Don't be a gitThe essentials you should know

about git to use correctly

Otto KekäläinenVincit Teatime 11.11.2015

Tampere, Finland

@ottokekalainen● 15+ years in open source● technology & business & advocacy● development with git since 2010

Git / t/ɡɪ”A silly, incompetent, stupid, annoying or childish person.”

http://en.wiktionary.org/wiki/git

"I'm an egotistical bastard, so I name all my projects after myself.

First Linux, now Git”

Linus Torvalds, PC World. 2012-07-14

Linus needed a new source code revision manager for Linux, and none of the available options in 2005 where

good enough, so he wrote his own in.

Kernel 2.6.12 was the first release managed by Git and version 1.0 of Git was released in December 2005.

Git popularity according to OpenHub.net

Git popularity according to Google Trends

git

svn

...but adoption would be faster if it was not so difficult to use.

Originally Linus did not intend end users to use Git directly, instead he tried to delegate to somebody else the task of making the actual command line

interface. We are still waiting for it...

Luckily Git has been simplified and documentation has improved over time, but some Git commands

still refer to Git internals that are difficult to grasp.

E.g. git-push: Update remote refs along with associated objects.

Git might feel difficult at first, but once you learn it, you never want to go back to anything less

flexible and powerful.

First obstacle: same terms with different meanings

git config --global user.name "John Doe"git config --global user.email johndoe@example.com

Define the author of commits

git pull# edit filesgit commit -am “Implemented X”git push

git loggit show

https://help.github.com/articles/generating-ssh-keys/

Learn the basics: http://try.github.com/

How to write a good commit message● Your attitude towards commit messages should be the same as for code:

it is written once, but read thousands of times.● Don't explain how was done, that is visible in the diff anyway. Explain

what the intention was and why it was made.● Use imperative form “Fix typo” (instead of “Fixed typo”)● Keep subject line short and sweet, under 72 chars. Body can be verbose. ● Use proper English. Capital letters. Reference issue identifiers is

possible.● Looking for a good example? How about one by Linus himself?

https://github.com/torvalds/linux/commit/fc90888

Use git commit -a --amend to you screwed up the commit and want to overwrite the latest

with an improved commit.

Completely delete the last commit:git reset --hard HEAD^

Delete all untracked and modified files:git clean -fdx && git reset --hard

Use branches and tags

Basically just labels that point to certain commit IDs.Each commit ID point to its parent, thus forms a chain.

All developers branch from master and then merge back.

New releases branch from master.

Support releases branch from release branch.

Image source: http://tleyden.github.io/blog/2014/04/09/a-successful-git-branching-model-with-enterprise-support/

Git layout strategy 1/2

● Start working on the master branch only. Don't create other branches until you really need them.

● Every new developer has their own branch (local clone).● Never break master. Push only stuff what works. Other

developers must be able to branch from master at any time and start working on something new, instead of having to deal with fixing a broken state that somebody else left in master.

Git layout strategy 2/2

● If you have a need to push your work that is half-way done, that is a good signal that you should create a new branch.

● Typically tree kind of branches:– feature branches

– bugfix branches

– personal branches

● Project layout: split code up in many separate files.– Lower likelihood of conflicting changes when branches merge.

Distributed version control?Example scenarios at

https://git-scm.com/about/distributed

Compare changes and create diffs

git show 166a2f284875

# standard show in patch format

git diff v1.2.3..HEAD translations/ > new-translations.diff

# diff of all changes in directory translation/ since tag v.1.2.3

git diff branch1 branch2 # compare two branches

git diff branch1..branch2 # same as above

git diff branch1...branch2 # tree dots: changes on branch2 since it diverged

GUI tool: gitk● graphical tool to view git history (and also uncommitted changes)● shows all commits, tags, branch names etc in a nice way● with gitk you don’t need to checkout the branch/file to view it● includes search feature for both commit messages and commit contents● run “gitk example/file.h” to see the git log that affects a single file● “git gui blame” can be used to view graphically via the context menu that opens when

clicking on a commit with the secondary mouse button● view diff between any two diffs: click on first commit normally, and then click with

secondary button on the second commit and select from context menu to view the diff● run “gitk mariadb-10.1.4..mariadb-10.1.5” to view diff between two commits● you can also view diff between different branches like this

