Git tutorial: reimplementing part of GIT in JavaScript
Please send remarks and suggestions to git-tutorial@suzanne.soy or simply fork this repository on GitHub.
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Credits and license
This article was written as part of my work for LIGO.
The main reference for this tutorial is the Pro Git book section on GIT internals.
This tutorial uses these libraries:
- CodeMirror 5.60.0, released under an
MIT license,
MIT License Copyright (C) 2017 by Marijn Haverbeke <marijnh@gmail.com> and others Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
- sha1.js, released under an
MIT license,
I offer these scripts for free use and adaptation to balance my debt to the open-source info-verse. You are welcome to re-use these scripts [under an MIT licence, without any warranty express or implied] provided solely that you retain my copyright notice and a link to this page. /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */ /* SHA-1 (FIPS 180-4) implementation in JavaScript (c) Chris Veness 2002-2019 */ /* MIT Licence */ /* www.movable-type.co.uk/scripts/sha1.html */ /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
- pako 2.0.3, released under an MIT license and the Zlib license, see the project page for details,
License ------- MIT - all files, except /lib/zlib folder ZLIB - /lib/zlib content (The MIT License) Copyright (C) 2014-2017 by Vitaly Puzrin and Andrei Tuputcyn Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. From zlib's README ============================================================================= Acknowledgments: The deflate format used by zlib was defined by Phil Katz. The deflate and zlib specifications were written by L. Peter Deutsch. Thanks to all the people who reported problems and suggested various improvements in zlib; they are too numerous to cite here. Copyright notice: (C) 1995-2013 Jean-loup Gailly and Mark Adler Copyright (c) <''year''> <''copyright holders''> This software is provided 'as-is', without any express or implied warranty. In no event will the authors be held liable for any damages arising from the use of this software. Permission is granted to anyone to use this software for any purpose, including commercial applications, and to alter it and redistribute it freely, subject to the following restrictions: 1. The origin of this software must not be misrepresented; you must not claim that you wrote the original software. If you use this software in a product, an acknowledgment in the product documentation would be appreciated but is not required. 2. Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software. 3. This notice may not be removed or altered from any source distribution. Jean-loup Gailly Mark Adler jloup@gzip.org madler@alumni.caltech.edu
- Viz.js (v1.8.2 which has a synchronous API), released under an MIT license.
Copyright (c) 2014-2018 Michael Daines Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
- FileSaver.js, released under an MIT license,
The MIT License Copyright © 2016 Eli Grey. Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
- Blob.js, released under an MIT license,
Copyright © 2014 Eli Grey. Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
- JSZip (v1.8.2 which has a synchronous API), dual-licensed under the MIT or GPLv3.
JSZip is dual licensed. You may use it under the MIT license *or* the GPLv3 license. The MIT License =============== Copyright (c) 2009-2014 Stuart Knightley, David Duponchel, Franz Buchinger, António Afonso Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. GPL version 3 ============= GNU GENERAL PUBLIC LICENSE Version 3, 29 June 2007 Copyright (C) 2007 Free Software Foundation, Inc. <http://fsf.org/> Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. Preamble The GNU General Public License is a free, copyleft license for software and other kinds of works. The licenses for most software and other practical works are designed to take away your freedom to share and change the works. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change all versions of a program--to make sure it remains free software for all its users. 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In order to encourage people to write their own implementation of a version control system, and improve upon the state of the art, the contents of this tutorial (the files index.html, git-tutorial.js, git-tutorial.css, deploy.sh and README, including the GIT implementation contained within) are dedicated to the Public Domain, using the Creative Commons CC0 1.0 Universal (CC0 1.0) Public Domain Dedication.
The intent is to enable everyone to freely reuse and share part or all of this material under any license, including the CC0, the MIT license, other open-source licenses or proprietary licenses, without any limitations. Crediting this original article is appreciated but not required.
This tutorial comes without any warranty, in particular there are a few incompatibilities (e.g. this implementation cannot read from repositories using pack files, and it is quite possible that some issues would cause it to produce repositories that are not 100% compatible with the official implementation of GIT), a few bugs (e.g. unicode and binary text might not be stored correctly), some security vulnerabilities (user input is not sanitized when displayed).
