Bisect for finding bug-introducing commits in Git - Step-by-Step Execution

Process Flow - Bisect for finding bug-introducing commits

Start Bisect

↓

Mark Good Commit

↓

Mark Bad Commit

↓

Git Chooses Mid Commit

↓

Test Mid Commit

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Good

↓

Mark Good

↓

Repeat Until One Commit Left

↓

Found Bug-Introducing Commit

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End Bisect

The bisect process narrows down the commit range by marking commits as good or bad, testing the middle commit each time, until it finds the exact commit that introduced the bug.

Execution Sample

Git

git bisect start
git bisect good v1.0
git bisect bad v2.0
# test commit chosen by git
# mark good or bad
# repeat until found

This sequence starts bisect, marks known good and bad commits, then tests commits in between to find the bug-introducing commit.

Process Table

Step	Action	Commit Tested	Result	Next Step
1	Start bisect	-	Bisect started	Mark good commit
2	Mark good commit	v1.0	Marked as good	Mark bad commit
3	Mark bad commit	v2.0	Marked as bad	Git picks middle commit
4	Test middle commit	v1.5	Bug present	Mark bad commit
5	Mark bad commit	v1.5	Marked as bad	Git picks new middle commit
6	Test middle commit	v1.25	Bug absent	Mark good commit
7	Mark good commit	v1.25	Marked as good	Git picks new middle commit
8	Test middle commit	v1.375	Bug present	Mark bad commit
9	Mark bad commit	v1.375	Marked as bad	Git picks new middle commit
10	Test middle commit	v1.3125	Bug absent	Mark good commit
11	Mark good commit	v1.3125	Marked as good	Git picks new middle commit
12	Test middle commit	v1.34375	Bug present	Mark bad commit
13	Mark bad commit	v1.34375	Marked as bad	One commit left
14	Bisect complete	v1.34375	Bug introduced here	End bisect

💡 Bisect ends when only one commit remains between good and bad, identifying the bug-introducing commit.

Status Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	After 7	Final
Good Commit	-	v1.0	v1.0	v1.25	v1.25	v1.3125	v1.3125	v1.34375	v1.34375
Bad Commit	-	v2.0	v1.5	v1.5	v1.375	v1.375	v1.34375	v1.34375	v1.34375
Current Tested Commit	-	v1.5	v1.25	v1.375	v1.3125	v1.34375	-	-	-
Bisect State	Not started	Started	In progress	In progress	In progress	In progress	In progress	Complete	Complete

Key Moments - 3 Insights

Why does git pick the middle commit between good and bad commits?

Git uses a binary search approach to quickly narrow down the commit range by testing the middle commit, as shown in steps 4, 6, 8, 10, 12 in the execution_table.

What happens if the tested commit is good?

If the tested commit is good, it becomes the new 'good' commit, narrowing the search range, as seen in steps 6, 10, and 11.

When does bisect end?

Bisect ends when only one commit remains between the good and bad commits, identifying the bug-introducing commit, as in step 14.

Visual Quiz - 3 Questions

Look at the execution_table at step 4. What was the result of testing commit v1.5?

A Bug present

B Bug absent

C Commit marked good

D Bisect ended

💡 Hint

Check the 'Result' column for step 4 in the execution_table.

At which step does the bisect process mark commit v1.25 as good?

A Step 5

B Step 6

C Step 7

D Step 8

💡 Hint

Look for 'Mark good commit' action with commit v1.25 in the execution_table.

If the commit tested at step 8 was good instead of bad, how would the next tested commit change?

A Git would pick a commit between v1.25 and v1.375

B Git would pick a commit between v1.375 and v1.5

C Git would pick a commit between v1.0 and v1.25

D Bisect would end immediately

💡 Hint

Refer to the binary search logic in the concept_flow and variable_tracker.

Concept Snapshot

git bisect start
Mark a known good commit: git bisect good <commit>
Mark a known bad commit: git bisect bad <commit>
Git tests middle commits between good and bad
Mark tested commits as good or bad
Repeat until one commit remains
This commit introduced the bug

Full Transcript

Git bisect helps find the commit that introduced a bug by using a binary search. You start bisect, mark a known good commit where the bug was absent, and a known bad commit where the bug is present. Git then picks the middle commit to test. If the bug is present, mark it bad; if absent, mark it good. This narrows the search range. Repeat testing and marking commits until only one commit remains between good and bad. This commit is the one that introduced the bug. The process is efficient because it halves the search space each time.

Practice

1. What is the main purpose of git bisect?

easy

A. To delete old commits from history

B. To merge two branches automatically

C. To create a new branch from the current commit

D. To find the first commit that introduced a bug

Solution

1. Step 1: Understand the role of git bisect

   Git bisect helps identify which commit introduced a bug by testing commits between a known good and a bad commit.

