The best version of Mercurial for Windows is TortoiseHg, which can be found at http://bitbucket.org/tortoisehg/stable/wiki/Home. This package has no external dependencies; it “just works”. It provides both command line and graphical user interfaces.

Lee Cantey publishes an installer of Mercurial for Mac OS X at http://mercurial.berkwood.com.

Because each Linux distribution has its own packaging tools, policies, and rate of development, it's difficult to give a comprehensive set of instructions on how to install Mercurial binaries. The version of Mercurial that you will end up with can vary depending on how active the person is who maintains the package for your distribution.

To keep things simple, I will focus on installing Mercurial from the command line under the most popular Linux distributions. Most of these distributions provide graphical package managers that will let you install Mercurial with a single click; the package name to look for is mercurial.

SunFreeWare, at http://www.sunfreeware.com, provides prebuilt packages of Mercurial.

Mercurial provides a built-in help system. This is invaluable for those times when you find yourself stuck trying to remember how to run a command. If you are completely stuck, simply run hg help; it will print a brief list of commands, along with a description of what each does. If you ask for help on a specific command (as below), it prints more detailed information.

$ hg help init
hg init [-e CMD] [--remotecmd CMD] [DEST]

create a new repository in the given directory

    Initialize a new repository in the given directory. If the given
    directory does not exist, it is created.

    If no directory is given, the current directory is used.

    It is possible to specify an ssh:// URL as the destination.
    See 'hg help urls' for more information.

options:

 -e --ssh        specify ssh command to use
    --remotecmd  specify hg command to run on the remote side

use "hg -v help init" to show global options

For a more impressive level of detail (which you won't usually need) run hg help -v. The -v option is short for --verbose, and tells Mercurial to print more information than it usually would.

Copying a repository is just a little bit special. While you could use a normal file copying command to make a copy of a repository, it's best to use a built-in command that Mercurial provides. This command is called hg clone, because it makes an identical copy of an existing repository.

$ hg clone https://bitbucket.org/bos/hg-tutorial-hello
destination directory: hello
requesting all changes
adding changesets
adding manifests
adding file changes
added 5 changesets with 5 changes to 2 files
updating working directory
2 files updated, 0 files merged, 0 files removed, 0 files unresolved

One advantage of using hg clone is that, as we can see above, it lets us clone repositories over the network. Another is that it remembers where we cloned from, which we'll find useful soon when we want to fetch new changes from another repository.

If our clone succeeded, we should now have a local directory called hello. This directory will contain some files.

$ ls -l
total 4
drwxrwxr-x 3 bos bos 4096 May  5 06:55 hello
$ ls hello
Makefile  hello.c

These files have the same contents and history in our repository as they do in the repository we cloned.

Every Mercurial repository is complete, self-contained, and independent. It contains its own private copy of a project's files and history. As we just mentioned, a cloned repository remembers the location of the repository it was cloned from, but Mercurial will not communicate with that repository, or any other, unless you tell it to.

What this means for now is that we're free to experiment with our repository, safe in the knowledge that it's a private “sandbox” that won't affect anyone else.

When we take a more detailed look inside a repository, we can see that it contains a directory named .hg. This is where Mercurial keeps all of its metadata for the repository.

$ cd hello
$ ls -a
.  ..  .hg  Makefile  hello.c

The contents of the .hg directory and its subdirectories are private to Mercurial. Every other file and directory in the repository is yours to do with as you please.

To introduce a little terminology, the .hg directory is the “real” repository, and all of the files and directories that coexist with it are said to live in the working directory. An easy way to remember the distinction is that the repository contains the history of your project, while the working directory contains a snapshot of your project at a particular point in history.

As English is a notoriously sloppy language, and computer science has a hallowed history of terminological confusion (why use one term when four will do?), revision control has a variety of words and phrases that mean the same thing. If you are talking about Mercurial history with other people, you will find that the word “changeset” is often compressed to “change” or (when written) “cset”, and sometimes a changeset is referred to as a “revision” or a “rev”.

While it doesn't matter what word you use to refer to the concept of “a changeset”, the identifier that you use to refer to “a specific changeset” is of great importance. Recall that the changeset field in the output from hg log identifies a changeset using both a number and a hexadecimal string.

This distinction is important. If you send someone an email talking about “revision 33”, there's a high likelihood that their revision 33 will not be the same as yours. The reason for this is that a revision number depends on the order in which changes arrived in a repository, and there is no guarantee that the same changes will happen in the same order in different repositories. Three changes a,b,c can easily appear in one repository as 0,1,2, while in another as 0,2,1.

Mercurial uses revision numbers purely as a convenient shorthand. If you need to discuss a changeset with someone, or make a record of a changeset for some other reason (for example, in a bug report), use the hexadecimal identifier.