Introduction
GIT is based on a simple model, with a lot of shorthands for common use cases. This model is sometimes hard to guess just from the everyday commands. To illustrate how GIT works, we'll implement a stripped down clone of GIT in a few lines of JavaScript. * empty lines and single closing braces excluded, a few more in total.
The Operating System's filesystem
Model of the filesystem
The Operating System's filesystem will be simulated by a very
simple key-value store. In this very simple filesystem, directories
are entries mapped to null
and files are entries mapped
to strings. The path to the current directory is stored in a separate
variable.
Filesystem access functions (read
, write
, mkdir
, exists
, remove
, cd
)
The filesystem exposes functions to read an entire file, create or replace an entire file, create a directory, test the existence of a filesystem entry, and change the current directory.
Filesystem access functions (listdir
)
It will be handy for some operations to list the contents of a directory.
Example working tree
Our imaginary user will create a proj
directory,
and start filling in some files.
A working tree designates the directory (and the subdirectories and files within) in which
the user will normally view and edit the files. GIT has commands to save the state of the working tree
(git commit), in order to be able to go back in time later on, and view older versions of the files.
The command git worktree
allows the user to create multiple working trees using the same
local repository. This effectively allows the user to easily have two or more versions of the project
side-by-side. GIT commands can be invoked in either copy. It is worth noting that the .git/
directory exists only in the original working tree; while it is safe to remove other worktrees (followed by
an invocation of git worktree prune
from one of the remaining working tree to let GIT
detect the deletion), the removal of the original working tree will discard ths .git/
directory, and all versions of the project that have not been published elsewhere (usually via
git push
) will be lost.
git init
(creating .git
)
The first thing to do is to initialize the GIT directory.
For now, only the .git
folder is needed, The rest
of the function implementing git init
will be
written later.
Click on the eval button to see the files and directories that were created so far.
git hash-object
(storing a copy of a file in .git
)
The most basic element of a GIT repository is an object. Objects have a type which can be
blob
(individual files), tree
(directories),
commit
(pointers to a specific version of the root directory,
with a description and some metadata) and tag
(named pointers to a specific commit,
with a description and some metadata).
When a file is added to the git repostitory, a compressed copy is stored in GIT's database,
in the .git/objects/
folder. This copy is a blob object.
The compressed copy is given a unique filename, which is obtained by hashing the contents of the original file.
Some filesystems have poor performance when a single directory contains a large number of files, and some filesystems
have a limit on the number of files that a directory may contain. To circumvent these issues, the first two characters
of the hash are used as the name of an intermediate directory: if a file's hash is 0a1bd…
, its compressed
copy will be stored in .git/objects/0a/1bd…
This function creates a file that looks like this:
The objects stored in the GIT database are compressed with zlib (using the "deflate" compression method). The filesystem view shows the marker deflated: followed by the uncompressed data. Click on the (un)compressed data to toggle between this pretty-printed view and the raw compressed data.
When creating some blob
objects, the result could be, for example:
This function reproduces faithfully the behaviour of (a subset of the options of)
the git hash-object
command which can be called on a real git command-line.
Adding a file to the GIT database
So far, our GIT database does not know about any of the user's
files. In order to add the contents of the README
file in
the database, we use git hash-object -w -t blob README
,
where -w
tells GIT to write the object in its
database, and -t blob
indicates that we want to create
a blob object, i.e. the contents of a file.
Click on the eval button to see the file that was created by this call.
You can notice that the database does not contain the name of the original file, only its content, stored under a unique identifier which is derived by hashing that content. Let's add the second user file to the database.
zlib
compression
GIT compresses objects with zlib. The deflate()
function used in
the script above comes from the pako 2.0.3 library.
