Python Tutorial =============== Introduction ------------ This is a step by step introduction to using Exscript in `Python `_. We'll assume that Exscript is already installed. You can confirm that your installation works by typing ``exscript --version`` into a terminal; if this prints the version number, your installation is complete. We will also assume that you have at least a little bit of programming experience, though most of the examples should be pretty simple. Exscript also has extensive :doc:`API documentation `, which may be used as a reference throughout this tutorial. Getting started --------------- As a first simple test, let's try to connect to a network device via SSH2, and execute the ``uname -a`` command on it. Create a file named ``start.py`` with the following content:: from Exscript.util.interact import read_login from Exscript.protocols import SSH2 account = read_login() # Prompt the user for his name and password conn = SSH2() # We choose to use SSH2 conn.connect('localhost') # Open the SSH connection conn.login(account) # Authenticate on the remote host conn.execute('uname -a') # Execute the "uname -a" command conn.send('exit\r') # Send the "exit" command conn.close() # Wait for the connection to close Awesome fact: Just replace ``SSH2`` by ``Telnet`` and it should still work with Telnet devices. As you can see, we prompt the user for a username and a password, and connect to ``localhost`` using the entered login details. Once logged in, we execute ``uname -a``, log out, and make sure to wait until the remote host has closed the connection. You can see one important difference: We used ``conn.execute`` to run ``uname -a``, but we used ``conn.send`` to execute the ``exit`` command. The reason is that *``conn.execute`` waits until the server has acknowledged that the command has completed*, while ``conn.send`` does not. Since the server won't acknowledge the ``exit`` command (instead, it just closes the connection), using ``conn.execute`` would lead to an error. Making it easier ---------------- While the above serves as a good introduction on how to use :mod:`Exscript.protocols`, it has a few drawbacks: #. It only works for SSH2 or for Telnet, but not for both. #. It contains a lot of unneeded code. #. You can't use the script to connect to multiple hosts. Let's solve drawbacks 1. and 2. first. Here is a shortened version of the above script:: from Exscript.util.start import quickstart def do_something(job, host, conn): conn.execute('uname -a') quickstart('ssh://localhost', do_something) As you can see, we made two major changes: #. We moved the code that executes ``uname -a`` into a function named ``do_something``. (Note: We could have picked any other name for the function.) #. We imported and used the :func:`Exscript.util.start.quickstart` function. :func:`Exscript.util.start.quickstart` does the following: #. It prompts the user for a username and a password. #. It connects to the specified host, using the specified protocol. #. It logs in using the given login details. #. It calls our ``do_something()`` function. #. When ``do_something()`` completes, it closes the connection. Running a script on multiple hosts ---------------------------------- In practice, you may want to run this script on multiple hosts, and optimally at the same time, in parallel. Using the :func:`Exscript.util.start.quickstart` function, this is now really easy:: from Exscript.util.start import quickstart def do_something(job, host, conn): conn.execute('uname -a') hosts = ['ssh://localhost', 'telnet://anotherhost'] quickstart(hosts, do_something, max_threads=2) We only changed the last lines of the script: #. We pass in two hosts, ``localhost`` and ``anotherhost``. Note that ``localhost`` uses SSH, and ``anotherhost`` speaks Telnet. #. We added the ``max_threads=2`` argument. This tells Exscript to open two network connections in parallel. If you run this script, it will again ask for the login details, and run ``do_something()`` for both hosts in parallel. Note that the login details are only asked once and used on both hosts - this may or may not be what you want. For instructions one using different login mechanisms please refer to the section on accounts later. Loading hosts from a text file ------------------------------ If you do not wish to hard code the host names into the script, you may also list them in a text file and load it using :func:`Exscript.util.file.get_hosts_from_file` as follows:: from Exscript.util.start import start from Exscript.util.file import get_hosts_from_file def do_something(job, host, conn): conn.execute('uname -a') hosts = get_hosts_from_file('myhosts.txt') start(hosts, do_something, max_threads=2) Reading login information ------------------------- Depending on how you would like to provide the login information, there are a few options. The first is by hard coding it into the hostname:: hosts = ['ssh://localhost', 'telnet://myuser:mypassword@anotherhost'] quickstart(hosts, do_something, max_threads=2) In this case, ``quickstart`` still prompts the user for his login details, but the entered information is only used on hosts that do not have a user/password combination included