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Inspecting History

Reading history is how you find when a change happened, who made it, and what a commit contains. Interviewers lean on this because revision selection (HEAD~, ^, A..B versus A...B) is exact, and the two-dot and three-dot ranges mean opposite things to git log and git diff.

Track: Core · Interview weight: High

Must-Know Facts

Fact Value Verify with
Compact log git log --oneline (short SHA plus subject) git log --oneline
Branch shape git log --oneline --graph --all git log --graph
Custom format git log --pretty=format:'%h %ad %an %s' --date=short git log
First parent back HEAD~1 (or HEAD~n for n steps) git rev-parse HEAD~1
Nth parent HEAD^2 selects the second parent of a merge git rev-parse HEAD^2
Log range A..B = reachable from B, not from A git log A..B
Log symmetric A...B = commits on either side but not both git log --left-right A...B
Diff two-dot git diff A..B = tip-to-tip (A versus B) git diff A..B
Diff three-dot git diff A...B = B against the merge base of A and B git diff A...B
Upstream @{upstream} (or @{u}) is the tracked remote branch git log @{u}..HEAD
One commit git show <rev> shows its message and diff git show HEAD
Line authorship git blame <file> names the last commit per line git blame <file>

Reading the Log

git log --oneline is the everyday view: one short SHA and subject per commit. Adding --graph --all draws the branch and merge structure across every ref, which is the fastest way to understand a repository's shape.

git log --oneline --graph --all

Output:

*   445c620 Merge feature/search
|\
| * 387de87 Add ranking
| * 63238c6 Add search
* | 3e6d7e9 Add feature b
|/
* c640379 Add feature a
* b8fc6cf Add app

The |\ shows the merge commit's two parents, and the two columns are the two lines of work that merged. A custom format prints exactly the fields wanted, which is what scripts and changelogs use.

git log --pretty=format:'%h %ad %an %s' --date=short -5

Output:

445c620 2026-09-17 Amina Yusuf Merge feature/search
3e6d7e9 2026-09-17 Amina Yusuf Add feature b
387de87 2026-09-17 Amina Yusuf Add ranking
63238c6 2026-09-17 Amina Yusuf Add search
c640379 2026-09-17 Amina Yusuf Add feature a

%h is the short SHA, %ad the author date, %an the author name and %s the subject; --date=short trims the timestamp to a date.

Revision Selection

A commit can be named by SHA, by branch or tag, or relative to another commit. ~ walks first parents, and ^ selects among a commit's parents.

git rev-parse --short HEAD~1
git rev-parse --short HEAD^2

Output:

3e6d7e9
387de87

HEAD~1 is one commit back along the first-parent line, and HEAD~n is n steps. HEAD^2 is the second parent, which exists only on a merge commit, so it selects the merged-in branch's tip. HEAD^ alone means HEAD^1, the first parent.

~ walks generations, ^ chooses a parent

HEAD~2 means "two first-parent steps back", the same as HEAD^^. HEAD^2 means "the second parent of HEAD". They read similarly and are the most common revision-selection mix-up in an interview.

Commit Ranges for the Log

For git log, A..B lists commits reachable from B but not from A, the standard "what is on B that A does not have".

git log --oneline main..topic

Output:

cda7b9f Topic two
d6ea02d Topic one

A...B (three dots) lists the symmetric difference: commits on either side but not on both. --left-right marks which side each came from.

git log --oneline --left-right main...topic

Output:

> cda7b9f Topic two
< 9ea0d5d Main one
> d6ea02d Topic one

< marks commits reachable from the left side (main), > from the right (topic). git log @{u}..HEAD uses the same two-dot form to list local commits not yet pushed.

The Diff Range Convention Is Reversed

The dot notation means the opposite for git diff, which is the classic trap. For git diff, two dots compare the two tips directly.

git diff main..topic

Output:

diff --git a/f b/f
index df967b9..f747ae3 100644
--- a/f
+++ b/f
@@ -1 +1,3 @@
 base
+topic1
+topic2
diff --git a/g b/g
deleted file mode 100644
index aa658b1..0000000
--- a/g
+++ /dev/null
@@ -1,2 +0,0 @@
-base
-main1

The tip-to-tip diff shows g as deleted, because g exists on main but not on topic. Three dots compare B against the merge base of A and B, showing only what topic changed since it forked.

git diff main...topic

Output:

diff --git a/f b/f
index df967b9..f747ae3 100644
--- a/f
+++ b/f
@@ -1 +1,3 @@
 base
+topic1
+topic2

Three-dot git diff is what a pull request shows: the branch's own changes, not the unrelated work that landed on main in the meantime.

