Learning Mercurial in Workflows

With Mercurial you can use a multitude of different workflows. This page shows some of them, including their use cases. It is intended to make it easy for beginners of version tracking to get going instantly and learn completely incrementally. It doesn't explain the concepts used, because there are already many other great resources doing that.

Alternatives to this guide and further reading:

• Tutorial - a more exhaustive tutorial.
• Mercurial: The Definitive Guide - a very detailed description of Mercurial including behind the scenes, an indepth article on the design of Mercurial.
• Understanding Mercurial - the concepts behind Mercurial.

Basic workflows

We go from simple to more complex workflows. Those further down build on previous workflows.

Log keeping

Use Case

The first workflow is also the easiest one: You want to use Mercurial to be able to look back when you did which changes.

This workflow only requires an installed Mercurial and write access to some file storage (you almost definitely have that :) ). It shows the basic techniques for more complex workflows.

Workflow

Prepare Mercurial

As first step, you should teach Mercurial your name. For that you open the file ~/.hgrc (or mercurial.ini in your home directory for Windows) with a text-editor and add the ui section (user interaction) with your username:

[ui]
username = Mr. Johnson <johnson@smith.com>

Initialize the project

Now you add a new folder in which you want to work:

$ hg init project

Add files and track them

Enter the project folder, create some files, then add and commit them.

$ cd project
$ echo 'print("Hello")' > hello.py
$ hg add
$ hg commit
(your default editor opens,
 enter the commit message,
 save and close.)

$ hg commit -m "[MESSAGE]"

You can then look into your initial history with hg log:

$ hg log

$ hg log -v

$ cd project
$ hg init

$ hg add file0* file10 file11 file12

Save changes

First do some changes:

$ echo 'print("Hello World")' > hello.py

see which files changed, which have been added or removed, and which aren't tracked yet

$ hg status

see the exact changes

$ hg diff

commit the changes.

$ hg commit

now an editor pops up and asks you for a commit message. Upon saving and closing the editor, your changes have been stored by Mercurial.

Your history now looks like this:

Copy and move files

When you copy or move files, you should tell Mercurial to do the copy or move for you, so it can track the relationship between the files.

Remember to commit after moving or copying. From the basic commands only commit creates a new revision

$ hg cp hello.py copy
$ hg mv hello.py target
$ hg diff # see the changes
$ hg commit
(enter the commit message)

Now you have two files, "copy" and "target", and Mercurial knows how they are related.

Check your history

$ hg log

This prints a list of changesets along with their date, the user who committed them (you) and their commit message.

To see a certain revision, you can use the -r switch (--revision). To also see the diff of the displayed revisions, there's the -p switch (--patch)

$ hg log -p -r 3

Lone developer with nonlinear history

Use case

The second workflow is still very easy: You're a lone developer and you want to use Mercurial to keep track of your own changes and optimize your workflow.

It works just like the log keeping workflow, with the difference that you go back to earlier changes at times and work onwards from there.

To start a new project, you initialize a repository, add your files and commit whenever you finished a part of your work.

Also you check your history from time to time, so see how you progressed.

Workflow

Basics from log keeping

Init your project, add files, see changes and commit them.

$ hg init project
$ cd project
$ (add files)
$ hg add # tell Mercurial to track all files
$ (do some changes)
$ hg diff # see changes
$ hg commit # save changes
$ hg cp # copy files or folders
$ hg mv # move files or folders
$ hg log # see history

Seeing an earlier revision

Different from the log keeping workflow, you'll want to go back in history at times and do some changes directly there, for example because an earlier change introduced a bug and you want to fix it where it occurred.

To look at a previous version of your code, you can use update. Let's assume that you want to see revision 1.

$ hg update 1

Now your code is back at revision 1, the second commit (Mercurial starts counting at 0). To check if you're really at that revision, you can use identify -n.

$ hg identify -n

To update to the most recent revision, you can use "tip" as revision name.

$ hg update tip

Fixing errors in earlier revisions

When you find a bug in some earlier revision you have two options: either you can fix it in the current code, or you can go back in history and fix the code exactly where you did it, which creates a cleaner history.

To do it the cleaner way, you first update to the old revision, fix the bug and commit it. Afterwards you merge this revision and commit the merge. Don't worry, though: Merging in mercurial is fast and painless, as you'll see in an instant.

Let's assume the bug was introduced in revision 1.

$ hg update 1
$ echo 'print("Hello Mercurial")' > hello.py
$ hg commit

Now the fix is already stored in history. We just need to merge it with the current version of your code.

$ hg merge

If there are conflicts use hg resolve - that's also what merge tells you to do in case of conflicts.

First list the files with conflicts

$ hg resolve --list

Then resolve them one by one. resolve attempts the merge again

$ hg resolve conflicting_file
(fix it by hand, if necessary)

Mark the fixed file as resolved

$ hg resolve --mark conflicting_file

Commit the merge, as soon as you resolved all conflicts. This step is also necessary when there were no conflicts!

