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Git Rebase and Amend
Posted on September 2, 2020 in Git by Matt Jennings

Amend a Commit

• Amend is a shortcut that lets you make changes to the previous commit
• Example code to add a file to the last commit:
  git commit -m 'Add a blog post about Python'
git add posts/python.txt
git commit --amend

What is Rebase?

• Image our tech_posts and master branch have diverged.
• We don’t want a message merge commit in our history.
• We can pull in all the later changes from master, and apply our commits on top of them by changing the parent commit.
• Rebase = give a commit a new parent
  (i.e. a new base commit)
• Doesn’t show a merge commit.
• The changes create linear path.
• Get a much cleaner and linear project history.
• Example code:
  git checkout tech_posts
git rebase master

Power of Rebasing

• Commits can be edited, removed, combined, re-ordered, inserted, BEFORE they’re “replayed” on top of the new HEAD.

Interactive Rebase (rebase -i)

• Interactive rebase opens an editor with a list of “todos”:
  • In the format of: <command> <commit> <commit msg>
  • git will pick commits in specificed order, or stop to take an action when editing or a conflict occurs
• Interactive rebase with a shortcut:
  • git rebase -i <commit_to_fix>^
  • (the ^ specifies the parent commit) 
• Interactive Rebase Options:
  • pick
    keep this commit
  • reword
    keep the commit, just change the message
  • edit
    keep the commit, but stop to edit more than the message
  • squash
    combine this commit with the previous one. stop to edit the message.
  • fixup
    combine this commit with the previous one. keep the previous commit message
  • exec
    run the command on this line after picking the previous commit
  • drop
    remove the commit (tip: if you remove this line, the commit will be dropped too!)

Tip: “Amend” any Commit with Fixup & Autosquash!

1. git add new files
2. git commit --fixup <SHA>
   This creates a new commit, the message starts with ‘fixup!’
3. git rebase -i autosquash <SHA>^
   git will generate the right todos for you! just save and quit.

Quit a Git Rebase

• At any time before rebase is done, if things are going wrong:
  git rebase --abort

Rebase Pro Tips

• Before you rebase/fixup/squash/reorder make a copy of your current branch:
  git branch my_branch_backup
• git branch will make a new branch, without switching to it
• If rebase “succeeds” but you messed up…
  git reset my_branch_backup --hard
• Before pushing work to a shared repo:
  Rebase to clean up the commit history

Warning: Never Rewrite Public History!

• Rebase commits are copies
• If other people are working on the same repository they would be working on different commits
• You could cause massive merger conflicts
• Even worse, you can cause people to lose their work!