Getting started with Mercurial for version control
======
Learn the basics of Mercurial, a distributed version control system
written in Python.
![][1]
[Mercurial][2] is a distributed version control system written in Python. Because it's written in a high-level language, you can write a Mercurial extension with a few Python functions.
There are several ways to install Mercurial, which are explained in the [official documentation][3]. My favorite one is not there: using **pip**. This is the most amenable way to develop local extensions!
For now, Mercurial only supports Python 2.7, so you will need to create a Python 2.7 virtual environment:
```
python2 -m virtualenv mercurial-env
./mercurial-env/bin/pip install mercurial
```
To have a short command, and to satisfy everyone's insatiable need for chemistry-based humor, the command is called **hg**.
```
$ source mercurial-env/bin/activate
(mercurial-env)$ mkdir test-dir
(mercurial-env)$ cd test-dir
(mercurial-env)$ hg init
(mercurial-env)$ hg status
(mercurial-env)$
```
The status is empty since you do not have any files. Add a couple of files:
```
(mercurial-env)$ echo 1 > one
(mercurial-env)$ echo 2 > two
(mercurial-env)$ hg status
? one
? two
(mercurial-env)$ hg addremove
adding one
adding two
(mercurial-env)$ hg commit -m 'Adding stuff'
(mercurial-env)$ hg log
changeset: 0:1f1befb5d1e9
tag: tip
user: Moshe Zadka <[moshez@zadka.club][4]>
date: Fri Mar 29 12:42:43 2019 -0700
summary: Adding stuff
```
The **addremove** command is useful: it adds any new files that are not ignored to the list of managed files and removes any files that have been removed.
As I mentioned, Mercurial extensions are written in Python—they are just regular Python modules.
This is an example of a short Mercurial extension:
```
from mercurial import registrar
from mercurial.i18n import _
cmdtable = {}
command = registrar.command(cmdtable)
@command('say-hello',
[('w', 'whom', '', _('Whom to greet'))])
def say_hello(ui, repo, **opts):
ui.write("hello ", opts['whom'], "\n")
```
A simple way to test it is to put it in a file in the virtual environment manually:
```
`$ vi ../mercurial-env/lib/python2.7/site-packages/hello_ext.py`
```
Then you need to _enable_ the extension. You can start by enabling it only in the current repository:
```
$ cat >> .hg/hgrc
[extensions]
hello_ext =
```
Now, a greeting is possible:
```
(mercurial-env)$ hg say-hello --whom world
hello world
```
Most extensions will do more useful stuff—possibly even things to do with Mercurial. The **repo** object is a **mercurial.hg.repository** object.
Refer to the [official documentation][5] for more about Mercurial's API. And visit the [official repo][6] for more examples and inspiration.
