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Once you have a connected SSH client, the entire world of SSH operations is open to you. Simply by asking, you can access remote-shell sessions, run individual commands, commence file-transfer sessions, and set up port forwarding. We will look at each of these operations in turn. First, SSH can set up a raw shell session for you, running on the remote end inside a pseudoterminal so that programs act like they normally do when they are interacting with the user at a terminal. This kind of connection behaves very much like a Telnet connection; take a look at Listing 16 5 for an example, which pushes a simple echo command at the remote shell, and then asks it to exit. Listing 16 5. Running an Interactive Shell Under SSH #!/usr/bin/env python # Foundations of Python Network Programming - 16 - ssh_simple.py # Using SSH like Telnet: connecting and running two commands import paramiko class AllowAnythingPolicy(paramiko.MissingHostKeyPolicy): def missing_host_key(self, client, hostname, key): return client = paramiko.SSHClient() client.set_missing_host_key_policy(AllowAnythingPolicy()) client.connect('127.0.0.1', username='test') # password='') channel = client.invoke_shell() stdin = channel.makefile('wb') stdout = channel.makefile('rb') stdin.write('echo Hello, world\rexit\r') print stdout.read() client.close()
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CHAPTER 16 TELNET AND SSH
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You will see that this awkward session bears all of the scars of a program operating over a terminal. Instead of being able to neatly encapsulate each command and separate its arguments in Python, it has to use spaces and carriage returns and trust the remote shell to divide things back up properly.
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Note All of the commands in this section simply connect to the localhost IP address, 127.0.0.1, and thus should work fine if you are on a Linux or Mac with an SSH server installed, and you have copied your SSH identity public key into your authorized-keys file. If, instead, you want to use these scripts to connect to a remote SSH server, simply change the host given in the connect() call.
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Also, if you actually run this command, you will see that the commands you type are actually echoed to you twice, and that there is no obvious way to separate these command echoes from the actual command output: Ubuntu 10.04.1 LTS Last login: Mon Sep 6 01:10:36 2010 from 127.0.0.9 echo Hello, world exit test@guinness:~$ echo Hello, world Hello, world test@guinness:~$ exit logout Do you see what has happened Because we did not wait for a shell prompt before issuing our echo and exit commands (which would have required a loop doing repeated read() calls), our command text made it to the remote host while it was still in the middle of issuing its welcome messages. Because the Unix terminal is by default in a cooked state, where it echoes the user's keystrokes, the commands got printed back to us, just beneath the Last login line. Then the actual bash shell started up, set the terminal to raw mode because it likes to offer its own command-line editing interface, and then started reading your commands character by character. And, because it assumes that you want to see what you are typing (even though you are actually finished typing and it is just reading the characters from a buffer that is several milliseconds old), it echoes each command back to the screen a second time. And, of course, without a good bit of parsing and intelligence, we would have a hard time writing a Python routine that could pick out the actual command output the words Hello, world from the rest of the output we are receiving back over the SSH connection. Because of all of these quirky, terminal-dependent behaviors, you should generally avoid ever using invoke_shell() unless you are actually writing an interactive terminal program where you let a live user type commands. A much better option for running remote commands is to use exec_command(), which, instead of starting up a whole shell session, just runs a single command, giving you control of its standard input, output, and error streams just as though you had run it using the subprocess module in the Standard Library. A script demonstrating its use is shown in Listing 16 6. The difference between exec_command() and a local subprocess (besides, of course, the fact that the command runs over on the remote machine!) is that you do not get the chance to pass command-line arguments as separate strings; instead, you have to pass a whole command line for interpretation by the shell on the remote end.
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