prefer ``git repack`` over ``git gc --aggressive``
the recommended parameters are ``git repack -a -d -f --depth=20
--window=250``. See `this detailed experiment
<http://vcscompare.blogspot.ru/2008/06/git-repack-parameters.html>`_
for explanation of the effects of these parameters.

From time to time run ``git fsck [--strict]`` to verify integrity of
the database. ``git fsck`` may produce a list of dangling objects;
that's not an error, just a reminder to perform regular maintenance.


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

Command-line options and arguments
----------------------------------

`git help cli
<https://www.kernel.org/pub/software/scm/git/docs/gitcli.html>`_
recommends not to combine short options/flags. Most of the times
combining works: ``git commit -av`` works perfectly, but there are
situations when it doesn't. E.g., ``git log -p -5`` cannot be combined
as ``git log -p5``.

Some options have arguments, some even have default arguments. In that
case the argument for such option must be spelled in a sticky way:
``-Oarg``, never ``-O arg`` because for an option that has a default
argument the latter means "use default value for option ``-O`` and
pass ``arg`` further to the option parser". For example, ``git grep``
has an option ``-O`` that passes a list of names of the found files to
a program; default program for ``-O`` is a pager (usually ``less``),
but you can use your editor::

    $ git grep -Ovim  # but not -O vim

BTW, if git is instructed to use ``less`` as the pager (i.e., if pager
is not configured in git at all it uses ``less`` by default, or if it
gets ``less`` from GIT_PAGER or PAGER environment variables, or if it
was configured with ``git config [--global] core.pager less``, or
``less`` is used in the command ``git grep -Oless``) ``git grep``
passes ``+/$pattern`` option to ``less`` which is quite convenient.
Unfortunately, ``git grep`` doesn't pass the pattern if the pager is
not exactly ``less``, even if it's ``less`` with parameters (something
like ``git config [--global] core.pager less -FRSXgimq``); fortunately,
``git grep -Oless`` always passes the pattern.


bash/zsh completion
-------------------

It's a bit hard to type ``git rebase --interactive --preserve-merges
HEAD~5`` manually even for those who are happy to use command-line,
and this is where shell completion is of great help. Bash/zsh come
with programmable completion, often automatically installed and
enabled, so if you have bash/zsh and git installed, chances are you
are already done - just go and use it at the command-line.

If you don't have necessary bits installed, install and enable
bash_completion package. If you want to upgrade your git completion to
the latest and greatest download necessary file from `git contrib
<https://git.kernel.org/cgit/git/git.git/tree/contrib/completion>`_.

Git-for-windows comes with git-bash for which bash completion is
installed and enabled.


bash/zsh prompt
---------------

For command-line lovers shell prompt can carry a lot of useful
information. To include git information in the prompt use
`git-prompt.sh
<https://git.kernel.org/cgit/git/git.git/tree/contrib/completion/git-prompt.sh>`_.
Read the detailed instructions in the file.

Search the Net for "git prompt" to find other prompt variants.


SSH connection sharing
----------------------

SSH connection sharing is a feature of OpenSSH and perhaps derivatives
like PuTTY. SSH connection sharing is a way to decrease ssh client
startup time by establishing one connection and reusing it for all
subsequent clients connecting to the same server. SSH connection
sharing can be used to speedup a lot of short ssh sessions like scp,
sftp, rsync and of course git over ssh. If you regularly
fetch/pull/push from/to remote repositories accessible over ssh then
using ssh connection sharing is recommended.

To turn on ssh connection sharing add something like this to your
~/.ssh/config::

    Host *
    ControlMaster auto
    ControlPath ~/.ssh/mux-%r@%h:%p
    ControlPersist 600

See `OpenSSH wikibook
<https://en.wikibooks.org/wiki/OpenSSH/Cookbook/Multiplexing>`_ and
`search <https://www.google.com/search?q=ssh+connection+sharing>`_ for
more information.

SSH connection sharing can be used at GitHub, GitLab and SourceForge
repositories, but please be advised that BitBucket doesn't allow it
and forcibly closes master connection after a short inactivity period
so you will see errors like this from ssh: "Connection to bitbucket.org
closed by remote host."


