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Workflow

For information about how your workflow should go using git to keep track of changes to the main repository, see Git Workflow

Understanding Git

For developers transitioning from a Centralized Version Control System (CVCS) such as SVN to a Distributed Version Control System (DVCS) such as Git, the Pro Git book can be useful.  A free version of the book can be read on-line or downloaded.

Another good resource is Think Like A Git, a site who's main page states "Git shouldn't be so hard to learn."

Finally, Git for Computer Scientists is an in depth explanation of how git works.

Git Merge Strategies

Git has many "strategies" you can use while merging. Read the following carefully:

 Git Merge Strategies
MERGE STRATEGIES
----------------

The merge mechanism ('git-merge' and 'git-pull' commands) allows the
backend 'merge strategies' to be chosen with `-s` option.  Some strategies
can also take their own options, which can be passed by giving `-X<option>`
arguments to 'git-merge' and/or 'git-pull'.

resolve::
	This can only resolve two heads (i.e. the current branch
	and another branch you pulled from) using a 3-way merge
	algorithm.  It tries to carefully detect criss-cross
	merge ambiguities and is considered generally safe and
	fast.

recursive::
	This can only resolve two heads using a 3-way merge
	algorithm.  When there is more than one common
	ancestor that can be used for 3-way merge, it creates a
	merged tree of the common ancestors and uses that as
	the reference tree for the 3-way merge.  This has been
	reported to result in fewer merge conflicts without
	causing mis-merges by tests done on actual merge commits
	taken from Linux 2.6 kernel development history.
	Additionally this can detect and handle merges involving
	renames.  This is the default merge strategy when
	pulling or merging one branch.
+
The 'recursive' strategy can take the following options:

ours;;
	This option forces conflicting hunks to be auto-resolved cleanly by
	favoring 'our' version.  Changes from the other tree that do not
	conflict with our side are reflected to the merge result.
+
This should not be confused with the 'ours' merge strategy, which does not
even look at what the other tree contains at all.  It discards everything
the other tree did, declaring 'our' history contains all that happened in it.

theirs;;
	This is opposite of 'ours'.

patience;;
	With this option, 'merge-recursive' spends a little extra time
	to avoid mismerges that sometimes occur due to unimportant
	matching lines (e.g., braces from distinct functions).  Use
	this when the branches to be merged have diverged wildly.
	See also linkgit:git-diff[1] `--patience`.

ignore-space-change;;
ignore-all-space;;
ignore-space-at-eol;;
	Treats lines with the indicated type of whitespace change as
	unchanged for the sake of a three-way merge.  Whitespace
	changes mixed with other changes to a line are not ignored.
	See also linkgit:git-diff[1] `-b`, `-w`, and
	`--ignore-space-at-eol`.
+
* If 'their' version only introduces whitespace changes to a line,
  'our' version is used;
* If 'our' version introduces whitespace changes but 'their'
  version includes a substantial change, 'their' version is used;
* Otherwise, the merge proceeds in the usual way.

renormalize;;
	This runs a virtual check-out and check-in of all three stages
	of a file when resolving a three-way merge.  This option is
	meant to be used when merging branches with different clean
	filters or end-of-line normalization rules.  See "Merging
	branches with differing checkin/checkout attributes" in
	linkgit:gitattributes[5] for details.

no-renormalize;;
	Disables the `renormalize` option.  This overrides the
	`merge.renormalize` configuration variable.

rename-threshold=<n>;;
	Controls the similarity threshold used for rename detection.
	See also linkgit:git-diff[1] `-M`.

subtree[=<path>];;
	This option is a more advanced form of 'subtree' strategy, where
	the strategy makes a guess on how two trees must be shifted to
	match with each other when merging.  Instead, the specified path
	is prefixed (or stripped from the beginning) to make the shape of
	two trees to match.

octopus::
	This resolves cases with more than two heads, but refuses to do
	a complex merge that needs manual resolution.  It is
	primarily meant to be used for bundling topic branch
	heads together.  This is the default merge strategy when
	pulling or merging more than one branch.

ours::
	This resolves any number of heads, but the resulting tree of the
	merge is always that of the current branch head, effectively
	ignoring all changes from all other branches.  It is meant to
	be used to supersede old development history of side
	branches.  Note that this is different from the -Xours option to
	the 'recursive' merge strategy.

subtree::
	This is a modified recursive strategy. When merging trees A and
	B, if B corresponds to a subtree of A, B is first adjusted to
	match the tree structure of A, instead of reading the trees at
	the same level. This adjustment is also done to the common
	ancestor tree.

Git and Eclipse

The latest version of Eclipse (indigo) comes with a URL for installing EGit.  The EGit eclipse plugin provides eclipse with git support.  A good place to start is the User's guild: http://wiki.eclipse.org/EGit/User_Guide.

The following guild is also useful for setting up your Git/Eclipse workflow: http://wiki.eclipse.org/Platform-releng/Git_Workflows.