$ hg commit

At this point, your fix is merged with all your other work, and you can just go on coding. Additionally the history shows clearly where you fixed the bug, so you'll always be able to check where the bug was.

So now you can initialize repositories, save changes, update to previous changes and develop in a nonlinear history by committing in earlier changesets and merging the changes into the current code.

Separate features

Use Case

At times you'll be working on several features in parallel. If you want to avoid mixing incomplete code versions, you can create clones of your local repository and work on each feature in its own code directory.

After finishing your feature you then pull it back into your main directory and merge the changes.

Workflow

Work in different clones

First create the feature clone and do some changes

$ hg clone project feature1
$ cd feature1
$ hg update 3
$ echo 'print("Hello feature1")' > hello.py
$ hg commit -m "Greet feature1"

Now check what will come in when you pull from feature1, just like you can use diff before committing. The respective command for pulling is incoming

$ cd ../project
$ hg incoming ../feature1

If you like the changes, you pull them into the project

$ hg pull ../feature1

Now you have the history of feature1 inside your project, but the changes aren't yet visible. Instead they are only stored inside a ".hg" directory of the project (more information on the store).

If you didn't do any changes in the project, while you were working on feature1, you can just update to tip (hg update tip), but it is more likely that you'll have done some other changes in between changes. In that case, it's time for merging.

Merge feature1 into the project code

$ hg merge

If there are conflicts use hg resolve - that's also what merge tells you to do in case of conflicts. After you merge, you have to commit explicitly to make your merge final

$ hg commit
(enter commit message, for example "merged feature1")

You can create an arbitrary number of clones and also carry them around on USB sticks. Also you can use them to synchronize your files at home and at work, or between your desktop and your laptop.

Rollback mistakes

Now you can work on different features in parallel, but from time to time a bad commit might sneak in. Naturally you could then just go back one revision and merge the stray error, keeping all mistakes out of the merged revision. However, there's an easier way, if you realize your error before you do another commit or pull: rollback.

Rolling back means undoing the last operation which added something to your history.

Imagine you just realized that you did a bad commit - for example you didn't see a spelling error in a commit message. To fix it you would use

$ hg rollback

And then redo the commit

$ hg commit -m "Merged Feature 1"

If you can use the command history of your shell and you added the previous message via commit -m "message", that following commit just means two clicks on the arrow-key "up" and one click on "enter".

Though it changes your history, rolling back doesn't change your files. It only undoes the last addition to your history.

But beware, that a rollback itself can't be undone. If you rollback and then forget to commit, you can't just say "give me my old commit back". You have to create a new commit.

Sharing changes

Use Case

Now we go one step further: You are no longer alone, and you want to share your changes with others and include their changes.

The basic requirement for that is that you have to be able to see the changes of others.

Mercurial allows you to do that very easily by including a simple webserver from which you can pull changes just as you can pull changes from local clones.

Workflow

Using the builtin webserver

This is the easiest way to quickly share changes.

First the one who wants to share his changes creates the webserver

$ hg serve

Now all others can point their browsers to his IP address (for example 192.168.178.100) at port 8000. They will then see all his history there and can decide if they want to pull his changes.

$ firefox http://192.168.178.100:8000

If they decide to include the changes, they just pull from the same URL

$ hg pull http://192.168.178.100:8000

At this point you all can work as if you had pulled from a local repository. All the data is now in your individual repositories and you can merge the changes and work with them without needing any connection to the served repository.

Sending changes by email

Often you won't have direct access to the repository of someone else, be it because he's behind a restrictive firewall, or because you live in different timezones. You might also want to keep your changes confidential and prefer internal email (if you want additional protection, you can also encrypt the emails, for example with GnuPG).

In that case, you can easily export your changes as patches and send them by email.

Another reason to send them by email can be that your policy requires manual review of the changes when the other developers are used to reading diffs in emails. I'm sure you can think of more reasons.

Sending the changes via email is pretty straightforward with Mercurial. You just export your changes and attach (or copy paste) it in your email. Your colleagues can then just import them.

First check which changes you want to export

$ cd project
$ hg log

We assume that you want to export changeset 3 and 4

$ hg export 3 > change3.diff
$ hg export 4 > change4.diff

Now attach them to an email and your colleagues can just run import on both diffs to get your full changes, including your user information.

To be careful, they first clone their repository to have an integration directory as sandbox

$ hg clone project integration
$ cd integration
$ hg import change3.diff
$ hg import change4.diff

That's it. They can now test your changes in feature clones. If they accept them, they pull the changes into the main repository

$ cd ../project
$ hg pull ../integration

$ hg bundle --base FIRST_REVISION_TO_BUNDLE changes.bundle

$ hg pull path/to/changes.bundle

Using a shared repository

Sending changes by email might be the easiest way to reach people when you aren't yet part of the regular development team, but it creates additional workload: You have to bundle the changes, send mails and then import the bundles manually. Luckily there's an easier way which works quite well: The shared push repository.