To view a zlib-compressed object in your *nix terminal, simply write this
declaration in your shell.
unzlib() { python -c \ "import sys,zlib; \ sys.stdout.buffer.write(zlib.decompress(open(sys.argv[1], 'rb').read()));" \ "$1" }
You can then inspect git objects as follows, using hexdump
to view the null bytes and other non-printable bytes.
unzlib .git/objects/95/d318ae78cee607a77c453ead4db344fc1221b7 | hexdump -Cv
Storing trees (list of hashed files and subtrees)
At this point GIT knows about the contents of both of the user's files, but it would be nice to also store the filenames. This is done by creating a tree object
A tree object can contain files (by associating the blob's hash to its name), or directories (by associating the hash of other subtrees to their name).
The mode (100644
for the file and 40000
for the folder) indicates the permissions, and is given in octal using the values used by *nix
In the contents of a tree, subdirectories (trees) are listed before files (blobs); within each group the entries are ordered alphabetically.
This function needs a small utility to convert hashes encoded in hexadecimal to raw bytes.
Example use of store_tree()
The following code, once uncommented, stores into the GIT database the trees for src
and for the root directory of the GIT project.
The store_tree()
function needs to be called for the contents of subdirectories
first, and that result can be used to store the trees of upper directories. In the next section,
we will write a function which takes a list of paths, constructs an internal representation of
the hierarchy, and stores the corresponding trees bottom-up.
Storing a tree from a list of paths
Making trees out of the subfolders one by one is cumbersome. The following utility function takes a list of paths, and builds a tree from those.
Storing a commit in the GIT database
Now that the GIT database contains the entire tree for the current version, a commit can be created. A commit contains
- the hash of the tree object,
- the hash of the previous commit, which is dubbed the
parent
(merge commits have two or more parents, and the initial commit has no parent commit), - information about the author (the person who initially wrote the code),
- information about the committer (the person who adds the code to the GIT database, often the same person as the author, but it can be a different person e.g. when someone else rewrites the history with a rebase or applies a patch recieved by e-mail),
- and a description.
The author and committer information contain
- the person's name,
- the person's email,
- the *nix timestamp at which the version was authored or committed,
- and the timezone for that timestamp.
Storing an example commit
It is now possible to store a commit in the database. This saves a copy of the tree along with some metadata about this version. The first commit has no parent, which is represented by passing the empty list.
resolving references
The next few subsections will introduce symbolic references
and other references like branch names, the special name HEAD
or tag names.
Most GIT commands accept as an argument a commit hash or a named reference to a hash. In order to implement those, we need to be able to resolve these references first.
Symbolic references are nothing more than regular files containing a hexadecimal
hash or a string of the form ref: path/to/other/symbolic/reference
.
The HEAD
reference is stored in .git/HEAD
, and can point
directly to a commit hash like
0123456789abcdef0123456789abcdef01234567,
or can point to another symbolic reference, in which case the .git/HEAD
file
will contain e.g. refs/heads/main
.
Branches are simple files stored in .git/refs/heads/name-of-the-branch
and usually contain a hash like
0123456789abcdef0123456789abcdef01234567.
Tags are identical to branches in terms of representation. It seems that the only difference
between tags and branches is the behaviour of git checkout
and similar commands.
These commands, as explained in the section about git checkout
below,
normally write ref: refs/heads/name-of-branch
in .git/HEAD
when
checking out a branch, but write the hash of the target commit when checking out a tag or
any other non-branch reference.
We'll start with a small utility to remove the newline at the end of a string. GIT references are usually files containing a hexadecimal hash, and following *NIX tradition these files finish with a newline byte. When reading these references, we need to get rid of the newline first.
git symbolic-ref
git symbolic-ref
is a low-level command which reads
(and in the official GIT implementation also writes and updates)
symbolic references given a path relative to .git/
.
For example, git symbolic-ref HEAD
will read the
contents of the file .git/HEAD
, and if that file starts
with ref:
, the rest of the line will be returned.
