Installing and Configuring Extensions

A vanilla install of Mercurial does not have many features. This is an intentional choice by Mercurial to keep the base install simple and free of foot guns.

Guided Installation of Extensions

If you have a copy of the Firefox source repository, you can run ./mach vcs-setup to run an interactive wizard that will guide you through the process of configuring Mercurial.


mach vcs-setup is the recommended way to configure Mercurial for use at Mozilla. The sections below effectively duplicate the work that mach vcs-setup does for you.

Installing Extensions

To install an extension, you’ll need to add a line to your Mercurial configuration file.

As a user, you care about the following configuration files:

  1. Your global hgrc
  2. A per-repository hgrc

Your user-global configuration file is ~/.hgrc. Settings in this file apply to all hg commands you perform everywhere on your system (for your current user).

Each repository that you clone or initialize has an optional .hg/hgrc file that provides repository-specific configurations.

Mercurial starts by loading and applying settings from global configuration files and then overlays configurations from each lesser-scoped files.


To learn more about Mercurial configuration files, run hg help config.

To install an extension, you add a line under the [extensions] section of a config file like the following:


This is saying activate the **foo* extension whose code is present at path/to/extension*.

Core Extensions That Almost Everyone Wants

Mercurial ships with a number of built-in extensions. Of these, every user will almost always want to install the following extensions:

Monitor the filesystem for changes using so operations querying filesystem state complete faster.


fsmonitor is highly recommended when interacting with the Firefox repository. It will make Mercurial commands faster.

Since core extensions are bundled with Mercurial, they have a special syntax that makes them easier to install:


Core Extensions to Perform History Rewriting

Out of the box, Mercurial only allows commits operations to be additive. If you make a mistake, the solution is to create a new commit that fixes it. You can’t rewrite old commits. You can’t change the order of existing commits. You can’t change the shape of the DAG of the commits.

These operations all have something in common: they rewrite history.


Mercurial doesn’t allow history rewriting by default because it is a major foot gun for people new to version control. A potential side-effect of history rewriting is data loss or confusion due to loss of state. Mercurial believes that these risks should be opt-in and has thus shipped without any history rewriting features enabled by default.

Mercurial ships with a number of built-in extensions that enable history rewriting:


Enables the hg histedit command, which brings up a text editor listing commits, allowing you to change order and specify actions to perform.

The functionality is roughly equivalent to git rebase -i.

Enables the hg rebase command, which allows you to splice commits across different chains in the DAG.
Enables you to delete changesets completely.

Core Extensions to Enable Different Workflows

Mercurial ships with some extensions that enable alternate workflows. These include:

Enables uncommitted work to be saved to a standalone file without being committed to the repository.


chg is a C wrapper for the hg command. Typically, when you type hg, a new Python process is created, Mercurial is loaded, and your requested command runs and the process exits.

With chg, a Mercurial command server background process is created that runs Mercurial. When you type chg, a C program connects to that background process and executes Mercurial commands.

chg can drastically speed up Mercurial. This is because the overhead for launching a new Python process is high (often over 50ms) and the overhead for loading Mercurial state into that process can also be high. With chg, you pay this cost once and all subsequent commands effectively eliminate the Python and Mercurial startup overhead. For example:

$ time hg --version
real    0m0.118s
user    0m0.100s
sys     0m0.015s

$ time chg --version
real    0m0.012s
user    0m0.000s
sys     0m0.004s

$ time hg export
real    0m0.137s
user    0m0.093s
sys     0m0.042s

$ time chg export
real    0m0.034s
user    0m0.000s
sys     0m0.004s

Here, we see ~100ms wall time improvement with chg activated. That may not sound like a lot, but you will notice.

If you have installed Mercurial through a package manager (such as Homebrew or APT) you may already have chg installed. For more information, see here.

3rd Party Extensions You Should Highly Consider


The evolve extension opens up new workflows that harness Mercurial’s ability to record how changesets evolve over time.

Typically, when history is rewritten, new commits are created and the old ones are discarded. With the evolve extension enabled, Mercurial instead hides the old commits and writes metadata holding the relationship between old and new commits. This metadata can be transferred between clients, allowing clients to make intelligent decisions about how to recover from rewritten history. For example, if a force push is performed, a client will know exactly what rebase to perform to mimic what was done elsewhere.

The evolve extension also enables useful Mercurial commands such as hg previous, hg next, and hg amend (which is a shortcut for hg commit --amend).