Once you decide that you'd like to call a particular revision a “release”, it's a good idea to record the identity of that revision. This will let you reproduce that release at a later date, for whatever purpose you might need at the time (reproducing a bug, porting to a new platform, etc).

$ hg init mytag
$ cd mytag
$ echo hello > myfile
$ hg commit -A -m 'Initial commit'
adding myfile

Mercurial lets you give a permanent name to any revision using the hg tag command. Not surprisingly, these names are called “tags”.

$ hg tag v1.0

A tag is nothing more than a “symbolic name” for a revision. Tags exist purely for your convenience, so that you have a handy permanent way to refer to a revision; Mercurial doesn't interpret the tag names you use in any way. Neither does Mercurial place any restrictions on the name of a tag, beyond a few that are necessary to ensure that a tag can be parsed unambiguously. A tag name cannot contain any of the following characters:

You can use the hg tags command to display the tags present in your repository. In the output, each tagged revision is identified first by its name, then by revision number, and finally by the unique hash of the revision.

$ hg tags
tip                                1:8046fdcc425d
v1.0                               0:a80eb2d666b7

Notice that tip is listed in the output of hg tags. The tip tag is a special “floating” tag, which always identifies the newest revision in the repository.

In the output of the hg tags command, tags are listed in reverse order, by revision number. This usually means that recent tags are listed before older tags. It also means that tip is always going to be the first tag listed in the output of hg tags.

When you run hg log, if it displays a revision that has tags associated with it, it will print those tags.

$ hg log
changeset:   1:8046fdcc425d
tag:         tip
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Tue May 05 06:55:45 2009 +0000
summary:     Added tag v1.0 for changeset a80eb2d666b7

changeset:   0:a80eb2d666b7
tag:         v1.0
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Tue May 05 06:55:45 2009 +0000
summary:     Initial commit

Any time you need to provide a revision ID to a Mercurial command, the command will accept a tag name in its place. Internally, Mercurial will translate your tag name into the corresponding revision ID, then use that.

$ echo goodbye > myfile2
$ hg commit -A -m 'Second commit'
adding myfile2
$ hg log -r v1.0
changeset:   0:a80eb2d666b7
tag:         v1.0
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Tue May 05 06:55:45 2009 +0000
summary:     Initial commit

There's no limit on the number of tags you can have in a repository, or on the number of tags that a single revision can have. As a practical matter, it's not a great idea to have “too many” (a number which will vary from project to project), simply because tags are supposed to help you to find revisions. If you have lots of tags, the ease of using them to identify revisions diminishes rapidly.

For example, if your project has milestones as frequent as every few days, it's perfectly reasonable to tag each one of those. But if you have a continuous build system that makes sure every revision can be built cleanly, you'd be introducing a lot of noise if you were to tag every clean build. Instead, you could tag failed builds (on the assumption that they're rare!), or simply not use tags to track buildability.

If you want to remove a tag that you no longer want, use hg tag --remove.

$ hg tag --remove v1.0
$ hg tags
tip                                3:788019dad621

You can also modify a tag at any time, so that it identifies a different revision, by simply issuing a new hg tag command. You'll have to use the -f option to tell Mercurial that you really want to update the tag.

$ hg tag -r 1 v1.1
$ hg tags
tip                                4:7027bc5755bd
v1.1                               1:8046fdcc425d
$ hg tag -r 2 v1.1
abort: tag 'v1.1' already exists (use -f to force)
$ hg tag -f -r 2 v1.1
$ hg tags
tip                                5:cdbd327f8987
v1.1                               2:97a73dafd9c0

There will still be a permanent record of the previous identity of the tag, but Mercurial will no longer use it. There's thus no penalty to tagging the wrong revision; all you have to do is turn around and tag the correct revision once you discover your error.

Mercurial stores tags in a normal revision-controlled file in your repository. If you've created any tags, you'll find them in a file in the root of your repository named .hgtags. When you run the hg tag command, Mercurial modifies this file, then automatically commits the change to it. This means that every time you run hg tag, you'll see a corresponding changeset in the output of hg log.

$ hg tip
changeset:   5:cdbd327f8987
tag:         tip
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Tue May 05 06:55:46 2009 +0000
summary:     Added tag v1.1 for changeset 97a73dafd9c0

To return to the outline I sketched at the beginning of the chapter, let's think about a project that has multiple concurrent pieces of work under development at once.

There might be a push for a new “main” release; a new minor bugfix release to the last main release; and an unexpected “hot fix” to an old release that is now in maintenance mode.

The usual way people refer to these different concurrent directions of development is as “branches”. However, we've already seen numerous times that Mercurial treats all of history as a series of branches and merges. Really, what we have here is two ideas that are peripherally related, but which happen to share a name.

