Git Bisect Demo

squares.py just spits out the square of an integer passed in as an argument. The function of the script is irrelevant as we are really looking at git. Running breakme.sh will create a buggy branch with 1000 dummy commits and 1 commit that introduces a regression in squares.py. This environment can be used to demonstrate the value of the git bisect command.

Create your buggy branch

Keep in mind this will blow away any local branch named buggy.

$ git clone https://github.com/shawnsi/bisectdemo.git 
$ ./breakme.sh

Bisect

Running squares.py will now show an obvious error.

$ python squares.py 2
Traceback (most recent call last):
  File "squares.py", line 7, in <module>
    print(integer**2)
TypeError: unsupported operand type(s) for ** or pow(): 'str' and 'int'

Now we can use git bisect to identify the first commit that introduced the regression. Start this bisection then indicate known good and bad commits.

$ git bisect start
$ git bisect good master
$ git bisect bad HEAD

Test the current commit and tell git whether its good or bad. The order of commands below is just an example. Your individual bisection may proceed differently.

$ python squares.py 2
4
$ git bisect good
$ python squares.py 2
Traceback (most recent call last):
  File "squares.py", line 7, in <module>
    print(integer**2)
TypeError: unsupported operand type(s) for ** or pow(): 'str' and 'int'
$ git bisect bad

Eventually git will widdle the commit log down to one commit that introduced the regression.

$ git bisect good
15644003d085cf1a35e2c4c4af654d9e386f811a is the first bad commit
commit 15644003d085cf1a35e2c4c4af654d9e386f811a
Author: Shawn Siefkas <[email protected]>
Date:   Thu Nov 14 09:23:55 2013 -0600

    Breaking argument type

:100644 100644 ff0746a9f0ac02303bd87ed7c3d1571776cbf28d ac23d6bb695e1563edbd3a3fcc0079e1be1793ae M      squares.py

Automatic Bisection

This example can be further condensed via git bisect run:

$ git bisect start
$ git bisect good master
$ git bisect bad buggy
$ git bisect run python squares.py 2