It is common for machines (as opposed to humans) to consume Mercurial.

This help topic describes some of the considerations for interfacing

machines with Mercurial.

Choosing an Interface

=====================

Machines have a choice of several methods to interface with Mercurial.

These include:

- Executing the ``hg`` process

- Querying a HTTP server

- Calling out to a command server

Executing ``hg`` processes is very similar to how humans interact with

Mercurial in the shell. It should already be familiar to you.

:hg:`serve` can be used to start a server. By default, this will start

a "hgweb" HTTP server. This HTTP server has support for machine-readable

output, such as JSON. For more, see :hg:`help hgweb`.

:hg:`serve` can also start a "command server." Clients can connect

to this server and issue Mercurial commands over a special protocol.

For more details on the command server, including links to client

libraries, see https://www.mercurial-scm.org/wiki/CommandServer.

:hg:`serve` based interfaces (the hgweb and command servers) have the

advantage over simple ``hg`` process invocations in that they are

likely more efficient. This is because there is significant overhead

to spawn new Python processes.

.. tip::

   If you need to invoke several ``hg`` processes in short order and/or

   performance is important to you, use of a server-based interface

   is highly recommended.

Environment Variables

=====================

As documented in :hg:`help environment`, various environment variables

influence the operation of Mercurial. The following are particularly

relevant for machines consuming Mercurial:

HGPLAIN

    If not set, Mercurial's output could be influenced by configuration

    settings that impact its encoding, verbose mode, localization, etc.

    It is highly recommended for machines to set this variable when

    invoking ``hg`` processes.

HGENCODING

    If not set, the locale used by Mercurial will be detected from the

    environment. If the determined locale does not support display of

    certain characters, Mercurial may render these character sequences

    incorrectly (often by using "?" as a placeholder for invalid

    characters in the current locale).

    Explicitly setting this environment variable is a good practice to

    guarantee consistent results. "utf-8" is a good choice on UNIX-like

    environments.

HGRCPATH

    If not set, Mercurial will inherit config options from config files

    using the process described in :hg:`help config`. This includes

    inheriting user or system-wide config files.

    When utmost control over the Mercurial configuration is desired, the

    value of ``HGRCPATH`` can be set to an explicit file with known good

    configs. In rare cases, the value can be set to an empty file or the

    null device (often ``/dev/null``) to bypass loading of any user or

    system config files. Note that these approaches can have unintended

    consequences, as the user and system config files often define things

    like the username and extensions that may be required to interface

    with a repository.

HGRCSKIPREPO

    When set, the .hg/hgrc from repositories are not read.

    Note that not reading the repository's configuration can have

    unintended consequences, as the repository config files can define

    things like extensions that are required for access to the

    repository.

Command-line Flags

==================

Mercurial's default command-line parser is designed for humans, and is not

robust against malicious input. For instance, you can start a debugger by

passing ``--debugger`` as an option value::

    $ REV=--debugger sh -c 'hg log -r "$REV"'

This happens because several command-line flags need to be scanned without

using a concrete command table, which may be modified while loading repository

settings and extensions.

Since Mercurial 4.4.2, the parsing of such flags may be restricted by setting

``HGPLAIN=+strictflags``. When this feature is enabled, all early options

(e.g. ``-R/--repository``, ``--cwd``, ``--config``) must be specified first

amongst the other global options, and cannot be injected to an arbitrary

location::

    $ HGPLAIN=+strictflags hg -R "$REPO" log -r "$REV"

In earlier Mercurial versions where ``+strictflags`` isn't available, you

can mitigate the issue by concatenating an option value with its flag::

    $ hg log -r"$REV" --keyword="$KEYWORD"

Consuming Command Output

========================

It is common for machines to need to parse the output of Mercurial

commands for relevant data. This section describes the various

techniques for doing so.

Parsing Raw Command Output

--------------------------

Likely the simplest and most effective solution for consuming command

output is to simply invoke ``hg`` commands as you would as a user and

parse their output.

The output of many commands can easily be parsed with tools like

``grep``, ``sed``, and ``awk``.

A potential downside with parsing command output is that the output

of commands can change when Mercurial is upgraded. While Mercurial

does generally strive for strong backwards compatibility, command

output does occasionally change. Having tests for your automated

interactions with ``hg`` commands is generally recommended, but is

even more important when raw command output parsing is involved.

Using Templates to Control Output

---------------------------------

Many ``hg`` commands support templatized output via the

``-T/--template`` argument. For more, see :hg:`help templates`.

Templates are useful for explicitly controlling output so that

you get exactly the data you want formatted how you want it. For

example, ``log -T {node}\n`` can be used to print a newline

delimited list of changeset nodes instead of a human-tailored

output containing authors, dates, descriptions, etc.

.. tip::

   If parsing raw command output is too complicated, consider

   using templates to make your life easier.

The ``-T/--template`` argument allows specifying pre-defined styles.

Mercurial ships with the machine-readable styles ``cbor``, ``json``,

and ``xml``, which provide CBOR, JSON, and XML output, respectively.

These are useful for producing output that is machine readable as-is.

(Mercurial 5.0 is required for CBOR style.)

.. important::

   The ``json`` and ``xml`` styles are considered experimental. While

   they may be attractive to use for easily obtaining machine-readable

   output, their behavior may change in subsequent versions.

   These styles may also exhibit unexpected results when dealing with

   certain encodings. Mercurial treats things like filenames as a

   series of bytes and normalizing certain byte sequences to JSON

   or XML with certain encoding settings can lead to surprises.

Command Server Output

---------------------

If using the command server to interact with Mercurial, you are likely

using an existing library/API that abstracts implementation details of

the command server. If so, this interface layer may perform parsing for

you, saving you the work of implementing it yourself.

Output Verbosity

----------------

Commands often have varying output verbosity, even when machine

readable styles are being used (e.g. ``-T json``). Adding

``-v/--verbose`` and ``--debug`` to the command's arguments can

increase the amount of data exposed by Mercurial.

An alternate way to get the data you need is by explicitly specifying

a template.

Other Topics

============

revsets

   Revisions sets is a functional query language for selecting a set

   of revisions. Think of it as SQL for Mercurial repositories. Revsets

   are useful for querying repositories for specific data.

   See :hg:`help revsets` for more.

share extension

   The ``share`` extension provides functionality for sharing

   repository data across several working copies. It can even

   automatically "pool" storage for logically related repositories when

   cloning.

   Configuring the ``share`` extension can lead to significant resource

   utilization reduction, particularly around disk space and the

   network. This is especially true for continuous integration (CI)

   environments.

   See :hg:`help -e share` for more.