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

The simplest way to publish a repository or a group of repositories is
``git daemon``. The daemon provides anonymous access, by default it is
read-only. The repositories are accessible by git protocol (git://
URLs). Write access can be enabled but the protocol lacks any
authentication means, so it should be enabled only within a trusted
LAN. See ``git help daemon`` for details.

Git over ssh provides authentication and repo-level authorisation as
repositories can be made user- or group-writeable (see parameter
``core.sharedRepository`` in ``git help config``). If that's too
permissive or too restrictive for some project's needs there is a
wrapper `gitolite <http://gitolite.com/gitolite/index.html>`_ that can
be configured to allow access with great granularity; gitolite is
written in Perl and has a lot of documentation.

Web interface to browse repositories can be created using `gitweb
<https://git.kernel.org/cgit/git/git.git/tree/gitweb>`_ or `cgit
<http://git.zx2c4.com/cgit/about/>`_. Both are CGI scripts (written in
Perl and C). In addition to web interface both provide read-only dumb
http access for git (http(s):// URLs). `Klaus
<https://pypi.python.org/pypi/klaus>`_ is a small and simple WSGI web
server that implements both web interface and git smart HTTP
transport; supports Python 2 and Python 3, performs syntax
highlighting.

There are also more advanced web-based development environments that
include ability to manage users, groups and projects; private,
group-accessible and public repositories; they often include issue
trackers, wiki pages, pull requests and other tools for development
and communication. Among these environments are `Kallithea
<https://kallithea-scm.org/>`_ and `pagure <https://pagure.io/>`_,
both are written in Python; pagure was written by Fedora developers
and is being used to develop some Fedora projects. `GitPrep
<http://gitprep.yukikimoto.com/>`_ is yet another GitHub clone,
written in Perl. `Gogs <https://gogs.io/>`_ is written in Go.
`GitBucket <https://gitbucket.github.io/gitbucket-news/about/>`_ is
written in Scala.

And last but not least, `GitLab <https://about.gitlab.com/>`_. It's
perhaps the most advanced web-based development environment for git.
Written in Ruby, community edition is free and open source (MIT
license).


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

There are many tools to convert Mercurial repositories to git. The
most famous are, probably, `hg-git <https://hg-git.github.io/>`_ and
`fast-export <http://repo.or.cz/w/fast-export.git>`_ (many years ago
it was known under the name ``hg2git``).

But a better tool, perhaps the best, is `git-remote-hg
<https://github.com/felipec/git-remote-hg>`_. It provides transparent
bidirectional (pull and push) access to Mercurial repositories from
git. Its author wrote a `comparison of alternatives
<https://github.com/felipec/git/wiki/Comparison-of-git-remote-hg-alternatives>`_
that seems to be mostly objective.

To use git-remote-hg, install or clone it, add to your PATH (or copy
script ``git-remote-hg`` to a directory that's already in PATH) and
prepend ``hg::`` to Mercurial URLs. For example::

    $ git clone https://github.com/felipec/git-remote-hg.git
    $ PATH=$PATH:"`pwd`"/git-remote-hg
    $ git clone hg::https://hg.python.org/peps/ PEPs

To work with the repository just use regular git commands including
``git fetch/pull/push``.

To start converting your Mercurial habits to git see the page
`Mercurial for Git users
<https://www.mercurial-scm.org/wiki/GitConcepts>`_ at Mercurial wiki.
At the second half of the page there is a table that lists
corresponding Mercurial and git commands. Should work perfectly in
both directions.

Python Developer's Guide also has a chapter `Mercurial for git
developers <https://docs.python.org/devguide/gitdevs.html>`_ that
documents a few differences between git and hg.


Git and GitHub
==============

`gitsome <https://github.com/donnemartin/gitsome>`_ - Git/GitHub
command line interface (CLI). Written in Python, work on MacOS, Unix,
Windows. Git/GitHub CLI with autocomplete, includes many GitHub
integrated commands that work with all shells, builtin xonsh with
Python REPL to run Python commands alongside shell commands, command
history, customizable highlighting, thoroughly documented.


Copyright
=========

This document has been placed in the public domain.


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