basic git intro

Basic Git Into

JivyGroup [email protected]

Why Git and not X?

• You can find many comparisons and discussions on line.. Will not talk about this here much

• Yes, Git is Hard since it’s Complex.

• It’s complex in order to be flexible and powerful

• Yes, SVN is simpler, until you need to do something complicated

Why Git and not X?

If simple was the only thing that matters we would still…

SVN ModelThe Server

User2Repo2

User2Repo1

CommitCheckoutUpdate

commit

trunk Branches

Branches/b1trunk

User1Repo1

trunk

b1

root

GIT ModelThe Server (just Another Repo)

User2 Repo

ClonePull

master branch1

origin/master

User1 Repo

origin/master

mybranchbranch123

branch123branch456

push

push

branch2

origin/branch2

ClonePull

PullUser2/branch456

pushWho is server and who is client?

Local Git

Single repository concepts

LOCAL

Basic Definitions

• Repository

– Collection of files and their history organized in folders, branches, tags

– Is stored in a normal file system somewhere

– Can be on a local machine or a remote machine

– To work with GIT you will need a repo somewhere

– When creating a new one… it’s empty

Example 1

Basic Definitions

• Workspace– Where you do work..

• Index– Snapshot of files in the workspace (can be some of

them.

– You add to the index files you change / remove etc.

• Commit– A snapshot of the workspace at some point

– Has unique revision number

– Knows the commit (commits) that it’s based on

workspace indexRepository

store

changes

staged add

commitCommitted and generated unique rev. number

changes

staged add

Simple Local GIT

Work

Add To Index

Commit

• Add / Remove• Change Files

• Add (same as “stage”) the changes• Not all files must be staged• Can stage changes to same file several times• Think “snapshot” of the work

• Changes from the index stored• Get a unique rev. number• Index emptied

Example 2

Branches

• Branch is a pointer to a commit

• A branch can point at other commits – it can move

• A branch is a way to organize working histories

• Since commits know which commits they are based on – branch represents a commit and what came before it.

Branches

• When doing a “commit” the current “branch” moves to point at the new commit

C3 C2 C1bx

On branch bx -> Commit of C4

C3 C2 C1bx C4

Example 3

Branches

• Here is a branch

• Creating a new branch just adds another “pointer” to the same commit

C3 C2 C1bx

C3 C2 C1

bx

by

Example 3

Branches

• Other branches are not affected by a commit

C3 C2 C1

bx

by

C3 C2 C1

bx

by

On branch bx -> Commit of C4

C4

Example 3

Branches

• Checking out a branch puts the snapshot (the commit) it points to into the workspace


store

Checkout branch yBranch y snapshot

Branch x snapshot

Now on branch y

Example 3

Branches

• master – is just another branch, but is the default one which gets created. (So you will always see it in a repo)

• Branching is cheap – feel free to branch a lot.

• Branch is a good way to work on multiple things in parallel

Merging

• Combining one ore more branches into the current branch

• Allows changes from other branches to be integrated into the current work history

• Usually generates a new commit which has more than one predecessor

• Other branches not affected

Merging

C3 C2 C1

bx

by

C4

Merging bx into by

C3 C2 C1

bx C4

by C5

C5

C6

Merging

• The branch “bx” was not affected

• Merge only changes the workspace. Still need to add (to index) and commit (“by”) to actually generate “c6”

C3 C2 C1

bx C4

by C5C6

Merging - Conflicts

• When merging, if there are conflicts - need to solve them.

• After solving, need to “add” the changes and commit the merged workspace

Example 3

C3 C2 C1

bx C4

by C5C6

Merging – fast forward

• Fast Forward – a type of merge which only requires moving the “branch” pointer

• Possible when the current branch is an ancestor of the merged branch

• No new “merge” commits need to be created


C3 C2 C1bx

by

C4

Merging bx into by

C3 C2 C1bx

by

C4

C4 is based on C3 so “by” which points to

C3 can move to point to C4

Example 4


C3 C2 C1

bx

by

C4

C4 is not based on C5, so cannot fast

forward.

C5

C3 C2 C1

bx

by

C4

C5

C6

Now its possible to ff by

In that case, merge…

Remote

More than one repository

REMOTE

Collaborating

• Clone – creates a local repository by copying another (remote) repository

• Repositories which were cloned can exchange changes between them

• Pull? For now forget about it. we will come back to it later

• Commits are always local. But can be sent to other repositories.

Fetch

• Fetch – gets changes from a remote repository that you don’t already have.

• Fetch gets the changes to the local repository but does not touch the index or workspace.

• After a fetch, usually a merge needs to take place (more on this later)

Remote Branches - fetch

• Remote branches represent the state of a branch on the repository it is fetched from

• It is like another branch when it comes to merging from

• You cannot checkout a remote branch, you branch it (create a local pointer to it)

• After fetching, a remote branch and it’s local branch counterpart may need to be merged

Remote Branches

My Machine The Server

C1

master

C0

cloneC1

origin/master

C0

master

This is a remote branch

origin is just a name for a “remote”

Example 5



Some changes on the server

C1

C0

master

C2C1

origin/master

C0

master

fetch



C1

C0

master

C2C1

origin/master

C0

masterC2

Example 5



C1

C0

master

C2C1

origin/master

C0

masterC2

Can merge!