If you are having problems getting Eclipse + Maven + EGit to work together, here is a post with various workarounds: http://stackoverflow.com/questions/4869815/importing-a-maven-project-into-eclipse-from-git.

How to get the Synapse-Repository-Services (or any) project from Github into Eclipse

  1. Create a GitHub user account
    1. Internal Sage developers will be added as a developer on the project and will be able to push directly
    2. External contributors should fork the repository and submit GitHub Pull Requests for code inclusion
  2. Clone the repository: git clone https://github.com/Sage-Bionetworks/Synapse-Repository-Services
    1. This clones the 'Synapse-Repository-Services' project to your local machine.  Repeat for other projects, e.g. SynapseWebClient.
  3. Import the project as a maven project
    1. File -> Import -> Maven -> Existing Maven Projects
    2. Next, in Root Directory enter the local path to your repository clone
    3. Next, Finish 
  4. Enable Git Team tracking
    1. Now right click on portal (project name) -> Team -> Share Project
    2. Next, Select repository type Git
    3. Next, Check the box "Use or create repository in parent folder of project
    4. Next, Finish

Installing Git on Windows w/Cygwin

While Cygwin comes with a version of Git bundled, we had problems getting to work.  As a workaround we installed the latest version of Git for windows :http://git-scm.com/downloads, which fixed our issues.  The following instructions were used:

Download and run the Git for windows: Git http://git-scm.com/downloads and run the Git-<version>-preview<date>.exe

  1. choose a simple destination locations such as: C:\git-1.7.10
  2. "How would you like to run Git from the command line?"
    1. We choose this option since Cygwin already had an old version of Git installed.
  3. "Which Secure Shell client woud you like git to use?"
    1. We chose "Use OpenSSH"
  4. "How would you like git to treat line ending?"
    1. We selected "Checkout Windows-style, commit Unix-style line endings"

Once you have Git installed, you will need to setup the PATH

First add an environment variable:

 GIT_HOME=C:\git-1.7.10 (use the same path you set in the first step of the installer)

Next append this to your PATH

;%GIT_HOME%/bin

Start Cygiwn

Determine where the old version of git is currently running from:

$ which git
/usr/bin/git

Since this old version of Git was not working we simply renamed it:

mv /usr/bin/git /usr/bin/git-old

After restarting Cygwin the new version of git was being used:

$ which git
/cygdrive/c/git-1.7.10/bin/git
$ git --version
git version 1.7.10.msysgit.1

 

Tips and Tricks

This is a compilation of some potentially useful git commands and/or aliases

 

1) Command Line commit graph

This is a nifty little command-line for displaying all of a repository's history in a pretty way from a terminal

git log --graph --full-history --all --color --pretty=tformat:"%x1b[31m%h%x09%x1b[32m%d%x1b[0m%x20%s%x20%x1b[33m(%an)%x1b[0m"

you can also create an alias for this:

git config --global alias.gr 'log --graph --full-history --all --color 
    --pretty=tformat:"%x1b[31m%h%x09%x1b[32m%d%x1b[0m%x20%s%x20%x1b[33m(%an)%x1b[0m"'

Remove the '–global' if you don't want this alias to be set globally, and only have it be available in one repository.

Now all you have to type is 'git gr'

 

2) Ignoring local changes to a tracked repository file

For example, if you have changed the pom.xml of your maven project to depend on a SNAPSHOT of the main PLFM libraries, or some other local change that you shouldn't ever check in to the main repository.  Read more about it on this blog.

git update-index --assume-unchanged FILENAME

To make the changes to the file noticed by git again:

git update-index --no-assume-unchanged FILENAME

PITFALL - must be run against every new checkout of the repository.  I'm not sure if this means when you swap branches, but I suspect it may.

3) Ignoring whitespace changes when a rebase fails

Sometimes when you go to do a rebase that should be simple, git fails and tells you that the entire file has changed.  It may output something like this.

M	pom.xml
<stdin>:15: trailing whitespace.
				<groupId>log4j</groupId>
<stdin>:16: trailing whitespace.
				<artifactId>apache-log4j-extras</artifactId>
<stdin>:17: trailing whitespace.
				<version>1.1</version>
<stdin>:18: trailing whitespace.
			</dependency>
<stdin>:19: trailing whitespace.
warning: squelched 1 whitespace error
warning: 6 lines add whitespace errors.

This indicates that there have been whitespace changes.  Unless you're using python however, these probably aren't relevant and letting git sort them out will work fine.  To do this, pass the --ignore-whitespace option to rebase.

Alternatively, you could run this command to make it so whitespace is automatically ignored unless you want it to be relevant:

git config --global apply.ignorewhitespace change

This is documented under the git-apply man page.

4) Recovering from terrible things happening to your git repository

Check out this blog post: http://www.programblings.com/2008/06/07/the-illustrated-guide-to-recovering-lost-commits-with-git/

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