Before getting into the details of how the command line works, and how you can access it over the network, we should pause and note that there exist many systems today for automating the entire process.If you have dozens or hundreds of machines to maintain and you need to start sending them all the same commands, then you might find that tools already exist —tools that already provide ways to write command scripts, push them out for execution across a cloud of machines, batch up any error messages or responses for your review, and even save commands in a queue to be re-tried later in case a machine is down and cannot be reached at the moment.
What are the options?
First, the Fabric library is very popular with Python programmers who need to run commands and
copy files to remote server machines. As you can see in
fabfile.py, a Fabric script calls very simple
functions with names like
run() to perform operations on the machines to which it
connects. But you can learn more about it at its web site: http://fabfile.org/.
fabfile.py is designed to be run by Fabric's own fab command-line tool, Fabric can also
be used from inside your own Python programs; again, consult their documentation for details.
from fabric.api import * def versions(): with cd('/usr/bin'): with settings(hide('warnings'), warn_only=True): for version in '2.4', '2.5', '2.6', '2.7', '3.0', '3.1': result = run('python%s -c "None"' % version) if not result.failed: print "Host", env.host, "has Python", version
Another project to check out is Silver Lining. It is still very immature, but if you are an experienced programmer who needs its specific capabilities, then you might find that it solves your problems well. This library goes beyond batching commands across many different servers: it will actually create and initialize Ubuntu servers through the “libcloud” Python API, and then install your Python web applications there for you. You can learn more about this promising project at http://cloudsilverlining.org/.
On the other hand, there is “pexpect.” While it is not, technically, a program that itself knows how to use the network, it is often used to control the system “ssh” or “telnet” command when a Python programmer wants to automate interactions with a remote prompt of some kind. This typically takes place in a situation where no API for a device is available, and commands simply have to be typed each time the command-line prompt appears. Configuring simple network hardware often requires this kind of clunky step-by-step interaction. You can learn more about “pexpect” here: http://pypi.python.org/pypi/pexpect.
Finally, there are more specific projects that provide mechanisms for remote systems administration. Red Hat and Fedora users might look at func, which uses an SSL-encrypted XML-RPC service that lets you write Python programs that perform system configuration and maintenance: https://fedorahosted.org/func/.