Exploring Previous Commits

Review Commit History

• git log -n --oneline — review the previous n commits
• git log --branches=* — view all commits across all branches
• git branch -a — return a list of all known branches

Access a Previous Commit

Use git checkout COMMIT-ID to load a previous commit.

Normally, HEAD points to main or some other local branch. When checking out a previous commit, HEAD no longer points to a branch; it is detached. It points to that commit instead.

Use git checkout BRANCH-NAME to get back to the branch you were working on.

Modifying the Most Recent Commit

To modify the most recent commit only:

1. Make the changes you want to make
2. Use git add to stage the changes
3. Use git comimt --amend to apply those changes to the most recent commit

Undoing Commits

For all examples, consider this commit history resulting from calling git log --oneline as the starting point:

872fa7e Try something crazy
a1e8fb5 Make some important changes to hello.txt
435b61d Create hello.txt
9773e52 Initial import

With Revert

Use git revert when dealing with a shared (remote) repo:

• It works on a single commit.
• It takes a specified commit, reverses the changes made in that commit, and creates a new commit for those reversed changes.
• It preserves the repo’s commit history.

Reverting the Most Recent Commit

Use git revert HEAD. The commit history now looks like this:

e2f9a78 Revert "Try something crazy"
872fa7e Try something crazy
a1e8fb5 Make some important changes to hello.txt
435b61d Create hello.txt
9773e52 Initial import

Although 872fa7e still exists in the history, e2f9a78 is a new commit that undoes changes the changes of 872fa7e.

Reverting an Earlier Commit

Use git revert COMMIT-ID.

With Reset

Use git reset when dealing with changes in a local repo only (changes that are committed but not pushed):

• It destroys all changes in the chain back to the targeted commit.
• It destroys commit history.

Executing git reset --hard a1e8fb5 results in the commit history now looking like this:

a1e8fb5 Make some important changes to hello.txt
435b61d Create hello.txt
9773e52 Initial import

Although e2f9a78 and 872fa7e no longer exist in the commit history, they still exist in the remote repo. A git push will now fail. Use git push origin BRANCH-NAME --force with consequences.

Checkout vs Reset

Assume this commit history:

Executing git checkout b results in the main ref still pointing to d and the HEAD ref pointing to b:

Executing git reset b results in the main AND HEAD refs pointing to b and no longer has history of c and d:

More on Reset

• git reset has three different reset modes that work on different combinations of the commit history, the staging index, and the working directory:
  

  

• It uses --mixed HEAD as its default option (executing git reset is equivalent to executing git reset --mixed HEAD)
• Documentation: https://www.atlassian.com/git/tutorials/undoing-changes/git-reset

Soft Reset

Using git reset --soft COMMIT-ID:

• The commit history ref pointers are updated to that of COMMIT-ID
• The staging index and working directory are left untouched

Mixed Reset

Using git reset --mixed COMMIT-ID:

• The commit history ref pointers are updated to that of COMMIT-ID
• The staging index is reset to match that of COMMIT-ID
• Any changes that have been undone from the staging index are moved to the working directory

Hard Reset

Using git reset --hard COMMIT-ID:

• The commit history ref pointers are updated to that of COMMIT-ID
• The staging index is reset to match that of COMMIT-ID
• The working directory is reset to match that of COMMIT-ID
• All pending work is lost

Rebasing

• Rebasing makes for a cleaner history.
• A rebased branch appears as if all the work happened in a series, even when it actually happened in parallel.
• You may rebase when contributing to a project to which you do not maintain.
• Do not rebase commits that exist outside of your repository.
• Do not rebase commits that people may have based work on.
• Do rebase local changes before pushing to clean up your work.