git log & git blame — Exploring History

git log

git log shows the commit history of the current branch — or any branch, range, file, or author you specify.

---

Basic Usage

git log                        # full log, newest first
git log --oneline              # one line per commit
git log --oneline --graph      # ASCII graph of branch topology
git log --oneline --graph --all  # all branches

Filtering by Author, Date, or Message

# By author
git log --author="Jane Smith"
git log --author="[email protected]"

# By date range
git log --since="2024-01-01" --until="2024-01-31"
git log --since="2 weeks ago"
git log --after="yesterday"

# By commit message keyword
git log --grep="JIRA-123"
git log --grep="fix" --grep="auth" --all-match   # both terms must appear

# By branch/range
git log main..feature/JIRA-123          # commits in feature not in main
git log feature/JIRA-123..main          # commits in main not in feature
git log origin/main..HEAD               # your unpushed commits

Filtering by File or Directory

# Commits that touched a specific file
git log -- src/main/java/com/example/TransactionService.java

# Commits that touched any file in a directory
git log -- src/main/java/com/example/service/

# Commits that added or removed a specific string (the pickaxe)
git log -S "findByUserIdAndCreatedAtBetween"   # exact string
git log -G "findBy.*Between"                   # regex

The pickaxe (-S / -G) is extremely powerful for finding exactly when a method was introduced or removed.

Pretty Formats

# Standard formats
git log --format=oneline
git log --format=short
git log --format=medium        # default
git log --format=full
git log --format=fuller

# Custom format
git log --format="%h  %ad  %an  %s" --date=short
# a3f9bc2  2024-01-15  Jane Smith  feat(transactions): add date range filter

# Show changed files per commit
git log --stat
git log --name-only
git log --name-status           # M/A/D status per file

Useful Aliases for log

# Pretty one-line graph with author and date
git log --oneline --graph --decorate \
  --format="%C(yellow)%h%C(reset) %C(cyan)%ad%C(reset) %C(green)%an%C(reset) %s" \
  --date=short

# Save as alias
git config --global alias.lg \
  "log --oneline --graph --decorate --all \
   --format='%C(yellow)%h%C(reset) %C(cyan)%ad%C(reset) %C(green)%an%C(reset) %s' \
   --date=short"

# Usage:
git lg

---

git blame

git blame annotates every line of a file with the commit SHA, author, and date of the last change to that line. It answers: "Who wrote this line, and when?"

git blame src/main/java/com/example/TransactionService.java

Output:

a3f9bc2 (Jane Smith   2024-01-15 10:23:11 +0700 42)     public Page<TransactionDto> findTransactions(
9f3e2d1 (Bob Johnson  2024-01-20 14:01:55 +0700 43)             UUID userId,
9f3e2d1 (Bob Johnson  2024-01-20 14:01:55 +0700 44)             LocalDate fromDate,
9f3e2d1 (Bob Johnson  2024-01-20 14:01:55 +0700 45)             LocalDate toDate,
a3f9bc2 (Jane Smith   2024-01-15 10:23:11 +0700 46)             Pageable pageable) {

Blame specific lines

git blame -L 40,60 src/main/java/com/example/TransactionService.java

Blame a specific revision (point in time)

git blame v1.1.0 -- src/main/java/com/example/TransactionService.java
git blame HEAD~10 -- src/main/java/com/example/TransactionService.java

Ignore whitespace changes

git blame -w src/main/java/com/example/TransactionService.java

Detect moved or copied lines (-C, -M)

Find the commit that originally wrote a line, even if it was later moved or copied to this file:

git blame -C -C src/main/java/com/example/TransactionService.java
# -C    detect lines copied from other files in same commit
# -C -C detect lines moved from any file across the whole history

---

Following a Line's History

Once git blame gives you a SHA, drill deeper:

# See the full diff of that commit
git show a3f9bc2

# See what the file looked like at that commit
git show a3f9bc2:src/main/java/com/example/TransactionService.java

# Then blame that commit's parent to see the line's history before that change
git blame a3f9bc2^ -- src/main/java/com/example/TransactionService.java

Repeat this to walk a line's full history backward through time.

---

git log Flags Summary

Flag	Meaning
--oneline	One line per commit
--graph	ASCII branch topology
--all	Include all branches and tags
--decorate	Show branch/tag labels on commits
--stat	Show files changed per commit
--patch / -p	Show full diff per commit
--follow	Follow file renames
-S <string>	Pickaxe: commits that added/removed string
-G <regex>	Pickaxe: commits matching regex in diff
--author	Filter by author
--grep	Filter by commit message
--since / --after	Filter by date
--until / --before	Filter by date
-N	Limit to N commits
--no-merges	Exclude merge commits
--merges	Show only merge commits

---

IDE Integration

In IntelliJ IDEA, right-click any line → Git → Annotate with Git Blame for an inline blame view with full commit details on hover. For VS Code, the GitLens extension provides the same. Use these for day-to-day blame — use the CLI for scripting and custom queries.

Interview Questions (Senior Level)

1. How do you investigate a production regression quickly using git log, -S/-G, and blame without chasing noise?
2. What are the limits of git blame in heavily refactored codebases, and how do you mitigate false ownership conclusions?
3. How would you build a forensic checklist to trace when a risky behavior entered the codebase?
4. Which history views do you standardize for release-readiness audits?

Short answer guide:

• Combine pickaxe search, focused ranges, and targeted file history.
• Use -w, -C, and prior revisions to follow moved code.
• Correlate commit diffs with incident timeline and deployment metadata.
• Keep repeatable log formats and tags for release cut verification.