The official implementation of GIT follows references recursively
and returns the path/to/file
of the last file of the
form ref: path/to/file
. In the example below,
git symbolic-ref HEAD
would
- read the file
proj/.git/HEAD
which containsref: refs/heads/main
, - follow that indirection and read the file
proj/.git/refs/heads/main
which containsref: refs/heads/other
- follow that indirection and read the file
proj/.git/refs/heads/other
which contains a hash - return the last file path that contained a
ref:
, i.e. return the stringrefs/heads/other
git rev-parse
git rev-parse
is another low-level command. It takes a symbolic reference or other reference,
and returns the hash. The difference with git symbolic-ref
is that symbolic-ref
follows indirections
to other references, and returns the last named reference in the chain of indirections, whereas rev-parse
goes one step further and returns the hash pointed to by the last named reference.
git branch
A branch is a pointer to a commit, stored in a file in .git/refs/heads/name_of_the_branch
.
The branch can be overwritten with git branch -f
. Also, as will be explained later,
git commit
can update the pointer of a branch.
When we call git branch main HEAD
or equivalently
git branch main 0123456789012345678901234567890123456789
,
a file containing that hash is created in .git/refs/heads/main
. This file acts as a pointer
to the branch, and this pointer can be read e.g. by git rev-parse
.
After creating the branch, we show how the file .git/refs/heads/main
can be overwritten
using git branch -f
HEAD
The HEAD
indicates the "current" commit. It is set at first as part of the git init
routine.
Usually, the HEAD
is a symbolic reference to a branch, i.e. the
file .git/HEAD
contains ref: refs/heads/name-of-branch
.
When checking out a commit by specifying its hash directly, or when checking out
a non-branch reference, the file .git/HEAD
contains the hash of the
commit instead.
The state in which .git/HEAD
contains a commit hash is called
"detached HEAD", and often sounds alarming to people who have not encountered this
before. As we will see in the following sections, the only difference between detached
HEAD and the normal state is that git commit
updates the branch to point
to the new commit in the normal mode of operation. When the HEAD
is detached,
it does not point to a specific branch, and git commit
updates the HEAD
directly instead, overwriting it with the new commit hash.
Since the HEAD is supposed to be a transient pointer, it is easy to lose track of the hash of an important commit. For example, the following sequence of operations:
git checkout 0123456789abcdef0123456789abcdef01234567 touch new_file git add new_file git commit -m 'This is a commit adding a new file' git checkout branch-of-feature-foobar
roughly means:
HEAD = 0123456789abcdef0123456789abcdef01234567 // overwrite the contents of the working tree with // the contents of commit 0123456789abcdef0123456789abcdef01234567 checkout(0123456789abcdef0123456789abcdef01234567) // create commit with the new file: HEAD = commit(…) // Checkout other branch HEAD = git_rev_parse('branch-of-feature-foobar')
The hash of the new commit which is stored in HEAD on the second step is overwritten in the third step. In order to later retrieve that specific version with the precious new_file, one needs that hash. It would be possible to note down these hashes in a simple text file, but GIT offers a mechanism for that: branches. After all, branches are merely named text files containing the hash of the latest commit in that line of work.
The hash of a commit created with git commit
does not only exist in the
HEAD file (when in detached HEAD) or in the current branch file (normal mode). The official
implementation of GIT keeps a log of the changes being made to the various references.
.git/logs/HEAD
contains a log of the hashes pointed to by .git/HEAD
,
and .git/logs/refs/heads/main
contains a log of the hashes pointed to by
.git/refs/heads/main
, and the commands git reflog
and
git reflog main
pretty-print these files.
There are a few more ways to find a lost commit hash, including a careful invocation of
git fsck
which checks that the files stored in .git/
are not
corrupted, and that no reference (to another reference or a commit, tree or blob) points
to a non-existing file. The git fsck --unreachable
option tells this command
to print all object hashes which are not pointed to indirectly by any named reference
(so-called unreachable objects, which are well-formed but are not indirectly linked to
from a branch or other kind of named pointer).
The reflog can be used to recover a lost hash but handling hashes manually like this is somewhat error-prone, and most new users are not aware of those features; for this reason GIT commands tend to display a warning when switching to a detached HEAD state.
git config
The official implementation of GIT stores the settings in various files (.git/config
within a repository,
~/.gitconfig
in the user's home folder, and several other places).
