Git flythrough

Version used in this document (at least)

[dzu@pollux u-boot]$ git --version
git version 1.5.3.1

Technical introduction

Underlying concepts

git is a very fast directory content manager
only one "data type" in the database with 4 sub-types blob, tree, commit, tag
every object is stored under its SHA1 hash (=content) -> Immediate integrity check possible
objects are never modified
blobs hold the actual data (without its name or mode!)
trees are mappings from (mode,name) tuples to objects (blobs or trees)
a commit refers to a tree and its parent commits
a tag refers to a commit optionally using a signature

Thus commits form a DAG (directed acyclic graph) forming the whole history.

A specific "version" (or state of the directory hierarchy) is thus fully identified by the SHA1 of a commit.

A full history can be signed by signing a single tag as there is no way to change history because all references are to content.

git repositories are fully symmetric, i.e. there is no master/slave but only a (light) "origin/clone" relation.

Programmatically git is a collection of small user space programs exposing all the "bolts and nuts" in the typical unix fashion (called "plumbing", e.g. git-cat-file).

[Sidenote: this allowed for very rapid prototyping in a script language and subsequently optimizing critical parts in C]

Git also includes "porcelain" commands (e.g. git-show") to work with the core plumbing programs. Other porcelains are possible of course.

Basic interaction with the object database

one always works on an "unfolded" tree object (git-checkout)
only efficiency this is cached in the index
Changs in the files can be checked very fast against the index
usually the index is fully transparent
make changes to the working tree and "import to the index" (git-add, git-remove)
finally commit the state to the database (git-commit)

Working examples

Cf. http://www.kernel.org/pub/software/scm/git/docs/

Check out master repository

Elegant ways top check out a repository:

Normal clone

[dzu@pollux dzu]$ git clone /home/git/u-boot
Initialized empty Git repository in /tmp/dzu/u-boot/.git/
remote: Generating pack...
remote: Done counting 44005 objects.
remote: Deltifying 44005 objects...
remote:  100% (44005/44005) done
Indexing 44005 objects...
remote: Total 44005 (delta 35236), reused 42526 (delta 33832)
 100% (44005/44005) done
Resolving 35236 deltas...
 100% (35236/35236) done

[dzu@pollux dzu]$ du -hs u-boot/
74M  u-boot/

Using alternates

[dzu@pollux dzu]$ rm -rf u-boot/
[dzu@pollux dzu]$ git clone -s /home/git/u-boot
Initialized empty Git repository in /tmp/dzu/u-boot/.git/

[dzu@pollux dzu]$ du -hs u-boot.
53M u-boot/
[dzu@pollux dzu]$

Using remotes in an U-Boot example

[dzu@pollux u-boot]$ git clone /home/git/u-boot
Initialized empty Git repository in /tmp/dzu/u-boot/u-boot/.git/
remote: Generating pack...
remote: Done counting 44005 objects.
remote: Deltifying 44005 objects...
remote:  100% (44005/44005) done
Indexing 44005 objects...
remote: Total 44005 (delta 35236), reused 42526 (delta 33832)
 100% (44005/44005) done
Resolving 35236 deltas...
 100% (35236/35236) done

[dzu@pollux u-boot]$ cd u-boot
[dzu@pollux u-boot]$ git-remote add -f 4xx git://www.denx.de/git/u-boot-ppc4xx
remote: Generating pack...
remote: Done counting 0 objects.
remote: Total 0 (delta 0), reused 0 (delta 0)
Unpacking 0 objects...
...
[dzu@pollux u-boot]$ git-remote add -f testing git://www.denx.de/git/u-boot-testing
...
[dzu@pollux u-boot]$

Powerful tools

Introducing yourself to git

git-config --global user.email dzu@denx.de
git-config --global user.name "Detlev Zundel"

Looking for whats there

git-branch [-a]
git-tag -l
git-log

"Seeking in time"

Only for viewing/compiling: git-checkout testing/master
To work, crate a (temp.) branch: git-checkout -b work U-Boot-1_2_0

Inspecting state

git-status
git-diff [--stat]

Resetting to "unfolded tree"

git-reset --hard HEAD

"Addressing" versions and insepcting "traces"

git-show testing/master:Makefile
git-show HEAD^

Comparing trees and files

git-diff --stat testing/master..origin/master
git-diff testing/master:Makefile origin/master:Makefile

Show history of files

git-log -p Makefile
git-blame Makefile
git-log -p U-Boot-1_2_0.. board/amcc/taishan

Commiting work

git-add
git-commit -s

git-commit -a -s

Generating patches

Custodian:

git-checkout -b work U-Boot_1_2_0
   [edit, change, commit]
git-format-patch -n `git-merge-base HEAD origin/master`
git-send-email [--thread] 00*

Maintainer:

[Collect patches in standard mailbox file mbox]
git-am mbox

Rebasing branches (and thus patches)

git-checkout work
git-rebase origin/master

Pushing and pulling

Update master branch (fetching + merging):

git-checkout master
git-pull

Sync up to all remotes also (no merging done):

git-fetch -a

Push changes through ssh:

git-push ssh://[user@]host.xz[:port]/path/to/repo.git/

Merging branches

git-checkout origin/master
git-merge testing/master

Removing branches and cleaning up

git-branch -d work
git-gc

Goodies

Find tags:

git-describe origin/master

Generate pull requests automatically

git-request-pull U-Boot-1_2_0 git://www.denx.de/git/u-boot-ppc4xx

git-bisect

Checkout "bad" version.

git-bisect start
git-bad
git-good ...

Now a position "in the middle" is checked out for testing. Tell the system by

git-bisect good or git-bisect bad

Git tells you when the culprit was isolated.

good - f5577aae4aa9f245c4c67308fe0f7b3cecf61c93
  (needs "fixup", e.g. touch examples/hello_world.{srec,bin})
bad - 8318fbf8cc30418b621ea9f39b84b4c1a08f003a

[dzu@pollux u-boot]$ git-bisect reset
[dzu@pollux u-boot]$ git-bisect start
[dzu@pollux u-boot]$ git-bisect good
f5577aae4aa9f245c4c67308fe0f7b3cecf61c93
[dzu@pollux u-boot]$ git-bisect bad
8318fbf8cc30418b621ea9f39b84b4c1a08f003a
Bisecting: 35 revisions left to test after this
[854bc8da75709f13dab4cfa6e9094c0cb49b5c5a] Add support for AMCC
Rainier PPX440GRx eval board
[dzu@pollux u-boot]$

... make ... bad 
... make ... bad
... make ... bad
... make ... good
... make ... good

[dzu@pollux u-boot]$ git-bisect bad
Bisecting: 0 revisions left to test after this
[d7c2a02dea550e8701d1151010a823df48ad5c98] Added simple_strtoul(),
getenv() and setenv() to the exported functions.
[dzu@pollux u-boot]$

Training