Where did this problem come from?¶

Let's find the commit that created the file src/version-control/what-is-a-repository.md. We could find this out using git log, but the point here is to illustrate how to use a script to find the commit that causes any arbitrary change to our repository.

Once the commit has been found, you can inspect it (using git show <commit>) to see all of the changes this commit introduced and the commit message that (hopefully) explains the reasons why this commit was made. This is one way in which your commit messages can act as a lab book.

Create a Python script called my_test.py with the following contents:

#!/usr/bin/env python3
from pathlib import Path
import sys

expected_file = Path('src') / 'version-control' / 'what-is-a-repository.md'

if expected_file.exists():
    # This file is the "new" thing that we want to detect.
    sys.exit(1)
else:
    # The file does not exist, this commit is "old".
    sys.exit(0)

For reference, here is an equivalent R script:

#!/usr/bin/Rscript --vanilla

expected_file <- file.path('src', 'version-control', 'what-is-a-repository.md')

if (file.exists(expected_file)) {
    # This file is the "new" thing that we want to detect.
    quit(status = 1)
} else {
    # The file does not exist, this commit is "old".
    quit(status = 0)
}

Select the commit range over which to search. We know that the file exists in the commit 3dfff1f (Add notes about committing early and often), and it did not exist in the very first commit (5a19b02).
Instruct Git to start searching with the following command:
```
git bisect start 3dfff1f 5a19b02
```
Note that we specify the newer commit first, and then the older commit.

Git will inform you about the search progress, and which commit is currently being investigated.
```
Bisecting: 7 revisions left to test after this (roughly 3 steps)
[92f1375db21dd8a35ca141365a477b963dbbf6dc] Add CC-BY-SA license text and badge
```

Instruct Git to use the script my_test.py to check each commit with the following command:

git bisect run ./my_test.py

It will continue to report the search progress and automatically identify the commit that we're looking for:

running  './my_test.py'
Bisecting: 3 revisions left to test after this (roughly 2 steps)
[9be780b8785d67ee191b2c0b113270059c9e0c3a] Briefly describe key version control concepts
running  './my_test.py'
Bisecting: 1 revision left to test after this (roughly 1 step)
[055906f28da146a2d012b7c1c0e4707503ed1b11] Display example commit message as plain text
running  './my_test.py'
Bisecting: 0 revisions left to test after this (roughly 0 steps)
[1251357ab5b41d511deb48cd5386cae37eec6751] Rename the "What is a repository?" source file
running  './my_test.py'
1251357ab5b41d511deb48cd5386cae37eec6751 is the first bad commit
commit 1251357ab5b41d511deb48cd5386cae37eec6751
Author: Rob Moss <robm.dev@gmail.com>
Date:   Sun Apr 17 21:41:43 2022 +1000

    Rename the "What is a repository?" source file

    The file name was missing the word "a" and did not match the title.

 src/SUMMARY.md                              |  2 +-
 src/version-control/what-is-a-repository.md | 18 ++++++++++++++++++
 src/version-control/what-is-repository.md   | 18 ------------------
 3 files changed, 19 insertions(+), 19 deletions(-)
 create mode 100644 src/version-control/what-is-a-repository.md
 delete mode 100644 src/version-control/what-is-repository.md

To quit the search and return to your current commit, run the following command:
```
git bisect reset
```
You can then inspect this commit by running the following command:
```
git show 1251357
```