Using Mercurial from scripts and automation

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.

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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.

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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.)

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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.