These files use a .ini
syntax
with key = value
lines grouped under some [section]
headings. The configuration above could be
stored in ~/.gitconfig
or .git/config
using the following syntax:
[user] name = Ada Lovelace email = ada@analyti.cal
The $EDITOR
variable is a traditional *NIX environment variable, and could e.g. be declared with
EDITOR=nano
in ~/.profile
or ~/.bashrc
.
git commit
The git commit
command stores a commit (metadata and a pointer to a tree
containing the files given on the command-line), and updates the HEAD
or
current branch to point to the new commit.
If the HEAD
points to a commit hash, then git commit
updates the HEAD
to point to the new commit.
Otherwise, when the HEAD
points to a branch, then the target branch (represented by a file named .git/refs/heads/the_branch_name
) is updated.
The official implementation of git commit
makes use of the index.
When a file is scheduled for the next commit using git add path/to/file
, it is added to
the index. The index is a representation of a collection of copies of files, which can efficiently be
compared to the working tree. It uses a different representation, but its role is very similar
to that of a tree object along with the subtrees and blob objects of individual files. When
git commit
is called without specifying any files, it creates a commit containing the
version of the files stored in the index.
In this simplified implementation, we only support creating commits by specifying all the files that
must be present in the commit (including unchanged files). This contrasts with the official implementation
which would create a tree containing the files from the current HEAD, as well as the added, modified or
deleted files specified by git add
or specified directly on the git commit
command-line.
git tag
Tags behave like branches, but are stored in .git/refs/tags/the_tag_name
and a tag is not normally modified. Once created, it's supposed to always point
to the same version.
GIT does offer a git tag -f existing-tag new-hash
command,
but using it should be a rare occurrence.
Intuitively, tags differ from branches in the following way: when checking out a branch,
and a subsequent commit is made, the branch is updated to point to the new commit's hash.
As we've seen in the implementation of git commit
, the difference is actually
in the contents of the .git/HEAD
file. If it is a symbolic reference (generally
a pointer to a branch), then the target of that reference is updated every time a new commit
is created. If the .git/HEAD
file contains the hash of a commit, then the
.git/HEAD
file itself is updated every time a new commit is created.
Therefore, tags and branches differ only in their usage and in the path under which they are
stored (.git/refs/heads/name-of-the-branch
vs. .git/refs/tags/name-of-the-tag
).
The file .git/HEAD
is overwritten by git commit
and git checkout
.
It is the latter command which will behave differently for tags and branches; git checkout branch-name
turns the HEAD into a symbolic reference, whereas git checkout tag-name
resolves the tag name to
a commit hash, and writes that hash directly into .git/HEAD
.
git checkout
The git checkout commit-hash-or-reference
command modifies the HEAD to point to the given commit,
and modifies the working tree to match the contents of the tree object pointed to by that commit.
Checkout, branches and other references
The HEAD does not normally point to a tag. Although nothing actually
prevents writing ref: refs/tags/v1.0
into .git/HEAD
, the GIT
commands will not automatically do this. For example, git checkout tag-or-branch-or-hash
will put a symbolic ref:
in .git/HEAD
only if the argument is a branch.
Checking out files
In order to replace the contents of the working tree with those of the given commit, we recursively compare the subtrees, deleting from the working tree the files or directories that are not present in the tree object, and overwriting the others.
The official implementation of GIT will record the diff between the current working tree
and the current commit, and will re-apply these changes on top of the freshly checked-out commit.
The official git checkout
command will print warnings and refuse to proceed when
these changes cannot be re-applied without conflict, encouraging the user to create a commit
containing this updated version or to stash the changes (effectively creating a temporary commit
containing this version, pointed to by .git/refs/stash
). Our simple implementation
will always overwrite the changes.
Assert
The checkout_tree()
function needs to read the commit, tree and blob objects from the
.git/
folder. The following sections will introduce some parsers for these objects.
The parsers will check that their input looks reasonably well-formed, using assert()
.