in the hostname. If you do not wish to hard code the login details into the hostname, you can also use the Exscript.Host object as shown in the following script:: from Exscript import Host, Account … account1 = Account('myuser', 'mypassword') host1 = Host('ssh://localhost') host1.set_account(account1) account2 = Account('myuser2', 'mypassword2') host2 = Host('ssh://otherhost') host2.set_account(account2) quickstart([host1 , host2], do_something, max_threads=2) This script still has the problem that it prompts the user for login details, even though the details are already known. By using :func:`Exscript.util.start.start` instead of :func:`Exscript.util.start.quickstart`, you can avoid the prompt, and optionally pass in a pre-loaded list of accounts as seen in the following code:: from Exscript.util.start import start from Exscript.util.file import get_hosts_from_file def do_something(job, host, conn): conn.execute('uname -a') accounts = [] # No account needed. hosts = get_hosts_from_file('myhosts.txt') start(accounts, hosts, do_something, max_threads=2) Instead of passing in no account at all, you may also create one in the script:: from Exscript import Account … accounts = [Account('myuser', 'mypassword')] … Or you may load it from an external file:: from Exscript.util.file import get_accounts_from_file … accounts = get_accounts_from_file('accounts.cfg') … Note that ``accounts.cfg`` is a config file with a defined syntax as seen in the API documentation for :func:`Exscript.util.file.get_accounts_from_file`. Logging ------- Exscript has built-in support for logging. In a simple case, just pass the ``stdout`` and ``stderr`` parameters for log and errors to ``start()`` or ``quickstart()`` and you are done:: with open('log.txt','w+') as fp: start(accounts, hosts, do_something, stdout=fp) Exscript creates one logfile per device. In the case that an error happened on the remote device, it creates an additional file that contains the error (including Python's traceback). Interacting with a device ------------------------- So far we only fired and forgot a command on a device, there was no true interaction. But Exscript does a lot to make interaction with a device easier. The first notable tool is :mod:`Exscript.util.match` - a module that builds on top of Python's regular expression support. Let's look at an example:: from Exscript.util.start import quickstart from Exscript.util.match import first_match def do_something(job, host, conn): conn.execute('uname -a') print "The response was", repr(conn.response) os, hostname = first_match(conn, r'^(\S+)\s+(\S+)') print "The hostname is:", hostname print "Operating system:", os quickstart('ssh://localhost', do_something) The experienced programmer will probably wonder what happens when :func:`Exscript.util.match.first_match` does not find a match. The answer is: It will return a tuple ``None, None``. In other words, no matter what happens, the one liner can not fail, because :func:`Exscript.util.match.first_match` always returns a tuple containing the same number of elements as there are groups (bracket expressions) in the regular expression. This is more terse than the following typical regular idiom:: match = re.match(r'^(\S+)\s+(\S+)', conn.response) if match: print match.group(1) Similarly, the following use of :func:`Exscript.util.match.any_match` can never fail:: from Exscript.util.start import quickstart from Exscript.util.match import any_match def do_something(job, host, conn): conn.execute('ls -l') for permissions, filename in any_match(conn, r'^(\S+).*\s+(\S+)$'): print "The filename is:", filename print "The permissions are:", permissions quickstart('ssh://localhost', do_something) :func:`Exscript.util.match.any_match` is designed such that it always returns a list, where each item contains a tuple of the same size. So there is no need to worry about checking the return value first. Advanced queueing and reporting ------------------------------- :class:`Exscript.Queue` is a powerful, multi-threaded environment for automating more complex tasks. It comes with features such as logging, user account management, and error handling that make things a lot easier. The above functions :func:`Exscript.util.start.start` and :func:`Exscript.util.start.quickstart` are just convenience wrappers around this queue. In some cases, you may want to use the :class:`Exscript.Queue` directly. Here is a complete example that also implements reporting: .. literalinclude:: ../demos/report/report.py Emulating a remote device ------------------------- Exscript also provides a dummy protocol adapter for testing purposes. It emulates a remote host and may be used in place of the Telnet and SSH adapters:: from Exscript.protocols import Dummy conn = Dummy() ... In order to define the behavior of the dummy, you may define it by providing a Python file that maps commands to responses. E.g.:: def echo(command): return command.split(' ', 1)[1] commands = ( ('ls -l', """ -rw-r--r-- 1 sab nmc 1906 Oct 5 11:18 Makefile -rw-r--r-- 1 sab nmc 1906 Oct 5 11:18 myfile """), (r'echo [\r\n]+', echo) ) Note that the command name is a regular expression, and the response may be either a string or a function.