[Twisted-Python] Conch: multiple commands

Paul Swartz paulswartz at gmail.com
Thu Feb 9 10:14:20 EST 2006

Brendan Simon wrote:
> I'm trying to use and understand the latest Twisted and Conch (I'm a 
> newbie).  I can get teh sample apps going fine but they all seem to send 
> one command and then exit.  I'm trying to workout how to send multiple 
> commands to the server from the same client application.
> The sshsimpleclient.py does send multiple commands (false, true, cat) 
> with separate channels.
> Is that the preferred way to send multiple commands?
> Doesn't that require some setup overhead every time I want to issue a 
> command to the server?

That's the way SSH requires sending multiple commands.  It does require 
some server overhead, but there's no way around that.

> Is it possible to open _one_ channel, and send commands in a 
> synchronously?  ie. wait for commands to return output and then issue 
> other commands if necesary??
> Maybe this goes against the grain of the Twisted Asynchronous Framework ???

You can send one command at a time, but you still have to write the code 

> If I assume that I have to open a new channel for each command, and I 
> have a 100 different commands, I could have 100 classes for each command 
> (seems a bit extreme) or have one class that gets the command passed to 
> it.  The problem I see with the latter is that I don't know how to wait 
> until the first command has got a reply and is finished.  It seems that 
> the channelOpen method returns straight away.  How can I control the 
> program execution to wait for the response, error or timeout?  Example:
>    def servicesStarted(self):
>        self.openChannel(CommandChannel.openChannel('ls /tmp/dir1')
>        # how do I wait for list of dir1 to finish?
>        self.openChannel(CommandChannel.openChannel('ls /tmp/dir2')
>        # how do I wait for list of dir2 to finish?
>        self.openChannel(CommandChannel.openChannel('ls /tmp/dir3')
>        # how do I wait for list of dir3 to finish?

What I'd do is also pass a Deferred to CommandChannel, that is 
call/errbacked when the command finishes.

def servicesStarted(self):
     d = defer.Deferred()
     self.openChannel(CommandChannel(d, "ls /tmp/dir1"))

def _cbFirstLs(self, result):
     print 'directory listing', result
     d = defer.Deferred()
     self.openChannel(CommandChannel(d, "ls /tmp/dir2"))

def _ebFirstLs(self, f):

etc., for all the commands you want to run.

Paul Swartz
paulswartz at gmail dot com
AIM: z3penguin

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