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5
Rebasing to Rewrite History Written by Chris Belanger

As you saw in the previous chapter, rebasing provides you with an excellent alternative to merging. But rebasing also gives you the ability to reorganize your repository’s history. You can reorder, rewrite commit messages and even squash multiple commits into a single, tidy commit if you like.

Just as you’d tidy up your code before pushing your local branch to a remote repository, rebasing lets you clean up your commit history before you push to remote. This gives you the freedom to commit locally as you see fit, then rearrange and combine your commits into a handful of semantically-meaningful commits. These will have much more value to someone (even yourself!) who has to comb through the repository history at some point in the future.

Note: Again, a warning: Rebasing in this manner is best used for branches that you haven’t shared with anyone else. If you must rebase a branch that you’ve shared with others, then you must work out an arrangement with everyone who’s cloned that repository to ensure that they all get the rebased copy of your branch. Otherwise, you’re going to end up with a very complicated repository cleanup exercise at the end of the day.

To start, extract the compressed repository from the starter directory to a convenient location on your machine then navigate into that directory from the command line.

Reordering commits

You’ll start by taking a look at Will’s wValidator branch. Execute the following to see what the current history looks like:

git log --all --decorate --oneline --graph

You’ll see the following at the top of your history graph:

* 45f5b4f (HEAD -> wValidator) Updated team acronym
* 15233a5 Added new maintainer to README.md
* 783031e Refactoring the main check function
* 6396aa8 Removing TODO
* 8e39599 check04: Checking diagonal sums
* 199e71d util06: Adding a function to check diagonals
* a28b9e3 check03: Checking row and column sums
* bdc8bc7 util05: Fixing comment indentation
* a4d6221 util04: Adding a function to check column sums
* 59fd06e util03: Adding function to check row sums
* 5f53302 check02: Checking the array contains the correct values
* 136dc26 Refactoring the range checking function
* 665575c util02: Adding function to check the range of values
* 0fc1a91 check01: checking that the 2D array is square
* 5ec1ccf util01: Adding the checkSqaure function

It’s not terrible, but this could definitely use some cleaning up. Your task is to combine those two trivial updates to README.md into one commit. You’ll then reorder the util* commits and the check* commits together and, finally, to combine those related commits into two separate, tidy commits.

Interactive rebasing

First up: Combine the two top commits into one, inside the current branch. You’re familiar with rebasing branches on top of other branches, but in this chapter, you’ll rebase commits on top of other commits in the same branch.

git rebase -i 783031e
pick 15233a5 Added new maintainer to README.md
pick 45f5b4f Updated team acronym

# Rebase 783031e..45f5b4f onto 783031e (2 commands)
#
# Commands:
# p, pick <commit> = use commit
# r, reword <commit> = use commit, but edit the commit message
# e, edit <commit> = use commit, but stop for amending
# s, squash <commit> = use commit, but meld into previous commit
# f, fixup <commit> = like "squash", but discard this commit's log message
# x, exec <command> = run command (the rest of the line) using shell
# b, break = stop here (continue rebase later with 'git rebase --continue')
# d, drop <commit> = remove commit
# l, label <label> = label current HEAD with a name
# t, reset <label> = reset HEAD to a label
# m, merge [-C <commit> | -c <commit>] <label> [# <oneline>]
# .       create a merge commit using the original merge commit's
# .       message (or the oneline, if no original merge commit was
# .       specified). Use -c <commit> to reword the commit message.
#
# These lines can be re-ordered; they are executed from top to bottom.
#
# If you remove a line here THAT COMMIT WILL BE LOST.
#
# However, if you remove everything, the rebase will be aborted.
#
# Note that empty commits are commented out

Squashing in an interactive rebase

Here, Git’s taken all of the commits past your rebase point, 15233a5 and 45f5b4f, and put them at the top of the file with some rather helpful comments down below.