2. Step 2: Compare with other git commands

   Merging branches, deleting commits, or creating branches are different git operations unrelated to bisect.

3. Final Answer:

   To find the first commit that introduced a bug -> Option D

4. Quick Check:

   git bisect = find bug commit [OK]

Hint: Bisect narrows down bug commit by testing between good and bad [OK]

Common Mistakes:

• Confusing bisect with merge or branch commands
• Thinking bisect deletes commits
• Assuming bisect creates branches

2. Which command correctly starts a bisect session assuming HEAD is bad and v1.0 is good?

easy

A. git bisect start v1.0 HEAD

B. git bisect start HEAD v1.0

C. git bisect begin HEAD v1.0

D. git bisect init v1.0 HEAD

Solution

1. Step 1: Recall git bisect start syntax

   The correct syntax is git bisect start <bad> <good>, where the first argument is the bad commit and the second is the good commit.

2. Step 2: Match the given commits

   Since HEAD is bad and v1.0 is good, the command is git bisect start HEAD v1.0.

3. Final Answer:

   git bisect start HEAD v1.0 -> Option B

4. Quick Check:

   git bisect start bad good = start [OK]

Hint: Start bisect with bad commit first, then good commit [OK]

Common Mistakes:

• Reversing good and bad commits in the command
• Using incorrect command like bisect begin or init
• Omitting the start keyword

3. Given this bisect session output:

Bisecting: 3 revisions left to test after this (roughly 2 steps)
Testing commit abc1234

If you run git bisect good and then git bisect bad on the next commit, what will git do next?

medium

A. Git will narrow down the range and checkout the next commit to test

B. Git will end the bisect session immediately

C. Git will reset to the original branch without changes

D. Git will throw an error because you cannot mark good then bad consecutively

Solution

1. Step 1: Understand bisect workflow

   Marking commits as good or bad tells git which side of the commit range to keep searching. After marking, git checks out the next commit to test.

2. Step 2: Analyze the given commands

   Marking one commit good, then the next bad, narrows the search range. Git continues bisecting until it finds the first bad commit.

3. Final Answer:

   Git will narrow down the range and checkout the next commit to test -> Option A

4. Quick Check:

   Mark good/bad -> git picks next commit [OK]

Hint: Mark commits good/bad to let git pick next test commit [OK]

Common Mistakes:

• Thinking bisect ends after one good and one bad mark
• Assuming git resets automatically after marking
• Believing marking good then bad consecutively causes error

4. You started a bisect session but accidentally marked a good commit as bad. What is the best way to fix this?

medium

A. Edit the .git/BISECT_LOG file manually to fix the mistake

B. Run git bisect skip on the wrong commit

C. Run git bisect good on the same commit to correct it

D. Run git bisect reset and start over

Solution

1. Step 1: Understand bisect marking correction

   Git bisect allows re-marking a commit to correct its status if marked incorrectly.

2. Step 2: Correct the marking

   Run git bisect good on the commit you accidentally marked as bad to update the bisect state.

3. Final Answer:

   Run git bisect good on the same commit to correct it -> Option C

4. Quick Check:

   Re-mark good/bad to correct [OK]

Hint: Re-mark the commit with git bisect good to fix [OK]

Common Mistakes:

• Thinking you must reset instead of re-marking
• Editing internal git files manually
• Using skip to fix a marking error

5. You have a large project with 1000 commits. You know commit abc123 is good and def456 is bad. You want to find the first bad commit using git bisect. Which sequence of commands correctly performs this task?

hard

A. git bisect start def456 abc123; git bisect bad def456; git bisect good abc123; test commits; git bisect reset

B. git bisect start abc123 def456; git bisect bad abc123; git bisect good def456; test commits; git bisect reset

C. git bisect start def456 abc123; git bisect good def456; git bisect bad abc123; test commits; git bisect reset

D. git bisect start abc123 def456; git bisect good def456; git bisect bad abc123; test commits; git bisect reset

Solution

1. Step 1: Start bisect with bad then good commit

   The correct syntax is git bisect start <bad> <good>. Since def456 is bad and abc123 is good, the command is git bisect start def456 abc123.

2. Step 2: Mark the known commits correctly

   Mark the bad commit with git bisect bad def456 and the good commit with git bisect good abc123. Then test intermediate commits as git suggests.

3. Step 3: Finish bisect session

   After finding the first bad commit, run git bisect reset to return to the original branch.

4. Final Answer:

   git bisect start def456 abc123; git bisect bad def456; git bisect good abc123; test commits; git bisect reset -> Option A

5. Quick Check:

   Start with bad then good, mark correctly, reset [OK]

Hint: Start bisect with bad then good commit, mark accordingly [OK]

Common Mistakes:

• Reversing good and bad commits in start command
• Marking commits with wrong status
• Skipping reset after bisect finishes