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!

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