# Git - definitions
# what is Git
- a distributed version control system
- a directory content management system
- a tree history storage system
- a stupid content tracker
# concepts
Comprendre Git - ancien blog de Xavier Pigeon - 2012
Article intéressant avec une approche travaillée sur l'implémentation interne :
Git est un :
- gestionnaire de contenu,
- stocké sous la forme d'un graphe acyclique d'objets,
- accessible par des références.
Git est structuré en 2 partie :
- le frontend, qui regroupe les commandes dites porcelain (add, commit, checkout, etc).
- le backend, développé par Linus Torvalds, qui réunit les commandes dites plumbing (hash-object, cat-file).
On retrouve cette distinction dans la littérature informatique et dans les formations : souvent, les formations pour débutants présentent le frontend, et le backend ne sera traité que par les formations avancées. Les dissocier de cette manière est une erreur, car en maîtrisant le backend, on maîtrisera bien mieux le frondend.
Concept #1 : Git est un gestionnaire de contenu. Concept #2 : La relation entre commits est un graphe acyclique (ou DAG). Concept #3 : Le contenu est accessible sous forme de références.
# repository
Named also a tree by some Git users.
It is the whole history of a project (a directory).
Composed by commits. Each node is a commit.
Created by git init command (or by git clone if you want to copy an existing repository).
Create a self-sufficient .git directory. It means that a simple zip on the repository root directory is enough to backup it.
# bare repository
A repository created with git init command is for working.
A repository created with git init --bare command is for sharing.
A bare repository doesn't contain a working directory nor a .git directory.
GitHub, GitLab, etc ..., create a bare repository on their side.
What is a bare Git repository : www.saintsjd.com
Notice that you can add to your remote list a non-bare repository but you will not be able to push in it if the branch where
you want to push is checked out in it, Git will deny the push (cause possible inconsistency).
It can be bypassed by setting the receive.denyCurrentBranch config variable in the receiver repo to 'ignore' or 'warn' but it is not recommended.
# working directory
difference between working copy and working directory - groups.google.com - git-users
Literally it is the directory where the user is working 😉
A git status on a working directory without any changes tracked by Git output this :
$ git status
On branch master
Your branch is up-to-date with 'origin/master'.
nothing to commit, working directory clean
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# remote
A remote is a distant repository (a repo on another computer).
# commit
Also named revision. It is a state of a directory. Each commit has one ore more parents commit. Git stores only the difference between current commit state and parents. So a commit = current project state - parents.
Comment faire un commit sans faire du Git (Alexandre Garnier) : Devoxx2016
# branch
A "branch" is an active line of development. The most recent commit on a branch is referred to as the tip of that branch. The tip of the branch is referenced by a branch head, which moves forward as additional development is done on the branch.
# tag
A ref pointing to a tag or commit object. In contrast to a head, a tag is not changed by a commit.
# merge
git merge is the action of merging a source branch in a destination branch.
If the destination branch have not diverged from the root of the source branch, the merge is a fast-forward.
-- M1
\
B1 - B2 - B3
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At the end, the two branches will points on the same commit.
-- M1 - B1 - B2 - B3
If the destination branch have diverged from the root of the source branch,
-- M1 - M2
\
B1 - B2 - B3
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Git will create a merge commit containing all the modifications.
-- M1 - M2 - - - M3
\ /
B1 - B2 - B3
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# rebase
git rebase allows user to rewrite the history.
-- M1 - M2
\
B1 - B2 - B3
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At this point, instead of merging, we can rewrite our branch history, the goal is to set the B1 parents to M2 instead of M1 :
-- M1 - M2
\
B1 - B2 - B3
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Now we can run a fast-forward merge to keep a clean linear history :
-- M1 - M2 - B1 - B2 - B3
# fetch
git fetch is the action to update a given remote repository data locally. (you have a local occurrence of all the remote you have added to your remote list).
# pull
git pull <remote-name> <branch-name> runs a git fetch on the given remote then a git merge on the local branch from the remote branch.
# push
Obviously, write access is mandatory on the remote
git push <remote-name> <branch-name> write the local branch commits on the remote branch.
The history must be clean enough to allow a fast forward or Git will reject the push.
# range operators
Used mostly with git log and git diff commands. Provide a tool to compare branches and tags.
Difference depends on the command context. Log work with commits where diff work with commit content.
# .. (dot dot)
$ git log A..B
# output all of the commits that B has that A doesn't have
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$ git diff A..B
# exactly the same as
$ git diff A B
# output all the difference between A and B, if A got diff that B doesn't have and B has diff that A doesn't have, it will output BOTH.
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# ... (dot dot dot)
$ git log A...B
# filter out all of the commits that both A and B share, thus only showing the commits that they don't both share
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$ git diff A...B
# output only the commits in B that A doesn't have (compare from the root of the 2 branches)
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# ancestry references
Used in a lot of commands stuck with HEAD ref to points to ancestors commits.
What's the difference between HEAD^ and HEAD~ in Git? : stackoverflow.com
~specifies ancestors^specifies parents
Ancestors are related to the current branch.
A commit can have more then one parent, caret is used to select the parent.
Let's say we have this tree :
-- A1 - A2 - - - A3
\ /
B1 - B2 - B3
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where HEAD is A3.
# ^ (caret)
HEAD^ is a shortand for HEAD^1
HEAD^=A2
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HEAD^^ = HEAD^1^1 = A1
HEAD^2 = B3
HEAD^2^1 = B2
HEAD^2^2 = nothing as B3 has only one parent (B2)
# ~ (tilde)
HEAD~ is a shortand for HEAD~1
HEAD~2 = A1
HEAD^2~2 = B1