How to use the command 'git bisect' (with examples)
The git bisect command is an invaluable tool in a developer’s toolkit, particularly when dealing with elusive bugs that have crept into a codebase. It leverages a binary search algorithm to efficiently locate the specific commit that introduced an issue. By creatively navigating through your Git commit history, git bisect allows you to pinpoint the problematic commit without having to inspect each one manually. This not only saves time but also ensures a systematic approach to troubleshooting, minimizing the potential for oversight.
Use case 1: Start a bisect session on a commit range bounded by a known buggy commit, and a known clean (typically older) one
Code:
git bisect start bad_commit good_commit
Motivation:
When faced with a bug that appears somewhere in the history of your project, it can be daunting to manually inspect each commit to find the exact point where things went wrong. This command sets up a bisect session by marking the boundary of your search: the “bad” commit where the bug is present and an earlier “good” commit where the code functioned correctly. This lays the groundwork for an efficient traversal of your commit history, dramatically reducing the number of individual commits you have to check, enabling you to focus your debugging efforts more precisely and conserve precious time and energy.
Explanation:
git bisect start: This initializes the bisect operation. It essentially signals Git that you’re starting a binary search for a commit.bad_commit: This argument specifies the most recent commit where the bug is known to exist. This acts as the upper boundary in your search.good_commit: This sets the lower boundary in your search, corresponding to a commit where the code was functioning as expected, with no signs of the bug at hand.
Example Output:
Bisecting: 4 revisions left to test after this (roughly 2 steps)
This output indicates that Git has determined there are roughly four more revisions to examine before it can isolate the faulty commit. Each “step” helps halve the search space, illustrating the efficiency of binary search in this context.
Use case 2: For each commit that git bisect selects, mark it as “bad” or “good” after testing it for the issue
Code:
git bisect good|bad
Motivation:
Once your bisect session is underway, Git will automatically check out a commit in the specified range. It becomes crucial to determine whether this particular commit exhibits the bug or not. Marking commits as “good” or “bad” based on tests provides essential feedback to the git bisect algorithm, allowing it to advance intelligently through the commit history. This interactive process is vital for refinement, as it continuously narrows down the search space until the problematic commit is identified.
Explanation:
git bisect good: This tells Git that the currently checked-out commit is free of the bug. Consequently, Git will search backward from this commit, excluding it from future checks.git bisect bad: This indicates that the bug is present in the current commit, prompting Git to continue the search towards earlier commits, including this one in the problem area.
Example Output when marking a commit as “bad”:
Bisecting: 2 revisions left to test after this (roughly 1 step)
Example Output when marking a commit as “good”:
Bisecting: 3 revisions left to test after this (roughly 1 step)
Both outputs reflect the progress in your binary search, showing how the space of potential problematic commits is continuously being narrowed.
Use case 3: After git bisect pinpoints the faulty commit, end the bisect session and return to the previous branch
Code:
git bisect reset
Motivation:
Upon successfully identifying the commit that introduced the bug, it’s essential to conclude the bisect session properly. This command returns you to your previous branch and working state, ensuring no lingering changes from the bisect process affect your ongoing development. Effectively, this tidies up your Git environment, restoring the repository to a known good state post-diagnosis, ready for further development or testing once the identified bug is implemented or resolved.
Explanation:
git bisect reset: It finalizes the bisect operation, effectively “cleaning up” after the series of automated checkouts performed during the bisect process. Without this, your current HEAD may remain at the problematic commit, potentially leading to confusion or erroneous modifications.
Example Output:
Previous HEAD position was 1234567... Initial commit message
Switched to branch 'main'
This output shows that git bisect reset has returned the working directory to its original state, ensuring the impact of the bisect operation is fully undone and allowing normal development activities to resume.
Use case 4: Skip a commit during a bisect (e.g., one that fails the tests due to a different issue)
Code:
git bisect skip
Motivation:
Sometimes, external factors such as merge conflicts, or tests that fail for reasons unrelated to the bug being hunted, can disrupt a bisect session. When encountering such commits, it’s often best to bypass them, ensuring the bisect process continues smoothly. This command allows developers to exclude problematic or irrelevant commits, keeping the bisect session on track and focused solely on identifying the commit that caused the bug.
Explanation:
git bisect skip: This command tells Git to disregard the currently checked-out commit, whether due to its failure being unrelated to the main bug, merge conflicts, or a lack of sufficient information to properly assess it. By doing so, it nudges the bisect session past these roadblocks, allowing the search for the true offending commit to continue without bias.
Example Output:
Bisecting: 3 revisions left to test after this (roughly 1 step)
The output reflects that Git has managed to work around skipped commits without disrupting the bisect process, maintaining the search within the boundaries set by remaining good and bad markings.
Use case 5: Display a log of what has been done so far
Code:
git bisect log
Motivation:
Tracking the progress and decisions made during a bisect session is crucial to ensure a structured approach is being taken and to review or share the diagnostic journey with collaborators. By using this command, developers can review which commits were deemed “good”, which were marked “bad”, and any skips that occurred. This transparency not only improves collaboration and documentation but can also be an invaluable debugging resource if the session needs to be revisited or if an error is suspected in the bisect process itself.
Explanation:
git bisect log: This command retrieves and displays a chronological record of all the actions taken during the current bisect session — from the noted starting bad and good commits, decisions made for each tested commit, to any skips. It acts essentially like an audit trail, capturing the steps taken to identify the problematic commit.
Example Output:
git bisect start
# bad: [1234567] Commit message indicating bug presence
git bisect bad 1234567
# good: [abcdef0] Prior commit message indicating no issue
git bisect good abcdef0
# bad: [89abcde] Another tested commit deemed faulty
git bisect bad 89abcde
...
This output provides a clear, concise snapshot of the bisect session’s evolution, documenting the journey from start to conclusion, or next step, in resolving the identified bug.
Conclusion:
By following these use cases, developers can efficiently utilize git bisect to isolate troublesome commits within a project’s history. Each example highlights a specific functionality of the command, empowering users to employ this binary search methodology to not only find and document the source of bugs with precision but also navigate around potential hindrances in a structured and smart manner. Utilizing git bisect judiciously can significantly enhance a developer’s capacity to trace and resolve coding issues, streamlining the debugging process across various project landscapes.