Till now we transferred all changes either via email or via pull. Now we use another option: pushing. As the name suggests it's just the opposite of pulling: You push your changes into another repository.

But to make use of it, we first need something we can push to.

By default hg serve doesn't allow pushing, since that would be a major security hole. You can allow pushing in the server, but that's no solution when you live in different timezones, so we'll go with another approach here: Using a shared repository, either on an existing shared server or on a service like BitBucket. Doing so has a bit higher starting cost and takes a bit longer to explain, but it's well worth the effort spent.

If you want to use an existing shared server, you can use serve there and allow pushing. Also there are some other nice ways to allow pushing to a Mercurial repository, including simple access via SSH.

Otherwise you first need to setup a BitBucket Account. Just signup at BitBucket. After signing up (and login) hover your mouse over "Repositories". There click the item at the bottom of the opening dialog which say "Create new".

Give it a name and a description. If you want to keep it hidden from the public, select "private"

$ firefox http://bitbucket.org

Now your repository is created and you see instructions for pushing to it. For that you'll use a command similar to the following (just with a different URL):

$ hg push https://bitbucket.org/ArneBab/hello/

(Replace the URL with the URL of your created repository. If your username is "Foo" and your repository is named "bar", the URL will be https://bitbucket.org/Foo/bar/)

Mercurial will ask for your BitBucket name and password, then push your code.

Voilà, your code is online.

To see what you would get if you would push, you can use outgoing. It works with local repositories in the same way as with shared ones, so you can test it with a local one:

$ hg outgoing ../feature1

Now it's time to tell all your colleagues to sign up at BitBucket, too.

After that you can click the "Admin" tab of your created repository and add the usernames of your colleagues on the right side under "Permission: Writers". Now they are allowed to push code to the repository.

(If you chose to make the repository private, you'll need to add them to "Permission: Readers", too)

If one of you now wants to publish changes, he'll simply push them to the repository, and all others get them by pulling.

Publish your changes

$ hg push https://bitbucket.org/ArneBab/hello/

Pull others changes into your local repository

$ hg pull https://bitbucket.org/ArneBab/hello/

People who join you in development can also just clone this repository, as if one of you were using hg serve

$ hg clone https://bitbucket.org/ArneBab/hello/ hello

That local repository will automatically be configured to pull/push from/to the online repository, so new contributors can just use hg push and hg pull without an URL.

Summary

Now let's take a step back and look where we are.

With the commands you already know, a bit reading of hg help <command> and some evil script-fu you can already do almost everything you'll ever need to do when working with source code history. So from now on almost everything is convenience, and that's a good thing.

First this is good, because it means, that you can now use most of the concepts which are utilized in more complex workflows.

Second it aids you, because convenience lets you focus on your task instead of focusing on your tool. It helps you concentrate on the coding itself. Still you can always go back to the basics, if you want to.

A short summary of what you can do which can also act as a short check, if you still remember the meaning of the commands.

create a project

$ hg init project
$ cd project
$ (add some files)
$ hg add
$ hg commit
(enter the commit message)

do nonlinear development

$ (do some changes)
$ hg commit
(enter the commit message)
$ hg update 0
$ (do some changes)
$ hg commit
(enter the commit message)
$ hg merge
$ (optionally hg resolve)
$ hg commit
(enter the commit message)

use feature clones

$ cd ..
$ hg clone project feature1
$ cd feature1
$ (do some changes)
$ hg commit
(enter the commit message)
$ cd ../project
$ hg pull ../feature1

share your repository via the integrated webserver

$ hg serve &
$ cd ..
$ hg clone http://127.0.0.1:8000 project-clone

export changes to files

$ cd project-clone
$ (do some changes)
$ hg commit
(enter the commit message)
$ hg export tip > ../changes.diff

import changes from files

$ cd ../project
$ hg import ../changes.diff

pull changes from a served repository (hg serve still runs on project)

$ cd ../feature1
$ hg pull http://127.0.0.1:8000

Use shared repositories on BitBucket

$ (setup bitbucket repo)
$ hg push https://bitbucket.org/USER/REPO
(enter name and password in the prompt)
$ hg pull https://bitbucket.org/USER/REPO

Let's move on towards useful features and a bit more advanced workflows.

Advanced workflows

Backing out bad revisions

Use Case

When you routinely pull code from others, it can happen that you overlook some bad change. As soon as others pull that change from you, you have little chance to get completely rid of it.

