Open Source For Geeks: Using Fabric Python command-line tool

Sunday, 26 April 2015

Using Fabric Python command-line tool

What is Fabric?

You can visit the fabric site . It gives the most apt description of what fabric is.

Fabric is a Python (2.5-2.7) library and command-line tool for streamlining the use of SSH for application deployment or systems administration tasks.

More specifically, Fabric is:

A tool that lets you execute arbitrary Python functions via the command line
A library of subroutines (built on top of a lower-level library) to make executing shell commands over SSH easy and Pythonic.

Prerequisites

Assuming you have fair knowledge of python basics. If not please go over the basics in the tutorial - The Python Tutorial
Also you mus have python installed. If not refer to the guide - The Hitchhiker’s Guide to Python!

Note : Fabric is supported only by Python versions (2.5-2.7).

Installing Fabric

Before you install Fabric you need to install modules needed to install other modules. For that you can either install -

pip or
setuptools

I am going to use pip to install fabric as it is the latest one. To install Fabric use the following command -

pip install fabric

and your fabric should be installed. You can see various related installation ways here. You can see the installed module in

<Python installation directory>\Lib\site-packages .

For me it is located in

C:\Python34\Lib\site-packages\fabric

You can see the fab. exe file in

C:\Python34\Scripts

which we have already added in the PATH environment variable [This is the same place where you have pip and easy_install executable]

Getting Started

Lets start with our usual "Hello World!" example. Create a file called fabfile.py and add following content in it -

def helloworld():
        """This is a test hello world fab method"""
        print("Hello World!")

To see list of available fabric commands do -

fab -l

For above file you should see something like -

[athakur@localhost athakur]$ fab -l
Available commands:

helloworld This is a test hello world fab method

Next it's time to execute the command itself. Execute

fab helloworld

and you should see Hello World! printed on the console.

Note : Any method that starts with an underscore (_) is a private method and cannot be used directly as fab command. It will even not be listed in fab -l command.

Consider following code -

def helloworld():
        """This is a test hello world fab method"""
        print("Hello World")
        _private_helloworld()

def _private_helloworld():
        """ This is private method and should not be listed with fab -l"""
        print("Private hello World")

Execute fab -l and you should again see the same output as above i.e you should not see _private_helloworld method listed there.

Executing fab helloworld should print following now -

[athakur@localhost athakur]$ fab helloworld
Hello World
Private hello World
Done.

Note : The fab tool simply imports your fabfile and executes the function or functions you instruct it to. There’s nothing magic about it – anything you can do in a normal Python script can be done in a fabfile!

Fabfile discovery

Fabric is capable of loading Python modules (e.g. fabfile.py) or packages (e.g. a fabfile/ directory containing an __init__.py). By default, it looks for something named (to Python’s import machinery) fabfile - so either fabfile/ or fabfile.py.

The fabfile discovery algorithm searches in the invoking user’s current working directory or any parent directories. Thus, it is oriented around “project” use, where one keeps e.g. a fabfile.py at the root of a source code tree. Such a fabfile will then be discovered no matter where in the tree the user invokes fab.

(More Details)

Fabric Tasks with arguments

Put the following content in fabfile.py

def helloworld(name="Aniket"):
        """This is a test hello world fab method. Syntax : fab helloworld:name=<your name>"""
        print("Hello World from %s!" %name)
        _private_helloworld()

def _private_helloworld():
        """ This is private method and should not be listed with fab -l"""
        print("Private hello World")

and run

fab helloworld

You should get

[athakur@localhost athakur]$ fab helloworld
Hello World from Aniket!
Private hello World
Done.

Or you can execute by giving name argument as

fab helloworld:name="John" or simply
fab helloworld:"John"

and you should get output as

[athakur@localhost athakur]$ fab helloworld:"John"
Hello World from John!
Private hello World
Done.

Running fabric tasks on other machines

Replace your fabfile with following contents :

from fabric.api import run

def host_name():
        run('uname -a')

and then run it. You should br prompted for hostname and then password for the user you are already logged in as.

[athakur@localhost athakur]$ fab host_name
No hosts found. Please specify (single) host string for connection: localhost
[localhost] run: uname -a
[localhost] Login password for 'athakur':
[localhost] out: Linux localhost 2.6.32-504.8.1.el6.x86_64 #1 SMP Wed Jan 28 21:11:36 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux

Done.
Disconnecting from localhost... done.

Troubleshooting

In Windows while installing Fabric you may get following error -

building 'Crypto.Random.OSRNG.winrandom' extension
warning: GMP or MPIR library not found; Not building Crypto.PublicKey._fastmath
error: Microsoft Visual C++ 10.0 is required (Unable to find vcvarsall.bat).

If you get this you will have to install Microsoft Visual C++ compiler. See Stack overflow answers in the 1st link of related Sections. This is windows specific issue. If you are using Linux this should not occur.

Or You can install MinGW (Minimalist GNU for Windows) as I did. You need to install msys package under MinGW

and add following entries in your PATH env variable.

C:\MinGW\bin
C:\MinGW\msys\1.0\bin [This is where you will find chmod executable]

Then run your command from normal windows command prompt.