Don't be a git

This is a git repository that is to be used to along with my presentation "Don't be a git" to learn a few things about git.

Topics

The following will be covered.

• git add/reset -p
• git rebase -i
• git bisect
• git rerere

-p flag (or the power flag)

The -p flag can be used on a lot of git commands but I recommend using it on git add and git reset. From the git docs -p or --patchdoes the following:

Interactively choose hunks of patch between the index and the work tree and add them to the index.
This gives the user a chance to review the difference before adding modified contents to the index.

For our example I would recommend checking out the master branch. Making a could of changes to the file meet-magento-romania.txt on lines 30 and 83 so that git will see them as two different patches. Then after that run git add -p.

When running this command you will have the following options for each patch

y - stage this hunk
n - do not stage this hunk
q - quit; do not stage this hunk nor any of the remaining ones
a - stage this hunk and all later hunks in the file
d - do not stage this hunk nor any of the later hunks in the file
g - select a hunk to go to
/ - search for a hunk matching the given regex
j - leave this hunk undecided, see next undecided hunk
J - leave this hunk undecided, see next hunk
k - leave this hunk undecided, see previous undecided hunk
K - leave this hunk undecided, see previous hunk
s - split the current hunk into smaller hunks
e - manually edit the current hunk
? - print help

You can also try this with git reset -p which should do the reverse of the add and unstage the changes patch at a time.

Keeping your history

For working with the rebase please checkout the branch diverged-branch. This branch should have a different history than the current master. To see the difference in history run git log --oneline --decorate --all --graph

So to update the diverged branch to be onto of the current master we should call git rebase -i origin/master. This will prompt you with the commits that will be moved onto the master head. For our example we should simply save and exit this prompt. There will be a conflict here and if you run git status you should see:

rebase in progress; onto 4d701d4
You are currently rebasing branch 'diverged-branch' on '4d701d4'.
  (fix conflicts and then run "git rebase --continue")
  (use "git rebase --skip" to skip this patch)
  (use "git rebase --abort" to check out the original branch)

Unmerged paths:
  (use "git reset HEAD <file>..." to unstage)
  (use "git add <file>..." to mark resolution)

        both modified:      meet-magento-romania.txt

Use your faviourate text editor to fix the conflicts in meet-magento-romania.txt run git add meet-magento-romania.txt and then git rebase --continue. It will ask you for a commit message and then will finish the rebase.

After the rebase is finished running git log --oneline --decorate --all --graph will show you that the branch diverged-branch is now 1 commit ahead of master.

Find the bug with bisect

So if we checkout the branch bisect-branch and open the file meet-magento-romania.txt we will see the following line:

* Anna Völkl (Looking for a security talk come see mine! - Love Anna),

To find the commit that added this change we can run git bisect. For this we will use the initial bad commit of c199955 and good commit of abf1494.

git bisect start
git bisect bad c199955
git bisect good abf1494

This will start the bisect process and move the head pointer to the first testing position. To check if this is good or bad simple open the meet-magento-romania.txt file and look for the change. In this case the change is still there so we call git bisect bad It will then move the head pointer again and we can check the file for the change. Again it is there so we call git bisect bad. Once more it moves the head pointer and we can check the file. This time the change is not there so we call git bisect good

We will then get a message about the found "bad" commit.

b77c7a8b466c1711cbd66e9d0b3093adc325d526 is the first bad commit
commit b77c7a8b466c1711cbd66e9d0b3093adc325d526
Author: Anna Völkl <miss-magento-security@annavoelkl.at>
Date:   Tue Oct 25 08:34:59 2016 +0000

    This is not a security issue nothing to see here

:100644 100644 5d320792604905fb0eccbfb8e81ecdc8534920a2 df928694bb4a8b2ea1ba7043884c93953105ed83 M      meet-magento-romania.txt

To finish the bisect run git bisect reset.

Reuse recorded resolution

To demo the recorded resolutions we will firstly checkout the correct branch rerere-branch. And then enable rerere with the command:

• git config --global rerere.enabled true

Now we will merge in the master branch with the command git merge master. You should see the following message if rerere was enabled correctly.

Auto-merging meet-magento-romania.txt
CONFLICT (content): Merge conflict in meet-magento-romania.txt
Recorded preimage for 'meet-magento-romania.txt'
Automatic merge failed; fix conflicts and then commit the resul

So we simply fix the conflict and commit

• vim meet-magento-romania.txt
• git add meet-magento-romania.txt
• git commit

This will have saved the resolved resolution for use again later.

Now we can run this on a second branch rerere-branch2. Again call git merge master but this time we should see the following:

Auto-merging meet-magento-romania.txt
CONFLICT (content): Merge conflict in meet-magento-romania.txt
Resolved 'meet-magento-romania.txt' using previous resolution.
Automatic merge failed; fix conflicts and then commit the result.

And on opening the file meet-magento-romania.txt you should see your resolution has been used. Again adding and commit the file should finish the merge.