The easiest way to isolate a “big picture” branch in Mercurial is in a dedicated repository. If you have an existing shared repository—let's call it myproject—that reaches a “1.0” milestone, you can start to prepare for future maintenance releases on top of version 1.0 by tagging the revision from which you prepared the 1.0 release.

$ cd myproject
$ hg tag v1.0

You can then clone a new shared myproject-1.0.1 repository as of that tag.

$ cd ..
$ hg clone myproject myproject-1.0.1
updating working directory
2 files updated, 0 files merged, 0 files removed, 0 files unresolved

Afterwards, if someone needs to work on a bug fix that ought to go into an upcoming 1.0.1 minor release, they clone the myproject-1.0.1 repository, make their changes, and push them back.

$ hg clone myproject-1.0.1 my-1.0.1-bugfix
updating working directory
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cd my-1.0.1-bugfix
$ echo 'I fixed a bug using only echo!' >> myfile
$ hg commit -m 'Important fix for 1.0.1'
$ hg push
pushing to /tmp/branch-repo1HDNuO/myproject-1.0.1
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files

Meanwhile, development for the next major release can continue, isolated and unabated, in the myproject repository.

$ cd ..
$ hg clone myproject my-feature
updating working directory
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cd my-feature
$ echo 'This sure is an exciting new feature!' > mynewfile
$ hg commit -A -m 'New feature'
adding mynewfile
$ hg push
pushing to /tmp/branch-repo1HDNuO/myproject
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files

In many cases, if you have a bug to fix on a maintenance branch, the chances are good that the bug exists on your project's main branch (and possibly other maintenance branches, too). It's a rare developer who wants to fix the same bug multiple times, so let's look at a few ways that Mercurial can help you to manage these bugfixes without duplicating your work.

In the simplest instance, all you need to do is pull changes from your maintenance branch into your local clone of the target branch.

$ cd ..
$ hg clone myproject myproject-merge
updating working directory
3 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cd myproject-merge
$ hg pull ../myproject-1.0.1
pulling from ../myproject-1.0.1
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files (+1 heads)
(run 'hg heads' to see heads, 'hg merge' to merge)

You'll then need to merge the heads of the two branches, and push back to the main branch.

$ hg merge
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
$ hg commit -m 'Merge bugfix from 1.0.1 branch'
$ hg push
pushing to /tmp/branch-repo1HDNuO/myproject
searching for changes
adding changesets
adding manifests
adding file changes
added 2 changesets with 1 changes to 1 files

In most instances, isolating branches in repositories is the right approach. Its simplicity makes it easy to understand; and so it's hard to make mistakes. There's a one-to-one relationship between branches you're working in and directories on your system. This lets you use normal (non-Mercurial-aware) tools to work on files within a branch/repository.

If you're more in the “power user” category (and your collaborators are too), there is an alternative way of handling branches that you can consider. I've already mentioned the human-level distinction between “small picture” and “big picture” branches. While Mercurial works with multiple “small picture” branches in a repository all the time (for example after you pull changes in, but before you merge them), it can also work with multiple “big picture” branches.

The key to working this way is that Mercurial lets you assign a persistent name to a branch. There always exists a branch named default. Even before you start naming branches yourself, you can find traces of the default branch if you look for them.

As an example, when you run the hg commit command, and it pops up your editor so that you can enter a commit message, look for a line that contains the text “HG: branch default” at the bottom. This is telling you that your commit will occur on the branch named default.

To start working with named branches, use the hg branches command. This command lists the named branches already present in your repository, telling you which changeset is the tip of each.

$ hg tip
changeset:   0:692c56a8362e
tag:         tip
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Tue May 05 06:55:16 2009 +0000
summary:     Initial commit

$ hg branches
default                        0:692c56a8362e

Since you haven't created any named branches yet, the only one that exists is default.

To find out what the “current” branch is, run the hg branch command, giving it no arguments. This tells you what branch the parent of the current changeset is on.

$ hg branch
default

To create a new branch, run the hg branch command again. This time, give it one argument: the name of the branch you want to create.

$ hg branch foo
marked working directory as branch foo
$ hg branch
foo

After you've created a branch, you might wonder what effect the hg branch command has had. What do the hg status and hg tip commands report?

$ hg status
$ hg tip
changeset:   0:692c56a8362e
tag:         tip
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Tue May 05 06:55:16 2009 +0000
summary:     Initial commit

Nothing has changed in the working directory, and there's been no new history created. As this suggests, running the hg branch command has no permanent effect; it only tells Mercurial what branch name to use the next time you commit a changeset.

When you commit a change, Mercurial records the name of the branch on which you committed. Once you've switched from the default branch to another and committed, you'll see the name of the new branch show up in the output of hg log, hg tip, and other commands that display the same kind of output.

$ echo 'hello again' >> myfile
$ hg commit -m 'Second commit'
$ hg tip
changeset:   1:554c1c20014c
branch:      foo
tag:         tip
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Tue May 05 06:55:17 2009 +0000
summary:     Second commit

The hg log-like commands will print the branch name of every changeset that's not on the default branch. As a result, if you never use named branches, you'll never see this information.