To narrow the output of hg log down to a single revision, use the -r (or --rev) option. You can use either a revision number or a hexadecimal identifier, and you can provide as many revisions as you want.

$ hg log -r 3
changeset:   3:0272e0d5a517
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Sat Aug 16 22:08:02 2008 +0200
summary:     Get make to generate the final binary from a .o file.

$ hg log -r 0272e0d5a517
changeset:   3:0272e0d5a517
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Sat Aug 16 22:08:02 2008 +0200
summary:     Get make to generate the final binary from a .o file.

$ hg log -r 1 -r 4
changeset:   1:82e55d328c8c
user:        mpm@selenic.com
date:        Fri Aug 26 01:21:28 2005 -0700
summary:     Create a makefile

changeset:   4:2278160e78d4
tag:         tip
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Sat Aug 16 22:16:53 2008 +0200
summary:     Trim comments.

If you want to see the history of several revisions without having to list each one, you can use range notation; this lets you express the idea “I want all revisions between abc and def, inclusive”.

$ hg log -r 2:4
changeset:   2:fef857204a0c
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Sat Aug 16 22:05:04 2008 +0200
summary:     Introduce a typo into hello.c.

changeset:   3:0272e0d5a517
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Sat Aug 16 22:08:02 2008 +0200
summary:     Get make to generate the final binary from a .o file.

changeset:   4:2278160e78d4
tag:         tip
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Sat Aug 16 22:16:53 2008 +0200
summary:     Trim comments.

Mercurial also honours the order in which you specify revisions, so hg log -r 2:4 prints 2, 3, and 4. while hg log -r 4:2 prints 4, 3, and 2.

While the summary information printed by hg log is useful if you already know what you're looking for, you may need to see a complete description of the change, or a list of the files changed, if you're trying to decide whether a changeset is the one you're looking for. The hg log command's -v (or --verbose) option gives you this extra detail.

$ hg log -v -r 3
changeset:   3:0272e0d5a517
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Sat Aug 16 22:08:02 2008 +0200
files:       Makefile
description:
Get make to generate the final binary from a .o file.

If you want to see both the description and content of a change, add the -p (or --patch) option. This displays the content of a change as a unified diff (if you've never seen a unified diff before, see the section called “Understanding patches” for an overview).

$ hg log -v -p -r 2
changeset:   2:fef857204a0c
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Sat Aug 16 22:05:04 2008 +0200
files:       hello.c
description:
Introduce a typo into hello.c.


diff -r 82e55d328c8c -r fef857204a0c hello.c
--- a/hello.c	Fri Aug 26 01:21:28 2005 -0700
+++ b/hello.c	Sat Aug 16 22:05:04 2008 +0200
@@ -11,6 +11,6 @@
 
 int main(int argc, char **argv)
 {
-	printf("hello, world!\n");
+	printf("hello, world!\");
 	return 0;
 }

The -p option is tremendously useful, so it's well worth remembering.

When you try to run hg commit for the first time, it is not guaranteed to succeed. Mercurial records your name and address with each change that you commit, so that you and others will later be able to tell who made each change. Mercurial tries to automatically figure out a sensible username to commit the change with. It will attempt each of the following methods, in order:

If all of these mechanisms fail, Mercurial will fail, printing an error message. In this case, it will not let you commit until you set up a username.

You should think of the HGUSER environment variable and the -u option to the hg commit command as ways to override Mercurial's default selection of username. For normal use, the simplest and most robust way to set a username for yourself is by creating a .hgrc file; see below for details.

Creating a Mercurial configuration file

To set a user name, use your favorite editor to create a file called .hgrc in your home directory. Mercurial will use this file to look up your personalised configuration settings. The initial contents of your .hgrc should look like this.

	Home directory on Windows
	When we refer to your home directory, on an English language installation of Windows this will usually be a folder named after your user name in C:\Documents and Settings. You can find out the exact name of your home directory by opening a command prompt window and running the following command. C:\> echo %UserProfile%

# This is a Mercurial configuration file.
[ui]
username = Firstname Lastname <email.address@example.net>

The “[ui]” line begins a section of the config file, so you can read the “username = ...” line as meaning “set the value of the username item in the ui section”. A section continues until a new section begins, or the end of the file. Mercurial ignores empty lines and treats any text from “#” to the end of a line as a comment.

Choosing a user name

You can use any text you like as the value of the username config item, since this information is for reading by other people, but will not be interpreted by Mercurial. The convention that most people follow is to use their name and email address, as in the example above.

	Note
	Mercurial's built-in web server obfuscates email addresses, to make it more difficult for the email harvesting tools that spammers use. This reduces the likelihood that you'll start receiving more junk email if you publish a Mercurial repository on the web.

When we commit a change, Mercurial drops us into a text editor, to enter a message that will describe the modifications we've made in this changeset. This is called the commit message. It will be a record for readers of what we did and why, and it will be printed by hg log after we've finished committing.

