SPUG: shell script event loops + wrapping shell utilities

[Tim Maher]

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	======================================================
	|  Tim Maher                   tim at timmaher.org      |
	|  SPUG Founder & Leader       spug at seattleperl.org  |
	|  Seattle Perl Users Group    www.seattleperl.org   |
	======================================================

       
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If you're on a *nix system, I recommend using Exceed for this kind of
thing.  Tcl configured, but it's not too hard.  There's an O'Reilly book
for it and everything.

Not to discourage a discussion on the subject, I'd very much like to know
how to do this efficiently in Perl.

-- Dan

On Thu, 18 Apr 2002, dancerboy wrote:

> So, I wanted to add some minor functionality to a couple of shell
> programs (telnet and ftp, specifically) and the simplest way seemed
> to be simply to wrap them in a Perl script, i/e simply make a Perl
> script that opens the program, and pipes input from the terminal
> (keystrokes) through to the program, and passes characters output
> from the program back to the terminal -- occasionally adding some
> extra keystrokes of its own (the added functionality) but mostly just
> acting as a "bidirectional pipe" between the program and the terminal.
>
> Here is the main wrapper part of the ftp version of the script
> (greatly stripped-down for clarity):
>
> ____________________________
> #!/usr/bin/perl
>
> use strict;
> use Fcntl;
> use IPC::Open3;
>
> $| = 1;
>
> my $from_term;
> my $from_shell;
> my $pipe_error;
>
> open(TTY, "+</dev/tty") or die "no tty: $!";
> fcntl( TTY, F_SETFL, O_NONBLOCK );
>
> my $pid = open3( \*TO_SHELL, \*FROM_SHELL, \*SHELL_ERR,
>          'ftp -v strangelight.com'
> );
>
> fcntl( FROM_SHELL, F_SETFL, O_NONBLOCK );
> fcntl( SHELL_ERR, F_SETFL, O_NONBLOCK );
>
> my $oldfh = select(FROM_SHELL); $| = 1; select($oldfh);
> $oldfh = select(SHELL_ERR); $| = 1; select($oldfh);
> $oldfh = select(TTY); $| = 1; select($oldfh);
>
> $SIG{PIPE} = sub { ++$pipe_error; };
>
> while ( not $pipe_error ) {
>                  while( defined( $from_term = getc(TTY) ) ) {
>                          print TO_SHELL $from_term;
>                  }
>                  while( defined( $from_shell = getc(FROM_SHELL) ) ) {
>                          print $from_shell;
>                  }
>                  while( defined( $from_shell = getc(SHELL_ERR) ) ) {
>                          print $from_shell;
>                  }
> }
>
> __END__
>
> Now, I have two questions.
>
> My first question is, admittedly, one of those "I could probably
> figure it out on my own but I'm lazy so I'll ask the folks on SPUG
> instead" questions:
>
> While my script works fairly well as-is, it's a real processor hog,
> as you can probably guess.  All of those getc() calls are
> non-blocking, so even when there's no input or output to process, the
> main while() loop still keeps executing over and over again, doing
> absolutely nothing as fast as it possibly can.  What's the
> best/simplest way to tell Perl to "go to sleep, but wake up as soon
> as something interesting happens"?
>
> My second question is one I've banged my head on for a while and
> haven't been able to figure out at all:
>
> Even with setting $|=1 on all the open pipes, I still don't always
> get my I/O flushed promptly -- in particular, the responses I read
> back from FROM_SHELL seem often to be one or two lines behind what I
> should be getting. (My work-around has been to send a no-op command
> like 'pwd', the response to which usually forces the lines that I
> want to see out of the buffer.)  What else can I do to get my pipes
> "piping hot" (as the perldocs say)?
>
> -jason
>
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