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Advanced Git

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5. Rebasing to Rewrite History
Written by Jawwad Ahmad & Chris Belanger

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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 you can continue using your magicSquareJS repository from the previous chapter as long as you’ve completed the previous challenge. Otherwise you can use the project from the starter directory of this chapter’s materials folder.

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 --oneline --graph wValidator

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

* dc24e14 (HEAD -> wValidator) Updated team acronym to teamWYXZC
* 72c4e8f Added Chris as a new maintainer to README.md
* 4c5274c Refactoring the main check function
* f0f212f Removing TODO
* d642b89 check04: Checking diagonal sums
* 791744c util06: Adding a function to check diagonals
* 556b640 check03: Checking row and column sums
* 75a6d1f util05: Fixing comment indentation
* 72ec86a util04: Adding a function to check column sums
* 34a656c util03: Adding function to check row sums
* 0efbaf2 check02: Checking the array contains the correct values
* 136dc26 (origin/zValidator) 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

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 five util* commits and the three check* commits together and, finally, you’ll 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 4c5274c

pick 72c4e8f Added Chris as a new maintainer to README.md
pick dc24e14 Updated team acronym to teamWYXZC

# Rebase 4c5274c..dc24e14 onto 4c5274c (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 [-C | -c] <commit> = like "squash" but keep only the previous
#                    commit's log message, unless -C is used, in which case
#                    keep only this commit's message; -c is same as -C but
#                    opens the editor
# 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.
#

Squashing in an interactive rebase

Here, Git’s taken all of the commits past your rebase point, 72c4e8f and dc24e14, and put them at the top of the file with some rather helpful comments down below.

pick 72c4e8f Added new maintainer to README.md
squash dc24e14 Updated team acronym

# This is a combination of 2 commits.
# This is the 1st commit message:

Added Chris as a new maintainer to README.md

# This is the commit message #2:

Updated team acronym to teamWYXZC

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
#
# Date:      Sat Aug 21 23:20:17 2021 -0500
#
# interactive rebase in progress; onto 4c5274c
# Last commands done (2 commands done):
#    pick 72c4e8f Added Chris as a new maintainer to README.md
#    squash dc24e14 Updated team acronym to teamWYXZC
# No commands remaining.
# You are currently rebasing branch 'wValidator' on '4c5274c'.
#
# 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:

Added Chris to README and updated team acronym

Successfully rebased and updated refs/heads/wValidator.

git log --oneline --graph

* 6c07391 (HEAD -> wValidator) Added Chris to README and updated team acronym
* 4c5274c 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 --graph

* 6c07391 (HEAD -> wValidator) Added Chris to README and updated team acronym
* 4c5274c Refactoring the main check function
* f0f212f Removing TODO
* d642b89 check04: Checking diagonal sums
* 791744c util06: Adding a function to check diagonals
* 556b640 check03: Checking row and column sums
* 75a6d1f util05: Fixing comment indentation
* 72ec86a util04: Adding a function to check column sums
* 34a656c util03: Adding function to check row sums
* 0efbaf2 check02: Checking the array contains the correct values
* 136dc26 (origin/zValidator) 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 0efbaf2 check02: Checking the array contains the correct values
pick 34a656c util03: Adding function to check row sums
pick 72ec86a util04: Adding a function to check column sums
pick 75a6d1f util05: Fixing comment indentation
pick 556b640 check03: Checking row and column sums
pick 791744c util06: Adding a function to check diagonals
pick d642b89 check04: Checking diagonal sums
pick f0f212f Removing TODO
pick 4c5274c Refactoring the main check function
pick 6c07391 Added Chris to README and updated team acronym

# Rebase 69670e7..6c07391 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 34a656c util03: Adding function to check row sums
pick 72ec86a util04: Adding a function to check column sums
pick 75a6d1f util05: Fixing comment indentation
pick 791744c util06: Adding a function to check diagonals
pick 0efbaf2 check02: Checking the array contains the correct values
pick 556b640 check03: Checking row and column sums
pick d642b89 check04: Checking diagonal sums
pick f0f212f Removing TODO
pick 4c5274c Refactoring the main check function
pick 6c07391 Added Chris to README and updated team acronym

Successfully rebased and updated refs/heads/wValidator.

* 0d6790c (HEAD -> wValidator) Added Chris to README and updated team acronym
* aed8cbb Refactoring the main check function
* 8e6d0a8 Removing TODO
* 734676e check04: Checking diagonal sums
* 798260c check03: Checking row and column sums
* 7dd347e check02: Checking the array contains the correct values
* 7a6b92d util06: Adding a function to check diagonals
* c2fae05 util05: Fixing comment indentation
* 680edea util04: Adding a function to check column sums
* 48b6bcf util03: Adding function to check row sums
* 5cdbb66 Refactoring the range checking function
* beb30fb util02: Adding function to check the range of values
* 92b5982 util01: Adding the checkSqaure function
* 6adde96 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 6adde96

pick 92b5982 util01: Adding the checkSqaure function

reword 92b5982 util01: Adding the checkSqaure function

// Hash updated from 5cdbb66 to 6a9a50d
* 6a9a50d Refactoring the range checking function

// Hash updated from beb30fb to 9923d78
* 9923d78 util02: Adding function to check the range of values

// Hash updated from 92b5982 to 0be4b0c and spelling updated
* 0be4b0c util01: Adding the checkSquare function

// Commit before rebase has same hash of 6adde96
* 6adde96 check01: checking that the 2D array is square

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 6adde96 check01: checking that the 2D array is square
pick 0be4b0c util01: Adding the checkSquare function
squash 9923d78 util02: Adding function to check the range of values
squash 6a9a50d Refactoring the range checking function
squash d761cfe util03: Adding function to check row sums
squash 98c44f8 util04: Adding a function to check column sums
squash 01688ee util05: Fixing comment indentation
squash fd28844 util06: Adding a function to check diagonals
pick 43a1bc1 check02: Checking the array contains the correct values
pick 60b55d2 check03: Checking row and column sums
pick 53a9b10 check04: Checking diagonal sums
pick 3a04bfb Removing TODO
pick d3eea6b Refactoring the main check function
pick 08c60cf Added Chris to README and updated team acronym

# 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

* f8d6e1b (HEAD -> wValidator) Added Chris to README and updated team acronym
* ededa27 Refactoring the main check function
* 2670142 Removing TODO
* d0bdabb check04: Checking diagonal sums
* 4424300 check03: Checking row and column sums
* 5e087af check02: Checking the array contains the correct values
* 1cb3ad3 Creating utility functions for Magic Square validation
* 6adde96 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 main

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

4. Demystifying Rebasing
6. Gitignore After the Fact
Have a technical question? Want to report a bug? You can ask questions and report bugs to the book authors in our official book forum here.
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