5.1.8. An Introduction to git

git basics

• good introductions:
  • Linus Torvalds on git
  • Randall Schwartz on git

• life before git (tarballs+patches, bitkeeper, many discussions about licence, reverse engineering of bk, seperate ways...")
  • then use what? (requirements: distributed, robust, fast) -> none were qualified
  • Linus: "I can write something better in two weeks". And he did!

• git as as an prime example of software engineering under Unix

• git objects are identified by SHA1 sums!
  • objects:
    • blobs (file contents)
    • trees (blobs or other trees)
    • commits (tree, parent commits, message)
    • tags (object (usally commit), name (2.6.26), message, signature)
  • one SHA1 commit describes complete history (uniquely, cryptographically strong)

• Denx git trees and gitweb

• Stacked git

• gitk, tig

• 3 stages: working dir -> index (stage area) git-object db

Configuration

introduce yourself:

$ git config --global user.name "Your Name Comes Here"
$ git config --global user.email you@yourdomain.example.com

Create a new repository

$ git-init-db
$ git-add .
$ git-commit

Working with existing repositories

Best practice: work on a branch and leave master branch unmodified (track upstream)

clone a repository:

$ git-clone git://www.denx.de/git/linux-denx.git

look at the history

$ git-log     # simple
$ tig         # tui

add a branch for hacking

$ git-checkout -b hacking HEAD

edit some files and look at changes in working dir

$ git diff

add modified files to the index:

$ git add file1 file2

delete some and mark them as deleted:

$ git rm file3

look at diffs of the modified files added to the index

$ git diff --cached

get some general statistics of which files were modified:

$ git status

to throw away the changes git reset HEAD, to reset the working directory git-checkout -f

commit the changes added to the index (will be prompted for commit message)

$ git commit    # -s adds Signed-off-by: ... line

alternatively without using the index as an intermediate stage:

$ git commit -a

unhappy with commit message?

$ git commit --amend

local branches

what branches have we got?

git branch

create a branch as a clone of the current branch:

$ git branch hacking

check out the contents of this branch:

git checkout hacking

hack around, make a mess of it and decide to throw it away

git checkout -f
git checkout master
git branch -d hacking

git for developers

• git mostly aims at making the maintainers life easier
• conflict: maintainers expect logical separation of features but developers need to
  • commit often (traceable development)
  • rework code they have already committed
• some tools, e.g. stacked git make things easier for developers
• git's solution: rebasing nice description here
  • idea is to commit often during development and later "squash" commits into logical ones

Example: rework last 5 commits

$ git-rebase --interactive HEAD~5

move around lines and change pick into squash in order to merge commit into previous one.

$ git rebase --abort

goodies

• whos fault is it: git-blame
• which commit introduced the bug: git-bisect
• maintain a clean repository: git-gc
• check repository for consistence: git-fsck
• remove all those untracked files: git-clean (=-df)

• tree too big?
  • use a shared repository (objects on demand) git-clone -s
  • git-clone --reference : reference a local repository
  • make a shallow copy: git-clone --depth