pick 15233a5 Added new maintainer to README.md
squash 45f5b4f Updated team acronym
# This is a combination of 2 commits.
# This is the 1st commit message:

Added new maintainer to README.md

# This is the commit message #2:

Updated team acronym

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
#
# Date:      Sun Jun 9 07:28:08 2019 -0300
#
# interactive rebase in progress; onto 783031e
# Last commands done (2 commands done):
#    pick 15233a5 Added new maintainer to README.md
#    squash 45f5b4f Updated team acronym
# No commands remaining.
# You are currently rebasing branch 'wValidator' on '783031e'.
#
# Changes to be committed:
#       modified:   README.md
#

Creating the squash commit message

In this case, you’ll just create your own. Clear this file as follows:

Updates to README.md
Successfully rebased and updated refs/heads/wValidator.
git log --all --decorate --oneline --graph
* 2492536 (HEAD -> wValidator) Updates to README.md
* 783031e Refactoring the main check function
git log -p -1
-This project is maintained by teamWYXZ:
+This project is maintained by teamWYXZC:
 - Will
 - Yasmin
 - Xanthe
 - Zack
+- Chris

Reordering commits

The asynchronous and messy nature of development means that sometimes you’ll need to reorder commits to make it easier to squash a set of commits later on. Interactive rebase lets you literally rearrange the order of commits within a branch. You can do this as often as you need, to keep your repository history clean.

git log --oneline
2492536 (HEAD -> wValidator) Updates to README.md
783031e Refactoring the main check function
6396aa8 Removing TODO
8e39599 check04: Checking diagonal sums
199e71d util06: Adding a function to check diagonals
a28b9e3 check03: Checking row and column sums
bdc8bc7 util05: Fixing comment indentation
a4d6221 util04: Adding a function to check column sums
59fd06e util03: Adding function to check row sums
5f53302 check02: Checking the array contains the correct values
136dc26 Refactoring the range checking function
665575c util02: Adding function to check the range of values
0fc1a91 check01: checking that the 2D array is square
5ec1ccf util01: Adding the checkSqaure function
69670e7 Adding a new secret
git rebase -i 69670e7
pick 5ec1ccf util01: Adding the checkSqaure function
pick 0fc1a91 check01: checking that the 2D array is square
pick 665575c util02: Adding function to check the range of values
pick 136dc26 Refactoring the range checking function
pick 5f53302 check02: Checking the array contains the correct values
pick 59fd06e util03: Adding function to check row sums
pick a4d6221 util04: Adding a function to check column sums
pick bdc8bc7 util05: Fixing comment indentation
pick a28b9e3 check03: Checking row and column sums
pick 199e71d util06: Adding a function to check diagonals
pick 8e39599 check04: Checking diagonal sums
pick 6396aa8 Removing TODO
pick 783031e Refactoring the main check function
pick 2492536 Updates to README.md

# Rebase 69670e7..2492536 onto 69670e7 (14 commands)

pick 0fc1a91 check01: checking that the 2D array is square
pick 5ec1ccf util01: Adding the checkSqaure function
pick 665575c util02: Adding function to check the range of values
pick 136dc26 Refactoring the range checking function
pick 59fd06e util03: Adding function to check row sums
pick a4d6221 util04: Adding a function to check column sums
pick bdc8bc7 util05: Fixing comment indentation
pick 199e71d util06: Adding a function to check diagonals
pick 5f53302 check02: Checking the array contains the correct values
pick a28b9e3 check03: Checking row and column sums
pick 8e39599 check04: Checking diagonal sums
pick 6396aa8 Removing TODO
pick 783031e Refactoring the main check function
pick 2492536 Updates to README.md
Successfully rebased and updated refs/heads/wValidator.
35aab2b (HEAD -> wValidator) Updates to README.md
3899829 Refactoring the main check function
c8d5335 Removing TODO
5d16107 check04: Checking diagonal sums
5c9e64d check03: Checking row and column sums
4018013 check02: Checking the array contains the correct values
f7a31a0 util06: Adding a function to check diagonals
851663d util05: Fixing comment indentation
6c857e4 util04: Adding a function to check column sums
5ad299c util03: Adding function to check row sums
2575920 Refactoring the range checking function
96fb378 util02: Adding function to check the range of values
55d4ded util01: Adding the checkSqaure function
ded7caa check01: checking that the 2D array is square
69670e7 Adding a new secret

