debug with binary search

The bisect tool helps to identify the commit that introduced a bug.

To start, you need to tell git when the code last ran without problems:

$ git bisect start
$ git bisect bad
# current commit has a bug
$ git bisect good v2.1
# v2.1 passes the test

git will then check out the commit that is halfway between the good and bad commits. You test the code and if there was no problem:

$ git bisect good
# test halfway between middle 
# and bad commit

If there was a problem:

$ git bisect bad
# test halfway between middle
# and good commit

You continue doing this until git identifies the first bad commit.

When you finish you need to reset to the original state:

$ git bisect reset