Reading compressed objects
The GIT objects which are stored in .git/objects
are compressed with zlib
, and need to be
uncompressed before they can be parsed. The actual implementation of GIT also stores some objects in packs. Packs
contain a large number of objects, and used a form of delta compression, which effectively stores objects as the diff with
another similar object, in order to optimize the disk space usage.
Our simplified implementation only deals with zlib-compressed objects, and cannot read from pack files. The function below extracts the type and length, which form the header present in all objects, and returns those along with the contents of the object.
Parsing tree objects
We will start by parsing tree objects. As a reminder, a tree object has the following form:
After the object header, we have a mode, a filename, a null byte and a hash consisting of 20 bytes.
The null byte cannot appear in the mode or filename, so we use this null + hash as a delimiter
(the non-greedy match ensures the null byte terminator will not match with a 00
byte in the hash)
The parse_tree
function above needs a small utility to convert hashes represented using
raw bytes to a hexadecimal representation.
Parsing commit objects
The following function is fairly long, but only parses lines of the form header-name header-value
(with some restrictions depending on the header), followed by a blenk line, and a free-form description.
Example checkout
Now that we can parse blobs objects, trees, and commits, it is now possible to checkout a given commit. The following operation will revert the working tree to the state that was copied in the initial commit.
git init
The git init
command creates the .git
directory and points .git/HEAD
to the default branch (a file which does not exist yet, as this branch does not contain any commit at this point).
The index
When adding files with git add
, GIT does not immediately create a commit object.
Instead, it adds the files to the index, which uses a binary format with lots of metadata.
The mock filesystem used here lacks most of these pieces of information, so the value 0
will be used for most fields. See this blog post
for a more in-depth study of the index.
Playground
The implementation is now sufficiently complete to create a small repository.
By clicking on "Copy commands to recreate in *nix terminal.", it is possible to copy a series of mkdir …
and printf … > …
commands that, when executed, will recreate the virtual filesystem on a real system. The resulting
folder is bit-compatible with the official git log
, git status
, git checkout
etc.
commands.
Conclusion
This article shows that a large part of the core of GIT can be re-implemented in a few source lines of code (* empty lines and single closing braces excluded, a few more in total).
Click here to copy all the code.
- Some of the features which may appear mysterious at first sight (e.g. detached HEAD) should be clearer with the knowledge of how GIT works behind the scenes.
- Furthermore, branches are often associated with an intuition (containers into which commits are added) which does not match the implementation (mutable pointers to commits).
- Finally, it is tempting to think of commits as patches. While
darcs
tries to expose an interface which matches this intuition, it is clear that the implementation of GIT considers commits as copies of the entire repository, and are linked to the previous version solely by theparent
metadata in the commit headers.
A few core commands like git diff
and git apply
are not described in this tutorial.
They are little more than improved versions of the classical *nix commands diff
and patch
.
Most other commands provided by GIT are merely convenience wrappers around these commands. For example, git cherry-pick
is simply a combination of git diff
between the tree of a commit and the tree of its parent, followed by git apply
to apply the patch and git commit
to create a new commit whose diff is equivalent to the diff of the original commit. As an other example, the command git rebase
performs as succession of cherry-pick
operations.
By keeping in mind the internal model of GIT, it becomes easier to understand the usual commands and their quirks. By undersanding the design philosophy behind the implementation, the day-to-day usage can become, hopefully, less surprising.
Suggested exercises
The reader willing to improve their grasp of GIT's mental model, and reduce their reliance on a few learned recipies, might be interested in the following warm-up exercises:
Inspection using git cat-file
Inspect an existing repository, starting with cat .git/HEAD
and using git cat-file -p some-hash
to pretty-print an object given its hash.
This will help sink in the points explained in this tutorial, and give a better understanding of the internals of GIT. This knowledge is helpful for day-to-day tasks, as the GIT commands usually perform simple changes to this internal representation. Understanding the representation better can demistify the semantics of the daily GIT commands. Furthermore, equipped with a better understanding of GIT's implementation, the dreamy reader will be tempted to compare this lack of intrinsic complexity with the apparent complexity, and be entitled to expect a better, less arcane user interface for a tool with such a simple implementation.
