git bisect run for automated bisect - Time & Space Complexity

Time Complexity: git bisect run for automated bisect

O(log n)

Understanding Time Complexity

When using git bisect run, we want to know how the time to find a bad commit grows as the number of commits increases.

We ask: How many steps does it take to find the problem as the commit list gets bigger?

Scenario Under Consideration

Analyze the time complexity of this git bisect automation snippet.

git bisect start
git bisect bad HEAD
git bisect good v1.0

git bisect run ./test_script.sh

# test_script.sh returns 0 if good, 1 if bad

This code starts bisecting between a known good and bad commit, then runs a test script automatically on each commit to find the first bad one.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Running the test script on selected commits.
• How many times: The test runs on a subset of commits chosen by bisect, reducing the search space each time.

How Execution Grows With Input

Each test run cuts the number of commits to check roughly in half.

Input Size (n commits)	Approx. Test Runs
10	4
100	7
1000	10

Pattern observation: The number of test runs grows slowly, about doubling the commits adds only one more test run.

Final Time Complexity

Time Complexity: O(log n)

This means the number of test runs grows slowly as the number of commits grows, making bisect efficient.

Common Mistake

[X] Wrong: "git bisect runs the test on every commit one by one."

[OK] Correct: Bisect smartly picks commits to test, cutting the search space in half each time, so it does not test all commits.

Interview Connect

Understanding how bisect reduces work helps you explain efficient problem-solving in code history, a useful skill in real projects.

Self-Check

"What if the test script takes longer as the commit list grows? How would that affect the overall time complexity?"

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