C1

C0

master

C2

C1

origin/master

C0

master

C2Now merged (fast forward in this case)

Example 5

push

• Will take local objects (commits, tags) which are required to make a remote branch complete – and send them.

• Will merge those local changes into the remote branch

• Will only do a “fast-forward” merge (other merge type, if required, will fail the push)

Remote Branches - Push


C1

C0

master

C2

C1

origin/master

C0

master

C2

C3

This commit is not on the remote

master



C1

C0

master

C2

C1

origin/master

C0

master

C2

C3

push



C1

C0

master

C2

C1

origin/master

C0

master

C2

Now merged and in a fast forward manner

C3 C3

Example 5

push

• If cannot do a fast-forward – will fail. Then, afetch + merge is required to allow the push.

• Once remote changes merged locally again a fast forward is possible and the push would work.



C1

C0

master

C2

C1

origin/master

C0

master

C2

This commit is not on the remote

master

C3 C4

This commit is not on the local master

But local origin/master does

not know…



C1

C0

master

C2

C1

origin/master

C0

master

C2

C3 C4

push

No can do, fetch + merge first.



C1

C0

master

C2

C1

origin/master

C0

master

C2

C3 C4

fetch



C1

C0

master

C2

C1

origin/master

C0

master

C2

C3 C4

The commit from the remote arrived

C4

This is what the remote knows about

the master branch



C1

C0

master

C2

C1

origin/master

C0

master

C2

C3

C4C4

C5

Now merged to local master



C1

C0

master

C2

C1

origin/master

C0

master

C2

C3

C4C4

C5

push



C1

origin/master

C0

master

C2

C3

C4

C5

Now merged since fast forward is possible

C1

C0

master

C2

C3

C4

C5

The commits from the local arrived

Example 6

Remote Branches

• Reminder - Remote branches represent a branch on a remote repository

• The branch origin/master for example is a local pointer to the “master” on “origin”

• It reflects what the local repository currently knows about the state of “master” on “origin”

• You cannot change them, but you can “checkout” to get a “remote tracking branch”

Remote Tracking Branches

• A local branch which is configured to “track” a remote branch

• Will allow commands like “fetch” and “push” to know where to send/get changes

• Just a helper, you could specify all the locations in the command args

• Helpful also to remember where a local branch points to

Remote Tracking Branches

• When cloning a remote repo will check out the remote “master”

• “origin” is default name for the remote you cloned from

• So “origin/master” is the remote branch and “master” is configured to track it

• master is a remote tracking branch

Remote Branches Summary


srv1/branch1Cx

branch1

Cx

branch1

CyCy

remote: srv1 -> https://the.server/...

points at

Local state of

tracks

created when “fetch” from srv1

Created when “checkout” srv1/branch1

https://the.server/

pull

• Now it’s time to talk about pull since it’s just a shortcut command to do:

1. Fetch from a remote

2. Merge changes from “remote branch” into the “remote tracking branch”

• Sometimes the tool would also allow to “commit” merges for you right after the pull (not part of pull, but a helper)


store

clone

Remote Repository

checkout

add

push

commit

fetchmerge

Pull (fetch+merge)

Summary of operations

Rebase

• Instead of merging, replays set of changes on top of another branch

• Affects the “rebased” branch only

• Changes the history of commits

• Can be dangerous

• Very useful to remove history clutter

• Simple rule, use locally only and for branches which you will never share

Rebase

C2

bx

by

C4

C3

C2

bx

by

C4

C3C5

Merge bx into by

C2

bx

by

C4

C3C3’

Rebase by on bx

Note: In both cases can now fast-forward bx

Committing will have c4 as the only ancestor of c3’

Committing will have c4 and c3 as two ancestors of c5

Rebase – when to use

• You have a local branch – has some changes committed to it

• Fetched a remote branch

• Instead of merging into remote branch –rebase on it

• Now merge (fast forward) into the remote branch

• Push happilly

Example 7

We did not discuss

• The “Pull Request”

• Suggested work flow for small teams

• GitHub and BitBucket

Go learn more…

Summary

• Git is complex, but flexible and powerful

• Git supports distributed teams very well and also is the standard de-facto for OSS projects

• Due to it’s flexibility, every team needs to decide on the workflow which works best for it

How to know more

• Read the Git Pro Book

• No really, its very good, explains most of the stuff well

• Look for blog posts with titles like “how I got started with git”

http://git-scm.com/book/en/v2

basic git intro

Software

new branch

commita branch

branch yexample

movea branch

new commitc3c2c1bxon

predecessorother branches

localbranchesother branches

index files