A change log is a file which contains a curated, chronologically ordered list of notable changes for each version of an open source project.

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To make it easier for users and contributors to see precisely what notable changes have been made between each release (or version) of the project.

Because software tools are for people. If you don’t care, why are you contributing to open source? Surely, there must be a kernel (ha!) of care somewhere in that lovely little brain of yours.

I talked with Adam Stacoviak and Jerod Santo on The Changelog (fitting, right?) podcast about why open source maintainers and contributors should care, and the motivations behind this project. If you can spare the time (1:06), it’s a good listen.

I’m glad you asked.

A good change log sticks to these principles:

• It’s made for humans, not machines, so legibility is crucial.
• Easy to link to any section (hence Markdown over plain text).
• One sub-section per version.
• List releases in reverse-chronological order (newest on top).
• Write all dates in YYYY-MM-DD format. (Example: 2012-06-02 for June 2nd, 2012.) It’s international, sensible, and language-independent.
• Explicitly mention whether the project follows Semantic Versioning.
• Each version should:
  • List its release date in the above format.
  • Group changes to describe their impact on the project, as follows:
    • Added for new features.
    • Changed for changes in existing functionality.
    • Deprecated for once-stable features removed in upcoming releases.
    • Removed for deprecated features removed in this release.
    • Fixed for any bug fixes.
    • Security to invite users to upgrade in case of vulnerabilities.

Always have an "Unreleased" section at the top for keeping track of any changes.

This serves two purposes:

• People can see what changes they might expect in upcoming releases
• At release time, you just have to change "Unreleased" to the version number and add a new "Unreleased" header at the top.

Alright…let’s get into it.

• Dumping a diff of commit logs. Just don’t do that, you’re helping nobody.
• Not emphasizing deprecations. When people upgrade from one version to another, it should be painfully clear when something will break.
• Dates in region-specific formats. Americans put the month first ("06-02-2012" for June 2nd, 2012, which makes no sense), while Brits use a robotic-looking "2 June 2012", yet pronounce it differently. "2014-06-02" works logically from largest to smallest, and doesn't overlap in ambiguous ways with other date formats, and thus is the recommended date format for change logs.

There’s more. Help me collect those unicorn tears by opening an issue or a pull request.

Sadly, no. But I want to change that.

This project contains what I hope will become the standard CHANGELOG file for all open source projects. Take a look at it, and please suggest improvements!

Well, if you can’t tell from the example above, CHANGELOG.md is the best convention so far.

Some projects also use HISTORY.txt, HISTORY.md, History.md, NEWS.txt, NEWS.md, News.txt, RELEASES.txt, RELEASE.md, releases.md, etc.

It’s a mess. All these names only makes it harder for people to find it.

Because log diffs are full of noise. Can we really expect every single commit in an open source project to be meaningful and self-explanatory? That seems like a pipe dream.

It’s difficult, because people follow wildly different formats and file names.

Vandamme is a Ruby gem created by the Gemnasium team and which parses many (but not all) open source project change logs.

"CHANGELOG" is the name of the file itself. It's a bit shouty but it's a historical convention followed by many open source project. Other examples of similar files include README, LICENSE, and CONTRIBUTING.

The uppercase naming (which in old operating systems made these files stick to the top) is used to draw attention to them. Since they're important metadata about the project, they could be useful to anyone intending to use or contribute to it, much like open source project badges.

When I refer to a "change log", I'm talking about the function of this file: to log changes.

Yanked releases are versions that had to be pulled because of a serious bug or security issue. Often these versions don't even appear in change logs. They should. This is how you should display them:

## 0.0.5 - 2014-12-13 [YANKED]

The [YANKED] tag is loud for a reason. It's important for people to notice it. Since it's surrounded by brackets it's also easier to parse programmatically.

This document is not the truth; it’s my carefully considered opinion, along with information and examples I gathered. Although I provide an actual CHANGELOG on the GitHub repo, I have purposefully not created a proper release or clear list of rules to follow (as SemVer.org does, for instance).

This is because I want our community to reach a consensus. I believe the discussion is as important as the end result.

So please pitch in.