To resolve that fundamental problem in any really distributed system, Mercurial offers you the backout command. Backing out a change means, that you tell Mercurial to create a commit which reverses the bad change. That way you don't get rid of the bad code in history, but you can remove it from new revisions.

Workflow

Let's assume the bad change was revision 3, and you already have one more revision in your repository. To remove the bad code, you can just backout of it. This creates a new change which reverses the bad change. After backing out, you can then merge that new change into the current code.

$ hg backout 3
$ hg merge
(potentially resolve conflicts)
$ hg commit
(enter commit message. For example: "merged backout")

That's it. You reversed the bad change. It's still recorded that it was once there (following the principle "don't rewrite history, if it's not really necessary"), but it doesn't affect future code anymore.

Collaborative feature development

Now that you can share changes and reverse them if necessary, you can go one step further: Using Mercurial to help in coordinating the coding.

The first part is an easy way to develop features together, without requiring every developer to keep track of several feature clones.

Use Case

When you want to split your development into several features, you need to keep track of who works on which feature and where to get which changes.

Mercurial makes this easy for you by providing named branches. They are a part of the main repository, so they are available to everyone involved. At the same time, changes committed on a certain branch don't get mixed with the changes in the default branch, so features are kept separate, until they get merged into the default branch.

Workflow

When someone in your group wants to start coding on a feature without disturbing the others, he can create a named branch and commit there. When someone else wants to join in, he just updates to the branch and commits away. As soon as the feature is finished, someone merges the named branch into the default branch.

Working in a named branch

Create the branch

$ hg branch feature1
(do some changes)
$ hg commit
(write commit message)

Update into the branch and work in it

$ hg update feature1
(do some changes)
$ hg commit
(write commit message)

Now you can commit, pull, push and merge (and anything else) as if you were working in a separate repository. If the history of the named branch is linear and you call "hg merge", Mercurial asks you to specify an explicit revision, since the branch in which you work doesn't have anything to merge.

Merge the named branch

When you finished the feature, you merge the branch back into the default branch.

$ hg update default
$ hg merge feature1
$ hg commit
(write commit message)

To see the active branches with hg branches. To mark a branch as inactive, for example because you finished implementing the feature, you can close it. Closed branches are hidden in hg branches as well as in hg heads.

$ hg update feature1
$ hg commit --close-branch -m "finished feature1"

And that's it. Now you can easily keep features separate without unnecessary bookkeeping.

Tagging revisions

Use Case

Since you can now code separate features more easily, you might want to mark certain revisions as fit for consumption (or similar). For example you might want to mark releases, or just mark off revisions as reviewed.

For this Mercurial offers tags. Tags add a name to a revision and are part of the history. You can tag a change years after it was committed. The tag includes the information when it was added, and tags can be pulled, pushed and merged just like any other committed change.

Workflow

Let's assume you want to give revision 3 the name "v0.1".

Add the tag

$ hg tag -r 3 v0.1

See all tags

$ hg tags

When you look at the log you'll now see a line in changeset 3 which marks the Tag. If someone wants to update to the tagged revision, he can just use the name of your tag

$ hg update v0.1

Now he'll be at the tagged revision and can work from there.

Removing history

Use Case

At times you will have changes in your repository, which you really don't want in it.

There are many advanced options for removing these, and most of them use great extensions (Mercurial Queues is the most often used one), but in this basic guide, we'll solve the problem with just the commands we already learned. But we'll use an option to clone which we didn't yet need.

This workflow becomes inconvenient when you need to remove changes, which are buried below many new changes. If you spot the them early enough, you can get rid of bad changes without too much effort, though.

Workflow

Let's assume you want to get rid of revision 2 and the highest revision is 3.

The first step is to use the "--rev" option to clone: Create a clone which only contains the changes up to the specified revision. Since you want to keep revision 1, you only clone up to that

$ hg clone -r 1 project stripped

Now you can export the change 3 from the original repository (project) and import it into the stripped one

$ cd project
$ hg export 3 > ../changes.diff
$ cd ../stripped
$ hg import ../changes.diff

If a part of the changes couldn't be applied, you'll see that part in *.rej files. If you have *.rej files, you'll have to include or discard changes by hand

$ cat *.rej
(apply changes by hand)
$ hg commit
(write commit message)

That's it. hg export also includes the commit message, date, committer and similar metadata, so you are already done.

Summary

So now you can work with Mercurial in private, and also share your changes in a multitude of ways.

Additionally you can remove bad changes, either by creating a change in the repository which reverses the original change, or by really rewriting history, so it looks like the change never occurred.

And you can separate the work on features in a single repository by using named branches and add tags to revisions which are visible markers for others and can be used to update to the tagged revisions.

With this we can conclude our practical guide.

More Complex Workflows

If you now want to check some more complex workflows, please have a look at the general workflows wikipage and the list of extensions.

To deepen your understanding, you can also check the basic concept overview.

Have fun with Mercurial!