Git Pre-Commit Presentation

gitprecommitpresentation.md

What's a Git Hook?

• A hook that's run before (or after) parts of Git commit, push, etc.
• Can reject or modify commits.
• "But I don't use the command line for Git, I use an app."
  • Doesn't matter - your app just uses them anyway!

Many Git-Hooks to Customize Git

• Client-side
  • pre-commit
  • prepare-commit-msg, ...
• Server-side
  • pre-receive, ...

How to Make a Git-Hook?

• Create shell scripts in your_repo/.git/hooks
• Git will launch them at the right time
• It's code. So keep it under git control!
  • (Yes, keeping git hooks in git made my head hurt, too. Work through it, you’ll thank me!)

Shell Script? Seriously?

This is BostonRB, not BostonBourne!

How about a shell script that invokes Ruby?

Creating a Ruby-based Git Hook

• Invoke Ruby at a shell script's hashbang
• Run your code
• puts the reasons (if any) for failure
• exit 1 to fail, otherwise pass

Example Shell Script Pre-Commit

My pre-commit file

My pre-commit Looks For...

• "Bad content" in lines I'm changing
• Ensuring relationships between files
  • If adding one of x, make sure that y is touched
• Bad Syntax (almost, I'll get back to that)

What Makes Content "Bad?"

• For me, that’s code that I probably don't want to check in.
• Again: not really bad, just code that I probably don't want to check in.
• Pre-commit is an easily-overridable, local check
• Doesn’t have to be exact
  • The whole point is to save me time and embarrassment.

Woah, that Seems Imprecise!

• That's OK, it's super-easy to override.
  • Do everything in your app, then
  • If app fails to commit, git commit -m "My Message" --no-verify
  • Resume in your app.

Things I Check for

Merge Conflict Markers

• We all eventually check in git merge conflicts.
• So why not look for strings like
  • <<<<<<<<< or
  • >>>>>>>>> or
  • =========
• If this was server-side, I'd be pedantic about line endings, etc. But it's not.

Debugger/Logger Calls

• console.debug, console.log
• binding.pry, binding.remote-pry

TextExpander-Generated Debugging Markers

• I never type in logger lines "by hand," I use TextExpander macros for that.
  • boblogd expands into:
  • Rails.logger.debug "\n\nTMP_DEBUG <insertion_point>\n\n"
  • Thanks for making that generic, Jonathan!
• So, my pre-commit searches for TMP_DEBUG

Private Key Giveaways

• PRIVATE_KEY
• ssh-rsa
• God No!

Other Things I Probably Don’t Want to Commit

• alert
  • Sometimes I do mean to commit a call to alert.
  • But for me this catches more mistakes than it flags "valid" calls. So, awesome!

Slightly More Advanced

If One File is Added, Make Sure Another Changes

• "Main" work project requires changes to production.rb when a JavaScript or CSS asset is added.
• Ensure that if .js, .coffee, .css, .scss, etc. is added, that production.rb is committed.
• Again, not foolproof, but who cares?

Toolchain Integration

Hint: Easy-Peasy

(Mostly) Usable in Tools

• Tools that use git (GitBox, Github, etc.) call client-side hooks "for free."
• Unfortunate idiosyncrasies. Example:
  • If a commit from GitBox fails because of a pre-commit hook, the commit message is deleted from the prompt :-(
  • So, I'm in the habit of Command-A, Command-C before I commit.

A Bad Flaw When Using Tools

• Mac OS X Apps don't know rbenv (or rvm) from Adam.
• So, which version of Ruby does you favorite App run?
• Yeah. So...
  • Ruby version used to run the shell script?
  • Ruby grammar used to validate files?
• There's probably some .plist-based solution.

Git for Git

Keep Your Hooks Under Git Control

• Keep a separate "master" repo for your git hooks.
• symlink them into your working repos.
• See the setup.sh file
• Keep your symlinking script in the git hook repo, too.
• Oww, my head!

By the Way...

• There is no way that you can use these git hook on the git-hooks repo.
• Think about it. Every violation will appear in that file!

Per-Repo Customization

• Some repo's (like my main work repo) have weird requirements.
• Q: How do I reconcile that with the "one master copy" philosophy?
• A: Define constants in per-project /git/config files.
• See the git config doc, then look at my script to see it in use.

Questions?

Contact Me:

• dev@bobgilmore.name
• @bob_gilmore
• Bob on GitHub