Git History and Time Travel

One of Git's most powerful features is its history tracking. In this section, we'll explore how to navigate through your project's history, inspect changes over time, and even rewrite history when necessary.

Exploring Your Git History

Viewing Commit History

The basic command to view history is:

git log

This shows a list of commits with their:

• SHA-1 hash (commit ID)
• Author
• Date
• Commit message

For a more compact view:

git log --oneline

This shows each commit on a single line with a shortened hash.

Visualizing Branch History

To see a graphical representation of your branch history:

git log --graph --oneline --all

This shows how branches and merges relate to each other. Adding --decorate shows branch and tag references:

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

Filtering History

You can filter the log to show only what you're interested in:

By date:

git log --after="2023-01-01" --before="2023-02-01"

By author:

git log --author="Jane Smith"

By content:

git log -S"login" # Commits that add or remove the string "login"

By file:

git log -- path/to/file.js

By message content:

git log --grep="fix bug"

Viewing Changes in Commits

To see the changes introduced in each commit:

git log -p

For a statistical summary of changes:

git log --stat

To see changes for a specific file:

git log -p -- path/to/file.js

Viewing Specific Commits

To show a single commit:

git show commit-hash

To see a specific version of a file:

git show commit-hash:path/to/file.js

Time Traveling Through Your Code

Checking Out Previous Versions

To temporarily switch to a previous state of your project:

git checkout commit-hash

This puts you in a "detached HEAD" state, where you can look around but changes won't be saved to any branch.

To check out a specific file from a previous commit:

git checkout commit-hash -- path/to/file.js

This adds the old version of the file to your staging area.

Working with Tags

Tags are named references to specific commits, commonly used for releases:

Creating a lightweight tag:

git tag v1.0.0

Creating an annotated tag with a message:

git tag -a v1.0.0 -m "Version 1.0.0 release"

Listing tags:

git tag

Checking out a tag:

git checkout v1.0.0

Comparing Different Versions

To see differences between two commits:

git diff commit1..commit2

To see differences for a specific file:

git diff commit1..commit2 -- path/to/file.js

To compare branches:

git diff main..feature-branch

Debugging with Git History

Finding Bugs with git bisect

Git bisect uses binary search to find which commit introduced a bug:

# Start the bisect process
git bisect start

# Mark the current commit as bad
git bisect bad

# Mark a known good commit
git bisect good a1b2c3d4

# Git will checkout a commit halfway between good and bad
# Test your code, then mark it:
git bisect good  # Or git bisect bad

# Continue until Git identifies the first bad commit
# When done, reset to your original branch
git bisect reset

This can save hours of debugging by narrowing down exactly when a bug was introduced.

Finding Who Changed a Line

To see who last modified each line of a file:

git blame filename.txt

This shows the commit hash, author, date, and content for each line. It's useful for understanding why code was written a certain way.

For more context, use the -C flag to detect code moved from other files:

git blame -C filename.txt

Recovering Lost Work

Using Reflog

Git keeps a record of where your HEAD and branch references have been for the last 30 days in the reflog:

git reflog

This helps you find commit hashes you might have lost through operations like resets or rebases.

To recover a lost commit:

git checkout -b recovery-branch commit-hash

Recovering Deleted Branches

If you accidentally deleted a branch, you can recover it using reflog:

# Find the commit at the tip of the deleted branch
git reflog

# Create a new branch at that commit
git checkout -b recovered-branch commit-hash

Recovering Uncommitted Changes

If you accidentally discarded changes with git checkout -- file:

1. Check if the changes are in the stash:

   git stash list
   git stash apply
   

2. On some systems, IDE local history might have saved a copy

3. Unfortunately, truly unstaged and uncommitted changes that were discarded cannot be recovered with Git

Rewriting History

⚠️ Warning: Rewriting history should be done with caution, especially if the commits have been pushed to a shared repository.

Amending the Last Commit

To modify your most recent commit:

git commit --amend

This opens an editor to change the commit message. To include staged changes in the amended commit:

git add .
git commit --amend

To keep the same message:

git commit --amend --no-edit

Reordering and Modifying Commits with Interactive Rebase

Interactive rebase lets you modify a series of commits:

git rebase -i HEAD~3  # Modify the last 3 commits

This opens an editor with a list of commits. You can:

• pick - keep the commit
• reword - change the commit message
• edit - stop and amend the commit
• squash - combine with previous commit
• fixup - combine with previous commit, discard message
• drop - remove the commit

For example, to combine three commits into one:

pick 1a2b3c4 Add user authentication
squash 2b3c4d5 Fix login form styling
squash 3c4d5e6 Add password validation

Save and close the editor, and Git will combine the commits.

Splitting Commits

To split a commit into multiple commits:

git rebase -i HEAD~3
# Change "pick" to "edit" for the commit you want to split
# Save and close

# Reset that commit but keep the changes in your working directory
git reset HEAD^

# Now add and commit the changes in smaller logical chunks
git add file1.js
git commit -m "First part of the feature"
git add file2.js
git commit -m "Second part of the feature"

# Continue the rebase
git rebase --continue

Removing Sensitive Data

If you accidentally committed sensitive data:

git filter-branch --force --index-filter "git rm --cached --ignore-unmatch path/to/sensitive-file" --prune-empty --tag-name-filter cat -- --all

For larger repositories, consider using BFG Repo-Cleaner, which is faster and simpler.

Important: After removing sensitive data, you'll need to force-push the changes. Also, be aware that the data may still exist in clones of your repository.

Best Practices for Managing History

When to Rewrite History

Appropriate times to rewrite history:

• Before pushing commits to a shared repository
• In a personal branch only you are working on
• To clean up your commits before creating a pull request
• To remove sensitive information

When NOT to rewrite history:

• After pushing to a shared repository
• On branches other people are working on
• On main or other important branches
• If you're unsure about what you're doing

Keeping a Clean History

A clean, meaningful commit history makes it easier to understand the project's evolution:

• Write descriptive commit messages
• Make each commit represent a logical change
• Squash "work in progress" commits before merging
• Rebase feature branches on main before merging
• Use merge strategies that maintain a clear history

Handling Public History

For public repositories or shared branches:

• Avoid rewriting history that has been pushed
• Use git revert to undo changes instead of rewriting
• Consider using pull requests for reviews instead of forcing clean history
• Document significant changes in commit messages and release notes

Advanced History Techniques

Cherry-Picking

To apply a specific commit from one branch to another:

git cherry-pick commit-hash

This creates a new commit on your current branch with the same changes as the specified commit.

Using Rerere (Reuse Recorded Resolution)

If you frequently encounter the same merge conflicts, Git's rerere feature can help:

git config --global rerere.enabled true

This tells Git to remember how you resolved a conflict so it can automatically resolve it the next time.

Creating Patches

To create a patch file for sharing changes without pushing to a repository:

git format-patch -1 HEAD

This creates a file like 0001-commit-message.patch.

To apply a patch:

git apply path/to/file.patch

Or to apply and create a commit:

git am path/to/file.patch

Conclusion

Git's history tracking provides an invaluable record of your project's evolution. By learning these tools for exploring and manipulating history, you can better understand how your code developed over time, debug issues more efficiently, and maintain a clean, meaningful history.

Remember that with great power comes great responsibility, especially when rewriting history. Always be cautious about changing commits that have been shared with others, and communicate clearly when you need to make such changes.

In the next section, we'll discuss Git best practices to help you maintain an efficient and productive workflow.