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
We will also assume that you have at least a little bit of programming experience, though most of the examples should be pretty simple. If you already understand how to use Perl’s Net::Telnet, you should feel right at home.
Exscript also has extensive Exscript, which may be used as a reference throughout this tutorial.
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
test.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
Telnet and it should
still work with Telnet devices.
As you can see, we prompt the user for a username and a password, and
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
The reason is that ``conn.execute`` waits until the server has
acknowledged that the command has completed, while
not. Since the server won’t acknowledge the
exit command (instead,
it just closes the connection), using
conn.execute would lead to an
Making it easier¶
While the above serves as a good introduction on how to use
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 -ainto a function named
do_something. (Note: We could have picked any other name for the function.)
- We imported and used the
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()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
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,
anotherhost. Note that
localhostuses SSH, and
- We added the
max_threads=2argument. 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
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
Exscript.util.start.start() instead of
Exscript.util.start.quickstart(), you can avoid the prompt,
and optionally pass in a pre-loaded list of accounts as seen in the
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') …
accounts.cfg is a config file with a defined syntax as
seen in the API documentation for
Exscript has built-in support for logging. In a simple case, just pass
stderr parameters for log and errors to
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
Exscript.util.match - a module
that builds on top of Python’s regular expression support. Let’s look at
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
Exscript.util.match.first_match() does not find a match. The
answer is: It will return a tuple
In other words, no matter what happens, the one liner can not fail,
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
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)
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¶
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
Exscript.util.start.quickstart() are just convenience wrappers
around this queue.
In some cases, you may want to use the
Here is a complete example that also implements reporting:
#!/usr/bin/env python from Exscript import Queue, Logger from Exscript.util.log import log_to from Exscript.util.decorator import autologin from Exscript.util.file import get_hosts_from_file, get_accounts_from_file from Exscript.util.report import status, summarize logger = Logger() # Logs everything to memory. @log_to(logger) @autologin() def do_something(job, host, conn): conn.execute('show ip int brie') # Read input data. accounts = get_accounts_from_file('accounts.cfg') hosts = get_hosts_from_file('hostlist.txt') # Run do_something on each of the hosts. The given accounts are used # round-robin. "verbose=0" instructs the queue to not generate any # output on stdout. queue = Queue(verbose=5, max_threads=5) queue.add_account(accounts) # Adds one or more accounts. queue.run(hosts, do_something) # Asynchronously enqueues all hosts. queue.shutdown() # Waits until all hosts are completed. # Print a short report. print status(logger) print summarize(logger)
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) 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.