Inspection of the files in .git/
Inspect a small existing repository, starting with cat .git/HEAD
and using the zlib
decompression
tool from the zlib
compression section. Larger repositories will make use
of GIT packs, which are compressed archives containing a number of objects. GIT packs only matter as an optimization of the
disk space used by large repositories, but other tools would be necessary to inspect those.
This should help understand the internal representation of GIT commits and branches, and should help having a instinctive idea of how the data store is modified by the various commands. This in turn could come in handy in case of apparent data loss (a lost stash or a checkout leaving an unreferenced commit on a detached HEAD), as this would help understand the work done by the various disaster-recovery one-liners that a quick panicked online search provides.
Creating a repository from scratch
Run git init new-directory
in a terminal, and create an initial single-file commit from scratch, using only
git hash-object
, printf
and overwriting .git/HEAD
and/or
.git/refs/heads/name-of-a-branch
. This will involve retracing the steps in this tutorial to create a blob
object for the file, a tree object to be the directory containing just that file, and a commit object.
This exercise should
help sink in the feeling that the internal representation of GIT commits is not very complex, and that many commands with
convoluted options have very simple semantics. For example, git reset --soft other-commit
is little more than
writing that other commit's hash in .git/refs/heads/name-of-the-current-branch
or .git/HEAD
.
Furthermore, equipped with an even better understanding of GIT's implementation, the dreamy reader will
be tempted to compare this lack of intrinsic complexity with the sheer complexity of the systems they are working with on
a day-to-day basis, and be entitled to expect better features in a versioning tool. After all, writing those
few lines of code to reimplement the core of a versioning tool shouldn't take more than a
couple of afternoons, surely our community can do better?
Using only basic GIT commands
For a couple of weeks, only use the GIT commands commit
, diff
, checkout
,
merge
, cherry-pick
, log
, clone
, fetch
and
push remote hash-of-commit:refs/heads/name-of-the-branch
. In particular, don't use rebase
which is just a wrapper around a sequence of cherry-pick
commands, don't use pull
which is
just a wrapper around fetch
and merge
, don't use git push
as-is and instead
explicitly give the name (origin) or URL of the remote, the hash of the commit to push, and the path that should be
updated on the remote (git push
while the main
branch is checked out locally is equivalent
to git push origin HEAD:refs/heads/main
, where HEAD
can be replaced by the actual hash of
the commit).
This should help sink in the feeling that the internals of GIT are very simple (most of these commands
are implemented in this tutorial, and the other ones are merely wrappers around enhanced versions of the *NIX commands
diff
, patch
and scp
), and that the rest of the GIT toolkit consists mostly of
convenience wrappers to help seasoned users perform common tasks more efficiently.
Understanding commits as copies of the root directory
Try not even using git cherry-pick
or git diff
a few times, instead make two copies the git
directoy, check out the two different commits in each copy, and use the traditional *NIX commands diff
and
patch
.
This should help sink in the feeling that commits are not diffs, but are actual (deduplicated)
copies of the entire project directory. GIT commits are quite similar to the age-old manual versioning technique of
copying the entire directory under a new name at each version, except that the metadata keeps track of which version
was the previous one (or which versions were merged together to obtain the new one), and the deduplication avoids
excessive space usage, as would be the case with cp --reflink
on a filesystem supporting Copy-On-Write (COW).
Branches as pointers: living without branches
For a couple of weeks, don't use any local branch, and stay in detached HEAD state all the time. When checking out a
colleague's work, use git fetch && git checkout origin/remote-branch
, and use the reflog and a text file
outside of the repository to keep track of the latest commit in a current "branch" instead of relying on GIT.
This should help sink in the feeling that branches are not containers in which commits pile up, but are merely pointers to the latest commit that are automatically updated.
Changelog and errata
- v1
- Initial version.
- v1.0.1
- Internal changes to provide IPFS links.
- v1.0.2
- Added a sitemap for download tools.