Rewording commit messages

If you take a look at the util01 commit message, you’ll notice that it’s misspelled as “Sqaure” instead of “Square”. As a word nerd, I can’t leave that the way it is. But I can quickly use interactive rebase to change that commit message.

git rebase -i ded7caa
pick 55d4ded util01: Adding the checkSqaure function
reword 55d4ded util01: Adding the checkSqaure function
4f4e308 util01: Adding the checkSquare function

Squashing multiple commits

Now that you have your utility functions all arranged contiguously, you can proceed to squash these commits into one.

git rebase -i 69670e7
pick ded7caa check01: checking that the 2D array is square
pick 4f4e308 util01: Adding the checkSquare function
squash 421c298 util02: Adding function to check the range of values
squash 96dc840 Refactoring the range checking function
squash 19e90e9 util03: Adding function to check row sums
squash c9d8aa3 util04: Adding a function to check column sums
squash 30f164a util05: Fixing comment indentation
squash 0bda95b util06: Adding a function to check diagonals
pick d34c59b check02: Checking the array contains the correct values
pick d235bf9 check03: Checking row and column sums
pick 00212f3 check04: Checking diagonal sums
pick ca6f8df Removing TODO
pick a4a05c0 Refactoring the main check function
pick a351e8a Updates to README.md
# This is a combination of 7 commits.
# This is the 1st commit message:

util01: Adding the checkSquare function

# This is the commit message #2:

util02: Adding function to check the range of values

# This is the commit message #3:

Refactoring the range checking function

# This is the commit message #4:

util03: Adding function to check row sums

# This is the commit message #5:

util04: Adding a function to check column sums

# This is the commit message #6:

util05: Fixing comment indentation

# This is the commit message #7:

util06: Adding a function to check diagonals

Creating utility functions for Magic Square validation
858e215 (HEAD -> wValidator) Updates to README.md
d42dd03 Refactoring the main check function
499c6ac Removing TODO
7530a8f check04: Checking diagonal sums
c98bb17 check03: Checking row and column sums
eec2df9 check02: Checking the array contains the correct values
2207949 Creating utility functions for magic square validation
ded7caa check01: checking that the 2D array is square
69670e7 Adding a new secret

Challenge 1: More squashing

You’d like to squash all of the check0x commits into one tidy commit. And you could follow the pattern above, where you first rearrange the commits in one rebase and then perform the squash in a separate rebase.

Challenge 2: Rebase your changes onto master

Now that you’ve squashed your work down to just a few commits, it’s time to get wValidator back into the master branch. It’s likely your first instinct is to merge wValidator back to master. However, you’re a rebase guru by this point, so you’ll rebase those commits on top of master instead:

Key points

  • git rebase -i <hash> starts an interactive rebase operation.
  • Interactive rebases in Git let you create a “script” to tell Git how to perform the rebase operation
  • The pick command means to keep a commit in the rebase.
  • The squash command means to merge this commit with the previous one in the rebase.
  • The reword command lets you reword a particular commit message.
  • You can move lines around in the rebase script to reorder commits.
  • Rebasing creates new commits for each original commit in the rebase script.
  • Squashing lets you combine multiple commits into a single commit with a new commit message. This helps keep your commit history clean.

Where to go from here?

Interactive rebase is one of the most powerful features of Git because it forces you to think logically about the changes you’ve made, and how those changes appear to others. Just as you’d appreciate cloning a repo and seeing a nice, illustrative history of the project, so will the developers that come after you.

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