Version Control with Mercurial: A Better Kind of Backup

Key Points

Automated Version Control	Version control is like an unlimited ‘undo’. Version control also allows many people to work in parallel.
Configuring Mercurial	Use `hg config` to configure a user name, email address, editor, and other preferences once per machine.
Creating a Repository	`hg init` initializes a repository.
Tracking Changes to Files	`hg status` shows the status of a repository. `hg add` puts files in the staging area. `hg commit` saves the staged content as a new commit in the local repository. Always write a log message when committing changes.
Making Changes	`hg diff` displays differences between commits. `hg log` shows the history of a repository.
Exploring History	Use `hg diff` to compare different versions of files.
Recovering Old Versions	Use `hg update` to examine old versions of files.
Ignoring Things	Create a `.hgignore` file to specify things that Mercurial should ignore.
Remote Repositories	Use `hg push` to push changes to remote repositories. Use `hg pull` to pull changes from remote repositories.
Working with Clone Repositories	Use `hg clone` to make a local copy of a remote repository.
Collaboration	Set access to ‘public’ or ‘private’ to control who can see what.
Merging Changes from Different Clones	Use `hg merge` to combine independent changes to the same set of files.
Merge Conflicts	Conflicts occur when two or more people change the same file(s) at the same time. The version control system does not allow people to overwrite each other’s changes blindly, but highlights conflicts so that they can be resolved.
Open Science	Open scientific work is more useful and more highly cited than closed.
Licensing	People who incorporate GPL’d software into their own software must make their software also open under the GPL license; most other open licenses do not require this. The Creative Commons family of licenses allow people to mix and match requirements and restrictions on attribution, creation of derivative works, further sharing, and commercialization. People who are not lawyers should not try to write licenses from scratch.
Hosting	Projects can be hosted on university servers, on personal domains, or on public forges. Rules regarding intellectual property and storage of sensitive information apply no matter where code and data are hosted.

A Better Kind of Backup

Create a ~/.hgrc (or %USERPROFILE%\Mercurial.ini on Windows) to configure a user name, email address, editor, and other preferences once per machine.
hg init initializes a repository.
hg status shows the status of a repository.
Files are stored in a project’s working directory (which users see), and the local repository (where committed snapshots are permanently recorded).
hg add tells Mercurial to track files.
hg commit creates a snapshot of the changes to 1 or more files in the local repository.
Always write a log message when committing changes.
hg diff displays differences between revisions.
hg revert recovers old versions of files.
The .hgignore file tells Mercurial what files to ignore.

Collaborating

A local Mercurial repository can be connected to one or more remote repositories.
Use the HTTPS protocol to connect to remote repositories until you have learned how to set up SSH.
hg push copies changes from a local repository to a remote repository.
hg pull copies changes from a remote repository to a local repository.
hg clone copies a remote repository to create a local repository with a remote called default automatically set up.

Conflicts

Conflicts occur when two or more people change the same file(s) at the same time.
The version control system does not allow people to blindly overwrite each other’s changes. Instead, it highlights conflicts so that they can be resolved.

Open Science

Open scientific work is more useful and more highly cited than closed.
People who incorporate GPL’d software into theirs must make theirs open; most other open licenses do not require this.
The Creative Commons family of licenses allow people to mix and match requirements and restrictions on attribution, creation of derivative works, further sharing, and commercialization.
People who are not lawyers should not try to write licenses from scratch.
Projects can be hosted on university servers, on personal domains, or on public forges.
Rules regarding intellectual property and storage of sensitive information apply no matter where code and data are hosted.

Glossary

changeset: A group of changes to one or more files that are committed to a version control repository in a single operation.
clone: To make a local copy of a version control repository. See also: fork.
commit: To record the current state of a set of files (a changeset) in a version control repository.
conflict: A change made by one user of a version control system that is incompatible with changes made by other users. Helping users resolve conflicts is one of version control’s major tasks.
fork: To clone a version control repository on a server.
HTTP: The Hypertext Transfer Protocol used for sharing web pages and other data on the World Wide Web.
infective license: A license such as the GPL that compels people who incorporate material into their own work to place similar sharing requirements on it.
Internet Service Provider (ISP): A company or other organization that provides access to the Internet. Many ISPs provide hosting services for websites.
merge: To reconcile two sets of changes to a repository.
protocol: A set of rules that define how one computer communicates with another. Common protocols on the Internet include HTTP and SSH.
remote: A version control repository other than the current one that the current one is somehow connected to or mirroring.
repository: A storage area where a version control system stores old revisions of files and information about who changed what, when.
resolve: To eliminate the conflicts between two or more incompatible changes to a file or set of files being managed by a version control system.
revision: A recorded state of a version control repository.
SSH: The Secure Shell protocol used for secure communication between computers.
timestamp: A record of when a particular event occurred.
version control: A tool for managing changes to a set of files. Each set of changes creates a new revision of the files; the version control system allows users to recover old revisions reliably, and helps manage conflicting changes made by different users.