git bisect run for automated bisect - Step-by-Step Execution

Process Flow - git bisect run for automated bisect

Start git bisect

↓

Mark bad commit

↓

Mark good commit

↓

Git picks midpoint commit

↓

Run test script automatically

↓

Script returns 0 (good) or non-0 (bad)

↓

Git marks commit good/bad based on script

↓

Repeat until first bad commit found

↓

Bisect ends with culprit commit

This flow shows how git bisect uses an automated test script to find the first bad commit by repeatedly testing midpoint commits.

Execution Sample

Git

git bisect start
git bisect bad
git bisect good v1.0
git bisect run ./test_script.sh

This sequence starts bisect, marks bad and good commits, then runs an automated test script to find the bad commit.

Process Table

Step	Commit Tested	Test Script Result	Git Marks	Next Action
1	commit_8f3a	0 (good)	good	Test midpoint commit_4b2c
2	commit_4b2c	1 (bad)	bad	Test midpoint commit_6d7e
3	commit_6d7e	1 (bad)	bad	Test midpoint commit_7a9f
4	commit_7a9f	0 (good)	good	Test midpoint commit_6f1b
5	commit_6f1b	1 (bad)	bad	Bisect complete
6	-	-	-	First bad commit is commit_6f1b

💡 Bisect ends when only one bad commit remains between good and bad marks.

Status Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
Current Commit	none	commit_8f3a	commit_4b2c	commit_6d7e	commit_7a9f	commit_6f1b	commit_6f1b
Test Result	none	0	1	1	0	1	1
Good Commits	v1.0	v1.0, commit_8f3a	v1.0, commit_8f3a	v1.0, commit_8f3a	v1.0, commit_8f3a, commit_7a9f	v1.0, commit_8f3a, commit_7a9f	v1.0, commit_8f3a, commit_7a9f
Bad Commits	HEAD	HEAD	HEAD, commit_4b2c	HEAD, commit_4b2c, commit_6d7e	HEAD, commit_4b2c, commit_6d7e	HEAD, commit_4b2c, commit_6d7e, commit_6f1b	HEAD, commit_4b2c, commit_6d7e, commit_6f1b

Key Moments - 3 Insights

Why does git bisect test the midpoint commit instead of testing commits sequentially?

Git bisect uses a binary search approach to quickly narrow down the first bad commit by testing the midpoint between known good and bad commits, as shown in steps 1-5 of the execution_table.

What does the test script's exit code mean during 'git bisect run'?

The test script returns 0 if the commit is good and non-zero if bad. Git uses this to mark commits accordingly, as seen in the 'Test Script Result' column of the execution_table.

How does git bisect know when to stop testing commits?

Git bisect stops when it narrows down to the first bad commit between good and bad marks, indicated in the last row of the execution_table where bisect completes.

Visual Quiz - 3 Questions

Look at the execution_table at Step 3, what was the test script result for commit_6d7e?

A 0 (good)

B Script did not run

C 1 (bad)

D Unknown

💡 Hint

Check the 'Test Script Result' column for Step 3 in the execution_table.

At which step does git bisect mark commit_7a9f as good?

A Step 2

B Step 4

C Step 5

D Step 1

💡 Hint

Look at the 'Git Marks' column in the execution_table for commit_7a9f.

If the test script returned 0 for commit_6f1b instead of 1, what would happen next?

A Git would mark commit_6f1b as good and continue bisecting

B Git would mark commit_6f1b as bad and stop bisecting

C Git would stop bisecting immediately

D Git would ignore the result and test another commit

💡 Hint

Refer to how git marks commits based on test script results in the execution_table.

Concept Snapshot

git bisect run automates finding a bad commit by running a test script.
Start bisect with 'git bisect start', mark bad and good commits.
Use 'git bisect run <script>' to test commits automatically.
Script exit code 0 means good, non-zero means bad.
Git bisect uses binary search to find the first bad commit quickly.

Full Transcript

Git bisect run automates the process of finding the first bad commit by running a test script on each commit. The process starts by marking a known bad commit and a known good commit. Git then tests the midpoint commit by running the script. If the script returns 0, the commit is marked good; if non-zero, it is marked bad. Git repeats this binary search until it finds the first bad commit. This method speeds up debugging by automating testing and narrowing down the culprit commit efficiently.

Practice

1. What is the main purpose of git bisect run in debugging?

easy

A. To manually mark commits as good or bad during bisect

B. To automate the process of finding the first bad commit using a test script

C. To reset the repository to the latest commit

D. To merge two branches automatically

Solution

1. Step 1: Understand the role of git bisect run

   This command automates the bisect process by running a test script on each commit to check if it is good or bad.

