Little toy git-bisect example

README.md

What's this?

This is a very contrived way to show off what git-bisect is good for.

How does it work?

This will create a file called test_file and then do a bunch of commits with one "bad" one randomly inserted somewhere in the middle. And the "bad" commit involves a deletion, so you can't find it using a simple git blame on the file.

Example

./generate_commits.rb
git bisect bad
git bisect good $(git log --oneline | grep "starting state" | cut -d ' ' -f 1)
grep teapot test_file
git bisect [good|bad]
grep teapot test_file
git bisect [good|bad]
...

And based on the result of grep teapot test_file, mark the commit as good or bad until you find the right commit.

You can also just do git bisect run grep teapot test_file and it'll go much more quickly and automatically.

Note

This example has been contrived to make it impossible to use git blame instead of needing to bisect, but it still doesn't require a bisect. You can just do git log -p -G teapot instead. I know this. I just made this to show off how bisect works, and didn't put in the extra work to show where you need to bisect.

generate_commits.rb

#!/usr/bin/env ruby

FILENAME = "test_file"

TOTAL_COMMITS = 100

COMMIT_MESSAGE = "totally innocuous commit"

INITIAL_TEXT = <<~EOT
  I'm a little teapot
  short and stout
  here is my handle
  here is my spout
EOT

WORD_LIST = %w[when I get all steamed up hear me shout]

def initial_commit
  File.open(FILENAME, mode="w") do |f|
    f.puts INITIAL_TEXT
  end

  system("git", "add", "--all")
  system("git", "commit", "--all", "-m", "starting state")
end

def blank_commit
  system("git", "commit", "--allow-empty", "-m", COMMIT_MESSAGE)
end

def adding_commit
  line = WORD_LIST.shuffle.first(5)
  File.open(FILENAME, mode="a") do |f|
    f.puts line.join(" ")
  end
  system("git", "commit", "--all", "-m", COMMIT_MESSAGE)
end

def removing_commit
  lines = IO.readlines(FILENAME)
  File.open(FILENAME, mode="w") do |f|
    f.print lines[0..-2].join()
  end
  system("git", "commit", "--all", "-m", COMMIT_MESSAGE)
end

def bad_commit
  lines = IO.readlines(FILENAME)
  File.open(FILENAME, mode="w") do |f|
    f.print lines[1..-1].join()
  end
  system("git", "commit", "--all", "-m", COMMIT_MESSAGE)
end

initial_commit

x = rand((TOTAL_COMMITS/10) .. (TOTAL_COMMITS/5))
bad_commit_index = rand(x .. (TOTAL_COMMITS-x))

1.upto(TOTAL_COMMITS) do |i|

  if i == bad_commit_index
    bad_commit
    next
  end

  if rand(2).even?
    adding_commit
  elsif rand(2).even?
    removing_commit
  else
    blank_commit
  end

end