Viewing History and Diffs¶

Overview¶

When production breaks after a deploy, you need to know what changed, when, and who. Git's history and diff tools are your audit trail — every Terraform apply, every manifest update, every Dockerfile tweak is traceable. This tutorial teaches git log, git diff, git show, git blame, and search techniques used in incident response and compliance reviews.

This is Tutorial 6 in Module 2: Essential Workflow of the REBASH Academy Git series.

Prerequisites¶

• Basic Git Workflow — Add, Commit, Push
• Understanding the Git Object Model
• A repository with multiple commits (lab repo or any clone)

Learning Objectives¶

By the end of this tutorial, you will be able to:

• Navigate history with git log and formatting options
• Compare commits, branches, and files with git diff
• Inspect single commits with git show
• Use git blame to find who last changed a line
• Search commit messages and code changes with git log -S and -G
• Visualize history graphs for merge-heavy repos
• Export diffs for review and compliance archives

History Navigation Diagram¶

flowchart TB
    HEAD["HEAD / main"]
    C1["commit N<br/>latest fix"]
    C2["commit N-1<br/>feature add"]
    C3["commit N-2<br/>initial"]

    HEAD --> C1 --> C2 --> C3

    LOG[git log] --> HEAD
    DIFF[git diff C2 C1] --> C1
    BLAME[git blame file] --> C2
    SHOW[git show C1] --> C1```

## Theory

### git log — The History Browser

Basic usage:

```bash
git log
git log --oneline -20
git log --graph --decorate --all

Useful filters:

git diff — Comparing States¶

Three-dot diff (main...feature) shows changes on feature since branching — common for PR review.

git show — One Commit Detail¶

git show HEAD
git show abc1234 --stat
git show v1.2.0:deploy/k8s/deployment.yaml

Shows commit metadata plus full diff. The commit:path syntax retrieves file content at that commit without checkout.

git blame — Line-by-Line Attribution¶

git blame deploy/terraform/main.tf
git blame -L 40,60 Dockerfile

Each line shows commit SHA, author, date. In incidents: "who bumped the replica count to 50?"

Ignore whitespace and move detection:

git blame -w -M README.md

Pickaxe Search — Finding When Code Changed¶

git log -S "replica_count" --oneline
git log -G "password.*=" --oneline

-S finds commits that changed occurrence count of a string. -G uses regex. Essential for tracing when a secret or config value was introduced.

Follow Renames¶

git log --follow -- renames/old-name.tf

Tracks file across renames — default git log path may miss history before rename.

Format Strings for Automation¶

git log --format='%H|%an|%ae|%s' -5

Machine-readable output for scripts parsing audit data.

Diff Tools and External Review¶

git difftool -d main feature
git diff main feature > review.patch

Configure difftool for vimdiff, meld, or VS Code. Patch files attach to tickets or email review.

Advanced Log Formatting for Audits¶

Compliance reviews often require machine-readable exports:

git log --since="2026-01-01" --until="2026-12-31" \
  --format='{"sha":"%H","author":"%an","email":"%ae","date":"%aI","subject":"%s"}' \
  -- terraform/

Filter by path for monorepos — only commits touching infrastructure:

git log --oneline --follow -- terraform/modules/vpc/

Combine with --first-parent main on merge-heavy repos to show linear main history without feature branch noise. Store export scripts in scripts/audit-git-log.sh and run quarterly for SOC 2 evidence collection.

Hands-on Lab¶

Step 1 – Create repository with history¶

Command:

mkdir -p /tmp/git-history-lab && cd /tmp/git-history-lab
git init -b main
echo "replicas: 1" > deploy.yaml && git add . && git commit -m "feat: initial deploy config"
echo "replicas: 3" >> deploy.yaml && sed -i '' 's/replicas: 1/replicas: 3/' deploy.yaml 2>/dev/null || sed -i 's/replicas: 1/replicas: 3/' deploy.yaml
git add deploy.yaml && git commit -m "fix: scale replicas to 3"
echo "memory: 512Mi" >> deploy.yaml && git add . && git commit -m "feat: add memory limit"
git log --oneline

Step 2 – Graph and decorated log¶

Command:

git log --graph --decorate --oneline --all
git log --stat -2

Step 3 – Compare commits¶

Command:

git diff HEAD~2 HEAD
git diff HEAD~1 HEAD -- deploy.yaml

Explanation: HEAD~2 means two commits before HEAD.

Step 4 – git show and file at commit¶

Command:

git show HEAD~1 -- deploy.yaml
git show HEAD~2:deploy.yaml

Step 5 – git blame¶

Command:

git blame deploy.yaml
git blame -L 1,3 deploy.yaml

Step 6 – Pickaxe search¶

Command:

git log -S "replicas: 3" --oneline
git log --grep="scale" --oneline

Step 7 – Clean up¶

Command:

cd /tmp && rm -rf git-history-lab

Commands & Code¶

Audit export script¶

#!/usr/bin/env bash
# git-audit-export.sh — export commit log for compliance
set -euo pipefail
REPO="${1:-.}"
SINCE="${2:-2026-01-01}"
cd "$REPO"
git log --since="$SINCE" \
  --format='commit %H%nAuthor: %an <%ae>%nDate: %ad%nSubject: %s%n' \
  --date=iso-strict \
  --name-status

Common Mistakes¶

Best Practices¶

Troubleshooting¶

Summary¶

• git log navigates history with filters for author, date, path, and message
• git diff compares working tree, staging area, commits, branches, and tags
• git show displays one commit; git blame attributes lines to authors and commits
• Pickaxe search (-S, -G) finds when specific strings entered or left the codebase
• History tools are essential for incident response, compliance audits, and code review

Interview Questions¶

1. What is the difference between git diff and git diff --staged?
2. How do you show the last 10 commits in one line each?
3. What does git blame do, and when would you use it?
4. Explain two-dot vs three-dot diff syntax.
5. How do you find which commit introduced a specific string?
6. What does git show COMMIT:path/to/file return?
7. How do you view history of a file that was renamed?
8. What git log options help visualize branch merges?
9. How would you export commit history for a compliance audit?
10. Why might git blame show unexpected authors?

Related Tutorials¶

• Basic Git Workflow — Add, Commit, Push (previous)
• Branching Fundamentals (next — Module 3)
• Git Bisect and Debugging History
• Cherry-pick and Reflog
• Git – Category Overview

References¶

• Pro Git Book – Viewing History
• git log documentation
• git diff documentation
• git blame documentation
• REBASH Academy – Git Overview