Git for debugging

Riaan Cornelius

a.k.a Code archaeology

TopicsBecause nobody likes a surprise

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• Code Archaeology• Git Blame• Git log• Pickaxe• Git show

• Example: Jenkins• Debugging with git bisect• Automating git bisect

• Bitbucket code review

Code ArchaeologyUncovering the history of your code

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• All code is always documented.

• Separating the interesting commits from the uninteresting ones.

• Git has tools to make this easier.

Code ArchaeologyUncovering the history of your code

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Git blameAnnotates each line in the given file with information from the revision which last modified the line.

With -w, blame ignores lines where only whitespace changed.With -M, blame detects moved or copied lines within a file.With -C, blame extends this move or copy detection to other files that were modified in the same commit.

Code ArchaeologyUncovering the history of your code

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Git logShows the commit logs.

With --date=short shows only the date, but not the time, in YYYY-MM-DD format.With --graph draws a text-based graphical representation of the commit history on the left hand side of the output. With --pretty you can specify a format.

Code ArchaeologyUncovering the history of your code

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PickaxeFor Detecting Addition/Deletion of Specified String. Actually a git log param: -S‘<block of text>’

When used with --pickaxe-regex, treat the <block of text> as an extended POSIX regular expression to match, instead of a literal string.

Code ArchaeologyUncovering the history of your code

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Git showFor commits it shows the log message and textual diff.

When used with --quiet, Don’t show the diff.With --pretty you can specify a format.

ExampleJenkins

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Uncovering some of Jenkin’s history

Why do we need a Jenkins repoWho wrote that code?

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Git blameWho wrote that code?

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Git showWhat info can we get from the commit?

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Git blameWho wrote that code?

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Git showWhat info can we get from the commit?

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PickaxeDigging deeper

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Git logSeeing the bigger picture

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git log

Git logSeeing the bigger picture

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Git logSeeing the bigger picture

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git log --graph --abbrev-commit --pretty=format:'%Cred%h%Creset –%C(yellow)%d%Creset %s %Cgreen(%cr) %C(bold blue)<%an>%Creset'

Git logMaking it prettier

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Git logAdvanced use case - churn

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git log --all -M -C --name-only --format='format:' "$@" | sort | grep -v '^$' | uniq -c | sort | awk 'BEGIN {print "count\tfile"} {print $1 "\t" $2}' | sort -g

Git bisectHunting down bugs

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Git BisectFind by binary search the change that introduced a bug.

Simple process:• Git bisect start <BAD> <GOOD>• Git bisect good | bad• Repeat until first bad commit found• Git bisect reset

Git bisectHunting down bugs

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Simple example.

After a night of drunken coding, the main.java file is missing. Find out what happened.

Git bisectHunting down bugs

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Git bisectHunting down bugs

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Git bisectHunting down bugs

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Git bisect (automated)Hunting down bugs the easy way

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How do we automate this.

Essentially same steps as previously, but we write a test script to determine whether the commit is good or bad.

• Git bisect start <BAD> <GOOD>• Git bisect run <TEST SCRIPT>• Wait for bad commit to be found• Git bisect reset

Git bisect (automated)Hunting down bugs the easy way

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Git bisect (automated)Hunting down bugs the easy way

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Bitbucket peer reviewWhy?

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Why peer review?

• Improved code quality.• Knowledge transfer.

Bitbucket peer reviewWhy?

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We know we should do it, but polls consistently show only about 50% of teams peer review.

Bitbucket peer reviewHow?

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Bitbucket peer reviewHow?

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Bitbucket peer reviewHow?

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In closingWhat should you take from this?

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Code archaeology• All code is always documented

Learn to use Git more productively• Git blame• Git log• Pickaxe

• Git show• Third party extensions• So many others!

Do peer reviews!

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Questions?

References:

Git man pages: http://git-scm.com/docsCode Archaeology: http://jfirebaugh.github.com/blog/2012/03/07/code-archaeology-with-gitGit tutorial: http://www.vogella.com/tutorials/Git/article.html

XKCD References you might have noticed (Less useful, but definitely more amusing)Ballmer peak: https://xkcd.com/323/Commit messages: https://xkcd.com/1296/