Git Internals and Advanced Operations

Git questions go beyond basic commands in DevOps/SRE interviews. Let's master the internals and advanced operations.

Git Object Model

Understanding Git's internals impresses interviewers:

Git stores 4 object types:
1. blob   - File contents
2. tree   - Directory structure
3. commit - Snapshot with metadata
4. tag    - Named reference to commit

# View object type
git cat-file -t <sha>

# View object content
git cat-file -p <sha>

# List all objects
find .git/objects -type f

How Commits Work

                    ┌──────────┐
                    │  commit  │
                    │ (sha: a1)│
                    └────┬─────┘
                         │ points to
                    ┌────▼─────┐
                    │   tree   │
                    │ (sha: b2)│
                    └────┬─────┘
              ┌──────────┴──────────┐
         ┌────▼─────┐          ┌────▼─────┐
         │   blob   │          │   tree   │
         │  (file)  │          │  (dir)   │
         └──────────┘          └──────────┘

Advanced Git Operations

Rebase vs Merge

Aspect	Merge	Rebase
History	Preserves all commits	Linear history
Conflicts	Once during merge	Per commit
Safety	Safe for shared branches	Rewrites history
When to use	Shared branches	Local/feature branches

# Interactive rebase (reorder, squash, edit commits)
git rebase -i HEAD~5

# Rebase onto main
git checkout feature
git rebase main

# NEVER rebase shared branches!
# "If in doubt, don't rebase"

Interview question: "When should you use rebase vs merge?"

Answer: Use rebase for local feature branches to maintain clean history. Use merge for shared/public branches to preserve history and avoid rewriting commits others depend on.

Cherry-Pick

# Apply specific commit(s) to current branch
git cherry-pick <commit-sha>

# Cherry-pick without committing
git cherry-pick --no-commit <sha>

# Cherry-pick range
git cherry-pick A^..B

Git Bisect

Find the commit that introduced a bug:

# Start bisect
git bisect start

# Mark current (broken) as bad
git bisect bad

# Mark known good commit
git bisect good <sha>

# Git checks out middle commit, test and mark:
git bisect good   # or
git bisect bad

# Continue until Git finds the first bad commit
# When done:
git bisect reset

Reflog - Your Safety Net

# View reflog (history of HEAD movements)
git reflog

# Recover "deleted" commits
git checkout -b recovery <sha-from-reflog>

# Undo a bad rebase
git reset --hard HEAD@{5}

Git Workflow Strategies

Trunk-Based Development

main ─────●─────●─────●─────●─────●
           \           /
            ●─────●───
          (short-lived feature branch)

Direct commits to main
Feature flags for incomplete features
Requires good CI/CD and testing
Used by Google, Facebook

GitFlow

main    ─────●───────────────●─────────
              \             /
develop ───●───●───●───●───●───●───●───
            \       \     /
feature      ●───●   ●───●

release              ●───●───●

Separate develop and main branches
Feature, release, hotfix branches
Better for scheduled releases
More complex, more overhead

GitHub Flow

main ─────●─────●─────●─────●─────●
           \         / \       /
feature     ●───●───   ●───●
              (PR)       (PR)

Simple: main + feature branches
Pull requests for review
Deploy from main
Good balance for most teams

Interview Questions

Q: "How would you recover a commit after running git reset --hard?"

# Check reflog for the lost commit
git reflog

# Find the commit SHA before the reset
# Create a branch from it
git checkout -b recovery <sha>

# Or reset back to it
git reset --hard <sha>

Q: "What's the difference between git reset and git revert?"

Operation	Effect	History	Use Case
`reset`	Moves HEAD, can discard commits	Rewrites	Local cleanup
`revert`	Creates new commit undoing changes	Preserves	Shared branches

# Reset: moves HEAD backward (rewrites history)
git reset --hard HEAD~3

# Revert: creates new commit (safe for shared)
git revert <sha>

Q: "You accidentally pushed secrets to a public repo. How do you fix it?"

# 1. Remove from history using filter-branch or BFG
git filter-branch --force --index-filter \
  "git rm --cached --ignore-unmatch secrets.env" HEAD

# Or use BFG Repo Cleaner (faster)
bfg --delete-files secrets.env

# 2. Force push (coordinate with team!)
git push --force-with-lease

# 3. Rotate all exposed credentials immediately!
# 4. GitHub may cache - contact support

Next, we'll dive into CI/CD pipeline design—a core DevOps/SRE competency. :::

Quiz