Installing Mercurial

Having a modern Mercurial installed is important. Features, bug fixes, and performance enhancements are always being added to Mercurial. Staying up to date on releases is important to getting the most out of your tools.


Mercurial has a strong commitment to backwards compatibility.

If you are scared that upgrading will break workflows or command behavior, don’t be. It is very rare for Mercurial to intentionally break backwards compatibility.

Installing on Windows

If you are a Firefox developer, you should install Mercurial indirectly through MozillaBuild.

If you are not a Firefox developer, download a Windows installer direct from the Mercurial project.

Installing on OS X

Mercurial is not installed on OS X by default. You will need to install it from a package manager or install it from source.

mach bootstrap

If you have a clone of a Firefox repository, simply run mach bootstrap to install/upgrade Mercurial. Keep in mind this will install all packages required for Firefox development. If this is not wanted, follow a set of instructions below.


Homebrew typically keeps their Mercurial package up to date. Install Mercurial through Homebrew by running:

$ brew install mercurial

You may want to run brew update first to ensure your package database is up to date.


MacPorts typically keeps their Mercurial package up to date. Install through MacPorts by running:

$ port install mercurial

From Source

See the section below about how to install Mercurial from source.

Installing on Linux, BSD, and other UNIX-style OSs

The instructions for installing Mercurial on many popular distributions are available on Mercurial’s web site. However, many distros don’t keep their Mercurial package reasonably current. You often need to perform a source install.

Installing from Source

Installing Mercurial from source is simple and should not be dismissed because it isn’t coming from a package.

Download a source archive from Mercurial. Alternatively, clone the Mercurial source code and check out the version you wish to install:

$ hg clone
$ cd hg
$ hg up 4.0

Once you have the source code, run make to install Mercurial:

$ make install

If you would like to install Mercurial to a custom prefix:

$ make install PREFIX=/usr/local
$ make install PREFIX=/home/gps


Mercurial has some Python C extensions that make performance-critical parts of Mercurial significantly faster. You may need to install a system package such as python-dev to enable you to build Python C extensions.


Are you concerned about a manual Mercurial install polluting your filesystem? Don’t be.

A Mercurial source install is fully self-contained. If you install to a prefix, you only need a reference to the PREFIX/bin/hg executable to run Mercurial. You can create a symlink to PREFIX/bin/hg anywhere in PATH and Mercurial should just work.

Verifying Your Installation

To verify Mercurial is installed properly and has a basic configuration in place, run:

$ hg debuginstall

If it detects problems, correct them.

If you have a clone of the Firefox repository, you are highly encouraged to run mach mercurial-setup to launch an interactive wizard that will help you optimally configure Mercurial for use at Mozilla.

Reasons to Upgrade

General Advice

Mercurial releases tend to be faster and have fewer bugs than previous releases. These are compelling reasons to stay up to date.

Avoid Mercurial versions older than 3.7.3 due to issues below.

Security Issues

Versions of Mercurial before 3.7.3 are vulnerable to multiple security issues that can lead to executing arbitrary code when cloning or pulling from repositories. Avoid versions older than 3.7.3!

Cloning and Pulling Performance

Mercurial 3.6 contains a number of enhancements to performance of cloning and pull operations, especially on Windows. Clone times for mozilla-central on Windows can be several minutes faster with 3.6.

Revset Performance

Mercurial 3.5 and 3.6 contained a number of performance improvements to revision sets. If you are a user of hg wip or hg smartlog, these commands will likely be at least 4x faster on Mercurial 3.6.

Revsets are used internally by Mercurial. So these improvements result in performance improvements for a hodgepodge of operations.

Tags Cache Performance

Mercurial 3.4 contains improvements to the tags cache that prevent it from frequently doing CPU-intensive computations in some workflows.


Users of evolve will have horrible performance due to the tags cache implementation in versions older than 3.4 and should upgrade to 3.4+.

Performance Issues with Large Repositories

Mercurial 3.0 through 3.1.1 contained a significant performance regression that manifests when cloning or pulling tens of thousands of changesets. These versions of Mercurial should be avoided when interacting with large repositories, such as mozilla-central.

Mercurial 3.3 introduced a class of performance regressions most likely encountered as part of running hg blame or hg graft. The regressions are largely fixed in 3.4.


Mercurial versions older than 3.2.3 should be avoided due to a security issue (CVE-2014-9390) impacting Windows and OS X users.

Supporting Old Versions

Mozilla has written a handful of Mercurial extensions. Supporting N versions of Mercurial is easier than supporting N+1 versions, especially as Mercurial’s API is rapidly evolving. It is extra work to support old versions when new versions work just fine.

Newer Wire Protocol

Mercurial 3.5 featured a new wire protocol that performs pushes and pulls more efficiently.

Cloning from Pre-Generated Bundle Files

Mercurial 3.6 supports transparently cloning from pre-generated bundle files. When you clone from, many of the larger repositories will be served from a CDN. This results in a faster and more reliable clone.