– gitk 10.1..HEAD vs gitk HEAD..10.1 vs gitk 10.1...HEAD

Other GUI tools● git citool – graphical tool to do selective commits easily

– you can click to select the files you want, individual hunks inside files, and even individual lines inside hunks

– you can also write commits inside the tool’s window

● git mergetool – tool for easy merging,– automatically launches the pre-configured 3-way merge tool for

all conflicts (e.g. meld)

Keep branch name visible in the bash prompt:https://github.com/ottok/tooling/blob/master/bashrc/bashrc.git

Git internals

Folder /.git contains everything,your / is just the working copy.

Folder /.git contains everything,your / is just the working copy.

When you commit a 2 MB file /example.png,your / will grow to 4 MB...

When you add a file,it goes to the staging area.

The file does not go into theactual history tree until the stage

is committed.

Git tracks content, not files!

Git history is immutable as each commit ID is the SHA-1 of the commit data and metadata (including the commit ID of parents). Changing any commit in the history will change the SHA-1 commit IDs of every following commit in the chain.

If you need to change something in the history, you have to rebase and make a new history.

Git commit IDs and rebaseOriginal git log --oneline1bf7024 MDEV-8991: bind-address appears twice in default my.cnfb2205c5 MDEV-9011: Redo log encryption does not workcf9e6b2 Fix test failures seen on buildbot.923827e MDEV-7949: Item_field::used_tables() takes 0.29% in OLTP RO239e0c5 MDEV-8551 compilation fails with 10.1.6

After git rebase -i HEAD^^^34350b9 MDEV-8991: bind-address appears twice in default my.cnff5f2dd9 MDEV-9011: Redo log encryption does not work531e1ac Fixed all bugs923827e MDEV-7949: Item_field::used_tables() takes 0.29% in OLTP RO239e0c5 MDEV-8551 compilation fails with 10.1.6

Example of how fast-forward works 1/2

● Branch “feature-branch-example” forked from master branch “10.1” and has 3 commits

Example of how fast-forward works 2/2

● Normal merge defaults to fast-forward in this case●

●

● Result of no fast-forward (git merge --no-ff)

Want to avoid “ugly” merge commits?

● git config pull.ff=only● git config alias.ff=merge --ff-only● Run git rebase master to rebase you work on the master branch

before pushing or making pull request– In MariaDB before submitting a pull request: git rebase 10.1– You changes will be based on current 10.1 head and easy to merge

● Run git merge on when importing changes from remote head only if you really want to merge

Cherry-pick as alternative to merge

git cherry-pick 166a2f28487530ead0cf813ce0252baa

The commit with given ID will be applied as a new commit on top of your current branch head (only commit ID updates).

Git bisect – find the commit that broke you app

git bisect bad # mark the checked out commit as bad

git bisect good mariadb-10.1.7 # mark the ref (=commit) good

git bisect run my-test.sh

Git automatically checks out every commit between “bad” and “good” and runs the test script.

Test script is supposed to return 0 for good commits and non-0 for bad commits.

Exceptions are exit codes 255 and 125 that have special meaning to git bisect.

When the run is completed git will tell you the exact commit between “good” and “bad” where the my-test.sh started to fail (exit code not 0).

Git hooks and CI

Git commit hook: stop bad code entering repo/.git/hooks$ cat pre-commit

#!/bin/bash

# Really fast check if syntax is at all parseable

for FILE in $(git diff --cached --name-only); do

if [[ "$FILE" =~ \.php$ ]]; then

php -l "$FILE" 1> /dev/null

if [[ $? -ne 0 ]]; then

echo -e "\e[1;33mAborting commit: PHP code contains syntax errors\e[0m" >&2

exit 1

fi

fi

done

Want to put a simple shared repository on any SSH capable server? Create a bare .git with no working files:

git init --bare

Want to have notifications when somebody commits?Put a shell script at .git/hooks/post-receive

Continuous integration

● Host your own Jenkins?● Travis-CI

Will Github (or Gitlab) profile pages replace developers CV's?

Something to think about

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