Once you've named a branch and committed a change with that name, every subsequent commit that descends from that change will inherit the same branch name. You can change the name of a branch at any time, using the hg branch command.

$ hg branch
foo
$ hg branch bar
marked working directory as branch bar
$ echo new file > newfile
$ hg commit -A -m 'Third commit'
adding newfile
$ hg tip
changeset:   2:b9e802e02f12
branch:      bar
tag:         tip
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Tue May 05 06:55:17 2009 +0000
summary:     Third commit

In practice, this is something you won't do very often, as branch names tend to have fairly long lifetimes. (This isn't a rule, just an observation.)

If you have more than one named branch in a repository, Mercurial will remember the branch that your working directory is on when you start a command like hg update or hg pull -u. It will update the working directory to the tip of this branch, no matter what the “repo-wide” tip is. To update to a revision that's on a different named branch, you may need to use the -C option to hg update.

This behavior is a little subtle, so let's see it in action. First, let's remind ourselves what branch we're currently on, and what branches are in our repository.

$ hg parents
changeset:   2:b9e802e02f12
branch:      bar
tag:         tip
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Tue May 05 06:55:17 2009 +0000
summary:     Third commit

$ hg branches
bar                            2:b9e802e02f12
foo                            1:554c1c20014c (inactive)
default                        0:692c56a8362e (inactive)

We're on the bar branch, but there also exists an older hg foo branch.

We can hg update back and forth between the tips of the foo and bar branches without needing to use the -C option, because this only involves going backwards and forwards linearly through our change history.

$ hg update foo
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ hg parents
changeset:   1:554c1c20014c
branch:      foo
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Tue May 05 06:55:17 2009 +0000
summary:     Second commit

$ hg update bar
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg parents
changeset:   2:b9e802e02f12
branch:      bar
tag:         tip
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Tue May 05 06:55:17 2009 +0000
summary:     Third commit

If we go back to the foo branch and then run hg update, it will keep us on foo, not move us to the tip of bar.

$ hg update foo
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ hg update
0 files updated, 0 files merged, 0 files removed, 0 files unresolved

Committing a new change on the foo branch introduces a new head.

$ echo something > somefile
$ hg commit -A -m 'New file'
adding somefile
created new head
$ hg heads
changeset:   3:04da8e502cbb
branch:      foo
tag:         tip
parent:      1:554c1c20014c
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Tue May 05 06:55:18 2009 +0000
summary:     New file

changeset:   2:b9e802e02f12
branch:      bar
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Tue May 05 06:55:17 2009 +0000
summary:     Third commit

As you've probably noticed, merges in Mercurial are not symmetrical. Let's say our repository has two heads, 17 and 23. If I hg update to 17 and then hg merge with 23, Mercurial records 17 as the first parent of the merge, and 23 as the second. Whereas if I hg update to 23 and then hg merge with 17, it records 23 as the first parent, and 17 as the second.

This affects Mercurial's choice of branch name when you merge. After a merge, Mercurial will retain the branch name of the first parent when you commit the result of the merge. If your first parent's branch name is foo, and you merge with bar, the branch name will still be foo after you merge.

It's not unusual for a repository to contain multiple heads, each with the same branch name. Let's say I'm working on the foo branch, and so are you. We commit different changes; I pull your changes; I now have two heads, each claiming to be on the foo branch. The result of a merge will be a single head on the foo branch, as you might hope.

But if I'm working on the bar branch, and I merge work from the foo branch, the result will remain on the bar branch.

$ hg branch
bar
$ hg merge foo
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
$ hg commit -m 'Merge'
$ hg tip
changeset:   4:c760b46b1a7b
branch:      bar
tag:         tip
parent:      2:b9e802e02f12
parent:      3:04da8e502cbb
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Tue May 05 06:55:18 2009 +0000
summary:     Merge

To give a more concrete example, if I'm working on the bleeding-edge branch, and I want to bring in the latest fixes from the stable branch, Mercurial will choose the “right” (bleeding-edge) branch name when I pull and merge from stable.

You shouldn't think of named branches as applicable only to situations where you have multiple long-lived branches cohabiting in a single repository. They're very useful even in the one-branch-per-repository case.

In the simplest case, giving a name to each branch gives you a permanent record of which branch a changeset originated on. This gives you more context when you're trying to follow the history of a long-lived branchy project.

If you're working with shared repositories, you can set up a pretxnchangegroup hook on each that will block incoming changes that have the “wrong” branch name. This provides a simple, but effective, defence against people accidentally pushing changes from a “bleeding edge” branch to a “stable” branch. Such a hook might look like this inside the shared repo's /.hgrc.

[hooks]
pretxnchangegroup.branch = hg heads --template '{branches} ' | grep mybranch