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Exploring History with git log

Module 01 45 min

Section Objectives​

  • Browse commit history with different formats
  • Filter history by author, date, file, or message
  • Visualize branches graphically
  • Search through the history for specific changes

Basic git log​

git log

Output:

commit d5a1b4c2e3f4a5b6c7d8e9f0a1b2c3 (HEAD -> main)
Author: Alice Dupont <alice@example.com>
Date:   Wed Mar 18 14:30:00 2026 +0000

    feat(auth): add user login endpoint

commit c2e8f3a1b2c3d4e5f6a7b8c9d0e1f2 (origin/main)
Author: Alice Dupont <alice@example.com>
Date:   Tue Mar 17 10:15:00 2026 +0000

    fix(utils): correct email normalization

Essential git log Formats​

# Compact: one line per commit
git log --oneline

# With branch/tag graph
git log --oneline --graph

# Full graph with all branches
git log --oneline --graph --decorate --all

# Detailed: stats (files changed, insertions, deletions)
git log --stat

# Detailed: full diff of each commit
git log -p        # or --patch

# Show only N last commits
git log -5
git log -10 --oneline

Sample Output of --oneline --graph --all​

* d5a1b4c (HEAD -> main) feat: add dashboard
* c2e8f3a fix: correct email validation
| * b7c9d1e (feature/login) feat: login form UI
| * a3f4b2c feat: start login feature
|/
* e1f2a3b Initial commit

Filtering History​

By Author​

# Commits by a specific author
git log --author="Alice"
git log --author="alice@example.com"

By Date​

# Since a specific date
git log --since="2026-01-01"
git log --after="2 weeks ago"
git log --after="yesterday"

# Before a specific date
git log --until="2026-03-01"
git log --before="1 month ago"

# Between two dates
git log --after="2026-01-01" --before="2026-03-01"

By Commit Message​

# Search for a keyword in messages
git log --grep="fix"
git log --grep="feat(auth)"

# Case insensitive search
git log --grep="login" -i

By File​

# Only commits that touched a specific file
git log -- README.md
git log -- src/auth/login.py

# Only commits that touched a folder
git log -- src/

# Commits that touched a file, even after it was renamed
git log --follow -- src/auth/login.py

By Content (Pickaxe)​

# Commits that ADDED or REMOVED a specific string
git log -S "password"        # Added/removed the word "password"
git log -S "def login"       # Added/removed the login function

# Commits where content CHANGED (regex)
git log -G "password.*hash"

Formatting Output​

Custom Format​

# Custom format
git log --format="%h %an %ar %s"
# %h = short hash
# %H = full hash
# %an = author name
# %ae = author email
# %ar = relative date ("3 days ago")
# %ad = absolute date
# %s = commit subject (first line)
# %b = body
# %d = refs (branches, tags)

# Example output:
# d5a1b4c Alice Dupont 2 days ago feat: add dashboard

Useful Formats​

# Compact with relative date
git log --format="%C(yellow)%h%C(reset) %C(blue)%ar%C(reset) %s %C(green)(%an)%C(reset)"

# Export to file for documentation
git log --format="%h|%an|%ar|%s" > commit-history.csv

git show — Details of a Commit​

# Show details of last commit
git show

# Show a specific commit
git show d5a1b4c
git show HEAD          # Last commit
git show HEAD~1        # Second-to-last commit
git show HEAD~3        # 4th-to-last commit

# Show a specific file in a commit
git show d5a1b4c:src/main.py

# Show only changed files (no diff)
git show --stat d5a1b4c
git show --name-only d5a1b4c

Relative References​

ReferenceMeaning
HEADCurrent commit (last on current branch)
HEAD~1 or HEAD~Parent commit (1 before HEAD)
HEAD~33 commits before HEAD
HEAD^First parent of HEAD (same as HEAD~1)
HEAD^2Second parent of HEAD (for merges)
main~55 commits before the tip of main
abc1234A specific commit by hash
v1.0A tag

git blame — Who Changed What?​

# Show who last modified each line of a file
git blame README.md

# Ignore whitespace changes
git blame -w README.md

# Show only specific lines
git blame -L 10,20 src/main.py

# Show at a specific commit
git blame d5a1b4c -- src/main.py

Output:

d5a1b4c (Alice  2026-03-18 14:30 +0000  1) # My Project
c2e8f3a (Bob    2026-03-10 09:15 +0000  2)
c2e8f3a (Bob    2026-03-10 09:15 +0000  3) ## Installation
a3f4b2c (Alice  2026-03-01 08:00 +0000  4)
a3f4b2c (Alice  2026-03-01 08:00 +0000  5) ```bash
git blame is not for blaming people!

Despite its name, git blame is a collaboration tool, not for blaming colleagues. Use it to understand why code was written a certain way, who to ask for context, and to locate the commit that introduced a change.

Searching History​

git log -S — Find a Change​

# When was a function added?
git log -S "def authenticate" --oneline

# When was a variable renamed?
git log -S "user_id" --oneline -- src/

# View the exact change
git log -p -S "def authenticate" --oneline

git bisect — Binary Search for a Bug​

# Start bisect
git bisect start

# Mark current commit as bad (bug present)
git bisect bad

# Mark an old commit as good (bug not present)
git bisect good v1.0

# Git checks out the middle commit — test it
# If the bug is present:
git bisect bad
# If the bug is absent:
git bisect good

# Git continues narrowing down until finding the exact commit
# At the end:
git bisect reset

Practical Aliases for Log​

Add these to your ~/.gitconfig:

git config --global alias.lg "log --oneline --graph --decorate --all"
git config --global alias.ll "log --oneline -15"
git config --global alias.lp "log -p -1"
git config --global alias.ls "log --stat -5"
git config --global alias.lf "log --format='%C(yellow)%h%C(reset) %C(blue)%ar%C(reset) %s %C(green)(%an)%C(reset)'"

Summary Table​

CommandDescription
git logFull history
git log --onelineCompact history
git log --graph --allVisual graph of all branches
git log --statHistory with stats
git log -pHistory with diffs
git log --author="X"Filter by author
git log --since="date"Filter by date
git log --grep="text"Filter by message
git log -S "code"Search by content
git log -- fileCommits affecting a file
git show <commit>Details of a commit
git blame <file>Who modified each line

Next Steps​