2. Step 2: Compare with other options

   Manual marking is done without run, resetting or merging are unrelated to bisect automation.

3. Final Answer:

   To automate the process of finding the first bad commit using a test script -> Option B

4. Quick Check:

   Automation with test script = B [OK]

Hint: Remember: git bisect run automates testing commits [OK]

Common Mistakes:

• Confusing manual bisect with automated bisect
• Thinking it resets or merges branches
• Assuming it marks commits without a script

2. Which of the following is the correct syntax to start an automated bisect with a test script named test.sh?

easy

A. git bisect start test.sh

B. git bisect automate test.sh

C. git bisect run ./test.sh

D. git bisect test ./test.sh

Solution

1. Step 1: Recall the correct command structure

   The correct syntax to run a test script during bisect is git bisect run <script>.

2. Step 2: Match options with syntax

   Only git bisect run ./test.sh matches the correct syntax with the script path ./test.sh.

3. Final Answer:

   git bisect run ./test.sh -> Option C

4. Quick Check:

   Correct syntax = D [OK]

Hint: Use git bisect run <script> to automate tests [OK]

Common Mistakes:

• Using 'start' instead of 'run'
• Omitting './' before script name
• Using incorrect verbs like 'automate' or 'test'

3. Given the following test script test.sh that exits with 1 if a file bug.txt exists, else exits 0:

#!/bin/sh
if [ -f bug.txt ]; then
  exit 1
else
  exit 0
fi

What will git bisect run ./test.sh interpret if bug.txt is present in a commit?

medium

A. The commit will be skipped

B. The commit is good

C. The bisect will stop with an error

D. The commit is bad

Solution

1. Step 1: Understand exit codes in test script

   The script exits with 1 if bug.txt exists, which means failure (bad commit).

2. Step 2: Interpret exit code in bisect context

   git bisect run treats exit code 0 as good and non-zero as bad, so exit 1 means bad commit.

3. Final Answer:

   The commit is bad -> Option D

4. Quick Check:

   Exit 1 = bad commit = A [OK]

Hint: Exit 0 means good; non-zero means bad in bisect scripts [OK]

Common Mistakes:

• Confusing exit 1 as good
• Assuming script errors stop bisect
• Thinking bisect skips commits on exit 1

4. You run git bisect run ./test.sh but it stops with the error: ./test.sh: Permission denied. What is the most likely cause?

medium

A. The test script is missing execute permission

B. The bisect command is misspelled

C. The repository is not initialized

D. The test script returns exit code 0

Solution

1. Step 1: Analyze the error message

   The error Permission denied means the shell cannot run the script, often due to missing execute permission.

2. Step 2: Check other options

   Bisect command spelling or repo initialization would cause different errors; exit code 0 does not cause this error.

3. Final Answer:

   The test script is missing execute permission -> Option A

4. Quick Check:

   Missing execute permission = A [OK]

Hint: Make script executable with chmod +x before running [OK]

Common Mistakes:

• Ignoring file permissions
• Assuming bisect command typo
• Thinking exit code causes command not found

5. You want to find the first bad commit that introduced a bug detected by a Python test script check_bug.py. The script returns exit code 0 if no bug and 1 if bug found. Which sequence of commands correctly automates bisecting?

hard

A. git bisect start; git bisect bad <bad_commit>; git bisect good <good_commit>; git bisect run python3 check_bug.py

B. git bisect start; git bisect good; git bisect bad <bad_commit>; git bisect run python3 check_bug.py

C. git bisect start; git bisect bad; git bisect good <good_commit>; git bisect run python3 check_bug.py

D. git bisect start; git bisect good <bad_commit>; git bisect bad <good_commit>; git bisect run python3 check_bug.py

Solution

1. Step 1: Understand bisect start sequence

   You must start bisect, mark a known bad commit first, then a known good commit, then run the test script.

2. Step 2: Match the correct order

   git bisect start; git bisect bad <bad_commit>; git bisect good <good_commit>; git bisect run python3 check_bug.py correctly marks bad commit first, then good commit, then runs the script with Python.

3. Final Answer:

   git bisect start; git bisect bad <bad_commit>; git bisect good <good_commit>; git bisect run python3 check_bug.py -> Option A

4. Quick Check:

   Bad then good then run script = C [OK]

Hint: Always mark bad commit first, then good, then run script [OK]

Common Mistakes:

• Swapping good and bad commit order
• Forgetting to start bisect
• Running script before marking commits