A..B and A...B are reversed between log and diff

For git log, A..B is a range and A...B is the symmetric difference. For git diff, A..B is tip-to-tip and A...B diffs against the merge base. Reviewing a branch uses git diff main...topic, never main..topic.

Inspecting One Commit and Blame

git show <rev> prints a single commit's metadata and its diff, and --stat reduces the diff to a per-file summary.

git show --stat --oneline 63238c6

Output:

63238c6 Add search
 search.py | 1 +
 1 file changed, 1 insertion(+)

git blame <file> attributes each line to the commit that last changed it, which finds when and why a specific line arrived.

git blame f

Output:

^700e3e7 (Amina Yusuf 2026-09-17 10:00:00 +0500 1) base
d6ea02d2 (Amina Yusuf 2026-09-17 10:05:00 +0500 2) topic1
cda7b9f4 (Amina Yusuf 2026-09-17 10:10:00 +0500 3) topic2

The ^ prefix on 700e3e7 marks a boundary commit, the oldest commit blame reached. To follow a line back past a rename, git blame -C -M tracks content moved between and within files.

Common Errors

fatal: ambiguous argument 'HEAD^2': unknown revision or path

Cause: HEAD^2 was requested on a non-merge commit, which has only one parent, or the caret was eaten by the shell.

Fix: use HEAD^2 only on a merge commit; quote the argument ('HEAD^2') so the shell does not interpret ^.

fatal: no such path 'topic' in HEAD

Cause: a range like main..topic was parsed as a path because topic is not a valid ref, often a typo or a deleted branch.

Fix: confirm the ref exists with git branch -a; use -- to separate revisions from paths when a name is ambiguous.

Interview Checkpoints

L1: How do you see the branch and merge structure of a repository quickly?

Say first: git log --oneline --graph --all draws every branch and merge as an ASCII graph with short SHAs and subjects.

Proof: the |\ and column layout show a merge commit's two parents and the two lines of work.

Follow-up: What does HEAD~2 select, and how is it different from HEAD^2?

L1: What is the difference between HEAD~2 and HEAD^2?

Say first: HEAD~2 walks two first-parent steps back (the same as HEAD^^); HEAD^2 selects the second parent of HEAD, which only exists on a merge commit.

Proof: git rev-parse HEAD^2 fails on a non-merge commit but resolves to the merged branch's tip on a merge.

Follow-up: How do you list only the commits on a branch that another branch does not have?

L2: List the commits on your branch that are not yet pushed.

Say first: git log @{u}..HEAD compares your branch against its upstream using a two-dot range.

Proof:

git log --oneline @{u}..HEAD

Follow-up: What does git log HEAD..@{u} show instead?

L2: Review only the changes a feature branch introduced, ignoring what landed on main since it forked.

Say first: git diff main...topic (three dots) diffs the branch against the merge base, which is exactly what a pull request shows.

Proof: git diff main..topic (two dots) would instead show unrelated main files as deleted.

Follow-up: Why is the dot convention reversed between git log and git diff?

L3: A line of code looks wrong and you need to know when and why it changed. Where do you start?

Say first: git blame the file to find the commit that last touched the line, then git show that commit for the reasoning in its message and diff.

Proof: git blame <file> names the commit per line; git show <sha> explains it; git log -S'<string>' finds when a string first appeared.

Follow-up: How do you make blame follow the line across a rename?

L4: For git log and git diff, what exactly do A..B and A...B compute?

Say first: for git log, A..B is commits reachable from B not A, and A...B is the symmetric difference; for git diff, A..B is tip-to-tip and A...B diffs B against the merge base of A and B.

Proof: git diff main..topic showed g deleted (tip-to-tip); git diff main...topic showed only topic's own additions.

Don't say: "Two dots and three dots mean the same thing in both commands."

Captured on macOS 26 with git 2.50.1 (throwaway local repositories), 2026-09.