Updated 2010-05-17 20:37:55 by AMG

AMG: As its name implies, multissh ssh'es to multiple hosts for the purpose of executing commands. This all by itself is a very trivial task, but the fun begins when you start to consider what to do with stdin, stdout, and stderr. multissh takes all input received on stdin and sends it to all remote hosts; all stdout and stderr text from the remote hosts is piped to the local stdout and stderr, with the hostname prepended.

New in version 0.2, multissh is also a Tcl package, consisting of a single command. I want you to guess what that command is named. As a package multissh is quite limited: basically if you're writing a Tcl script using multissh, you can call it as a command rather than [exec]'ing it, and that's all. The script can't supply input (besides the command line) and can't see the output or error messages. But if there is interest, multissh can be made much nicer.

Perhaps an example will help. Here's a "screenshot":
 [[email protected]|~]# multissh "localhost bravo charlie" uptime
 localhost:  17:51:49 up  4:52,  2 users,  load average: 0.00, 0.00, 0.00
 bravo:  17:50:52 up  2:29,  1 user,  load average: 0.00, 0.00, 0.00
 charlie:  17:51:05 up  2:27,  1 user,  load average: 0.00, 0.00, 0.00
 [[email protected]|~]#

Well, there you have it. Now for stdin handling:
 [[email protected]|~]# multissh "localhost bravo charlie" "cat > file"
 illustrative verbiage
 [[email protected]|~]# multissh "localhost bravo charlie" "cat < file"
 localhost: illustrative verbiage
 bravo: illustrative verbiage
 charlie: illustrative verbiage
 [[email protected]|~]# multissh "localhost bravo charlie" "rm file"
 [[email protected]|~]#

Amazing, really. stderr works too:
 [[email protected]|~]# multissh "localhost bravo charlie" "rm file"
 localhost: rm: cannot remove `file': No such file or directory
 charlie: rm: cannot remove `file': No such file or directory
 bravo: rm: cannot remove `file': No such file or directory
 [[email protected]|~]#

Although not very well. :^) I don't get access to stderr until I [close] the ssh channels, so all error messages are delayed until the session is closed.

There's a related problem. Some programs, for example sort, generate output after getting EOF on stdin. But Tcl doesn't support closing a child process channel's stdin and continuing to listen to its stdout/stderr, so it can never see any output that happens after stdin EOF. That's bad. On the other hand, if I didn't tell you about this problem, you'd probably never notice it--- how often do you run sort on remote hosts, anyway? Just be wary for the time being, and maybe someday this will be fixed.

Here's the TODO/ideas list. If you have any comments or implementation tips, write them here or email them to me (Andy Goth).

  • Continue investigation of closing only stdin and leaving stdout/stderr open.
  • Continue investigation of getting stderr text without closing the channel.
  • Double-check that the standalone/sourced test is correct.
  • Make reentrant by eliminating globals or creating "instances".
  • Allow the caller to supply handlers for various events.
  • Convert the [puts]s into default event handlers.
  • Allow the caller to supply an alternate source for stdin data.
  • Optionally return immediately and expose [node_wait] in some form.
  • Be more configurable in general.
  • Write a GUI frontend.

And another thing...

Lately I have written several tools that all need to be callable both from Tcl and the shell using more or less the same interface. I'm considering factoring out the common code into a package. Basically such a package would need to handle detecting whether the code was executed or sourced, outputting through stdout/stderr versus [return]/[error], and generating shell-style or Tcl-style lists. Is anyone interested? I don't think it's very much code, but I have it duplicated in a lot of places.

AMG, update: Tcl 8.6 makes it possible to do several of the most important things on my TODO list. In particular, I can separately close stdin and stdout by means of TIP #332 "Half-Close for Bidirectional Channels" [1]. Also I can use TIP #304 "A Standalone chan pipe Primitive for Advanced Child IPC" to access stderr as it gets generated.
 What: multissh
 Where: http://andy.junkdrome.org/devel/multissh/
 Description: Program/package for simultaneously executing commands on
              multiple remote hosts.
 Version: 0.2
 Updated: 27 October 2005
 License: GPL 2.0 or later
 Contact: Andy Goth <[email protected]>