$ hg commit

The editor that the hg commit command drops us into will contain an empty line or two, followed by a number of lines starting with “HG:”.

This is where I type my commit comment.

HG: Enter commit message.  Lines beginning with 'HG:' are removed.
HG: --
HG: user: Bryan O'Sullivan <bos@serpentine.com>
HG: branch 'default'
HG: changed hello.c

Mercurial ignores the lines that start with “HG:”; it uses them only to tell us which files it's recording changes to. Modifying or deleting these lines has no effect.

Since hg log only prints the first line of a commit message by default, it's best to write a commit message whose first line stands alone. Here's a real example of a commit message that doesn't follow this guideline, and hence has a summary that is not readable.

changeset:   73:584af0e231be
user:        Censored Person <censored.person@example.org>
date:        Tue Sep 26 21:37:07 2006 -0700
summary:     include buildmeister/commondefs. Add exports.

As far as the remainder of the contents of the commit message are concerned, there are no hard-and-fast rules. Mercurial itself doesn't interpret or care about the contents of the commit message, though your project may have policies that dictate a certain kind of formatting.

My personal preference is for short, but informative, commit messages that tell me something that I can't figure out with a quick glance at the output of hg log --patch.

If we run the hg commit command without any arguments, it records all of the changes we've made, as reported by hg status and hg diff.

	A surprise for Subversion users
	Like other Mercurial commands, if we don't supply explicit names to commit to the hg commit, it will operate across a repository's entire working directory. Be wary of this if you're coming from the Subversion or CVS world, since you might expect it to operate only on the current directory that you happen to be visiting and its subdirectories.

If you decide that you don't want to commit while in the middle of editing a commit message, simply exit from your editor without saving the file that it's editing. This will cause nothing to happen to either the repository or the working directory.

Once we've finished the commit, we can use the hg tip command to display the changeset we just created. This command produces output that is identical to hg log, but it only displays the newest revision in the repository.

$ hg tip -vp
changeset:   5:b6fed4f21233
tag:         tip
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Tue May 05 06:55:53 2009 +0000
files:       hello.c
description:
Added an extra line of output


diff -r 2278160e78d4 -r b6fed4f21233 hello.c
--- a/hello.c	Sat Aug 16 22:16:53 2008 +0200
+++ b/hello.c	Tue May 05 06:55:53 2009 +0000
@@ -8,5 +8,6 @@
 int main(int argc, char **argv)
 {
 	printf("hello, world!\");
+	printf("hello again!\n");
 	return 0;
 }

We refer to the newest revision in the repository as the tip revision, or simply the tip.

By the way, the hg tip command accepts many of the same options as hg log, so -v above indicates “be verbose”, -p specifies “print a patch”. The use of -p to print patches is another example of the consistent naming we mentioned earlier.

To get started, let's clone our original hello repository, which does not contain the change we just committed. We'll call our temporary repository hello-pull.

$ cd ..
$ hg clone hello hello-pull
updating working directory
2 files updated, 0 files merged, 0 files removed, 0 files unresolved

We'll use the hg pull command to bring changes from my-hello into hello-pull. However, blindly pulling unknown changes into a repository is a somewhat scary prospect. Mercurial provides the hg incoming command to tell us what changes the hg pull command would pull into the repository, without actually pulling the changes in.

$ cd hello-pull
$ hg incoming ../my-hello
comparing with ../my-hello
searching for changes
changeset:   5:b6fed4f21233
tag:         tip
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Tue May 05 06:55:53 2009 +0000
summary:     Added an extra line of output

Bringing changes into a repository is a simple matter of running the hg pull command, and optionally telling it which repository to pull from.

$ hg tip
changeset:   4:2278160e78d4
tag:         tip
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Sat Aug 16 22:16:53 2008 +0200
summary:     Trim comments.

$ hg pull ../my-hello
pulling from ../my-hello
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
(run 'hg update' to get a working copy)
$ hg tip
changeset:   5:b6fed4f21233
tag:         tip
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Tue May 05 06:55:53 2009 +0000
summary:     Added an extra line of output

As you can see from the before-and-after output of hg tip, we have successfully pulled changes into our repository. However, Mercurial separates pulling changes in from updating the working directory. There remains one step before we will see the changes that we just pulled appear in the working directory.

Pulling specific changes

It is possible that due to the delay between running hg incoming and hg pull, you may not see all changesets that will be brought from the other repository. Suppose you're pulling changes from a repository on the network somewhere. While you are looking at the hg incoming output, and before you pull those changes, someone might have committed something in the remote repository. This means that it's possible to pull more changes than you saw when using hg incoming. If you only want to pull precisely the changes that were listed by hg incoming, or you have some other reason to pull a subset of changes, simply identify the change that you want to pull by its changeset ID, e.g. hg pull -r7e95bb.

We have so far glossed over the relationship between a repository and its working directory. The hg pull command that we ran in the section called “Pulling changes from another repository” brought changes into the repository, but if we check, there's no sign of those changes in the working directory. This is because hg pull does not (by default) touch the working directory. Instead, we use the hg update command to do this.

$ grep printf hello.c
	printf("hello, world!\");
$ hg update tip
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ grep printf hello.c
	printf("hello, world!\");
	printf("hello again!\n");

It might seem a bit strange that hg pull doesn't update the working directory automatically. There's actually a good reason for this: you can use hg update to update the working directory to the state it was in at any revision in the history of the repository. If you had the working directory updated to an old revision—to hunt down the origin of a bug, say—and ran a hg pull which automatically updated the working directory to a new revision, you might not be terribly happy.

Since pull-then-update is such a common sequence of operations, Mercurial lets you combine the two by passing the -u option to hg pull.

If you look back at the output of hg pull in the section called “Pulling changes from another repository” when we ran it without -u, you can see that it printed a helpful reminder that we'd have to take an explicit step to update the working directory.

To find out what revision the working directory is at, use the hg parents command.

$ hg parents
changeset:   5:b6fed4f21233
tag:         tip
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Tue May 05 06:55:53 2009 +0000
summary:     Added an extra line of output

If you look back at Figure 2.1, “Graphical history of the hello repository”, you'll see arrows connecting each changeset. The node that the arrow leads from in each case is a parent, and the node that the arrow leads to is its child. The working directory has a parent in just the same way; this is the changeset that the working directory currently contains.

To update the working directory to a particular revision, give a revision number or changeset ID to the hg update command.

$ hg update 2
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg parents
changeset:   2:fef857204a0c
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Sat Aug 16 22:05:04 2008 +0200
summary:     Introduce a typo into hello.c.

$ hg update
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg parents
changeset:   5:b6fed4f21233
tag:         tip
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Tue May 05 06:55:53 2009 +0000
summary:     Added an extra line of output

If you omit an explicit revision, hg update will update to the tip revision, as shown by the second call to hg update in the example above.

Mercurial lets us push changes to another repository, from the repository we're currently visiting. As with the example of hg pull above, we'll create a temporary repository to push our changes into.

$ cd ..
$ hg clone hello hello-push
updating working directory
2 files updated, 0 files merged, 0 files removed, 0 files unresolved

The hg outgoing command tells us what changes would be pushed into another repository.

$ cd my-hello
$ hg outgoing ../hello-push
comparing with ../hello-push
searching for changes
changeset:   5:b6fed4f21233
tag:         tip
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Tue May 05 06:55:53 2009 +0000
summary:     Added an extra line of output

And the hg push command does the actual push.

$ hg push ../hello-push
pushing to ../hello-push
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files

As with hg pull, the hg push command does not update the working directory in the repository that it's pushing changes into. Unlike hg pull, hg push does not provide a -u option that updates the other repository's working directory. This asymmetry is deliberate: the repository we're pushing to might be on a remote server and shared between several people. If we were to update its working directory while someone was working in it, their work would be disrupted.

What happens if we try to pull or push changes and the receiving repository already has those changes? Nothing too exciting.

$ hg push ../hello-push
pushing to ../hello-push
searching for changes
no changes found

When we clone a repository, Mercurial records the location of the repository we cloned in the .hg/hgrc file of the new repository. If we don't supply a location to hg pull from or hg push to, those commands will use this location as a default. The hg incoming and hg outgoing commands do so too.

If you open a repository's .hg/hgrc file in a text editor, you will see contents like the following.

[paths]
default = http://www.selenic.com/repo/hg

It is possible—and often useful—to have the default location for hg push and hg outgoing be different from those for hg pull and hg incoming. We can do this by adding a default-push entry to the [paths] section of the .hg/hgrc file, as follows.

[paths]
default = http://www.selenic.com/repo/hg
default-push = http://hg.example.com/hg

The commands we have covered in the previous few sections are not limited to working with local repositories. Each works in exactly the same fashion over a network connection; simply pass in a URL instead of a local path.

$ hg outgoing https://bitbucket.org/bos/hg-tutorial-hello
comparing with https://bitbucket.org/bos/hg-tutorial-hello
searching for changes
changeset:   5:b6fed4f21233
tag:         tip
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Tue May 05 06:55:53 2009 +0000
summary:     Added an extra line of output

In this example, we can see what changes we could push to the remote repository, but the repository is understandably not set up to let anonymous users push to it.

$ hg push http://bitbucket.org/bos/hg-tutorial-hello
pushing to http://bitbucket.org/bos/hg-tutorial-hello
searching for changes
ssl required