[CMI.PM] Trapping STDOUT in-process?

[Mitch Kutzko]

Perfect, Ed -- Exactly what I was looking for.

Thanks!

Mitch

At 09:19 PM 10/7/2004 -0500, Ed Thomson wrote:
> Hey Mitch-
> 
> qx// will read the entire output into a buffer, and will wait for the 
> process to complete before it continues.  So, the program has exited by 
> the time you get to your for() loop.
> 
> Instead of running the command, then running the for loop over the 
> output, try opening the program's output directly and running what's in 
> the for() loop inside a while loop reading the input.
> 
> Eg:
> open(TRACEROUTE, "traceroute $target 2>&1 |") || die;
> while(<TRACEROUTE>)
> {
>    .. print the lines ..
> }
> close(TRACEROUTE);
> 
> I think this will help -- good luck!
> 
> -Ed
> 
> Mitch Kutzko wrote:
> > Hi, folks -- Is there a way for me to read STDOUT on the fly, as it
> > happens, before the command I'm execing is complete?
> > 
> > Background:
> > 
> > I need to write a script that returns the names of all the hosts involved
> > in a traceroute.  This is fairly simple:
> > 
> > ------------------------------------------------------------
> > #!/usr/bin/perl -w
> > 
> > use strict;
> > 
> > my $target = "qbridge.bu.edu";  # Default target
> > if (defined $ARGV[0]) {         # If they gave a different target
> >         $target = $ARGV[0];
> > }
> > 
> > $target = lc($target);          # Lowercase it
> > 
> > # Get the output text for parsing, and supress console output
> > my @results = qx/traceroute $target/;
> > my @errors = qx/traceroute $target 2>&1/;
> > 
> > if (!@results && @errors) {             # What went wrong?
> >         print "Uh-oh:\n at errors\n";
> >         exit;
> > }
> > 
> > 
> > my $lth = scalar @results;      # Get number of lines (= # of hops)
> > my $hostname = `hostname -f`;   # Get local  hostname (-f may not work
some
> > OSs)
> > chomp $hostname;                # Remove trailing space
> > 
> > print "Results: $lth hops from $hostname to $target\n";
> > 
> > # Iterate over lines/hops to get hostnames
> > my $ii=0;
> > for ($ii=0;$ii<$lth;$ii++) {
> >         my @spot = split ' ', $results[$ii];    # Break on word boundaries
> >         $spot[1] = lc($spot[1]);                # Lowercase the name
> >         print "$ii) $spot[1]\n";                # Print it
> > }
> > 
> > exit;
> > ------------------------------------------------------------
> > 
> > 
> > If the host is pingable, or if the host doesn't actually exist, this works
> > fine.
> > 
> > The problem is the case in which this is likely to be used most often, in
> > which a legitimate host doesn't have a route to it at the moment (which is
> > why the diagnostic is being run in the first place), and so generates
> > output like this:
> > 
> > [mitch at jhereg advisor]$ traceroute dyn09.iacc-t21.ndsu.nodak.edu
> > traceroute to dyn09.iacc-t21.ndsu.nodak.edu (134.129.71.204), 30 hops max,
> > 38 byte packets
> >  1  netdev98-254 (141.142.98.254)  12.878 ms  0.381 ms  0.341 ms
> >  2  sbr0-vlan116.gw.ncsa.edu (141.142.0.65)  0.459 ms  0.543 ms  0.409 ms
> >  3  198.17.196.2 (198.17.196.2)  3.025 ms  2.980 ms  3.038 ms
> >  4  chin-mren-ge.abilene.ucaid.edu (198.32.11.97)  3.042 ms  3.178 ms
> > 3.010 ms
> >  5  iplsng-chinng.abilene.ucaid.edu (198.32.8.77)  175.039 ms  237.742 ms
> > 256.378 ms
> >  6  mn-abilene.northernlights.gigapop.net (192.42.152.170)  27.838 ms
> > 27.764 ms  27.763 ms
> >  7  ndsu-i2r.northernlights.gigapop.net (192.42.152.173)  34.522 ms
34.154
> > ms  34.337 ms
> >  8  vlan-107.cc.ndsu.NoDak.edu (134.129.107.100)  44.434 ms  34.303 ms
> > 34.304 ms
> >  9  * * *
> > 10  * * *
> > 
> > 
> > The problem with my script above is that it relies on the command actually
> > either completing or failing.  This case is "in-process", and so does
neither.
> > 
> > When run via my script, no output occurs at all, as the qx// never
comes back.
> > 
> > So my question is:
> > 
> > Is there a way for me to read STDOUT on the fly, as it happens, before the
> > command I'm execing is complete?
> > 
> > Thanks!
> > 
> > Mitch
> > --
> > Mitch Kutzko | mitch at dast.nlanr.net | mitch at ncsa.uiuc.edu | 217-333-1199
> > http://hobbes.ncsa.uiuc.edu/
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> 
--
Mitch Kutzko | mitch at dast.nlanr.net | mitch at ncsa.uiuc.edu | 217-333-1199
http://hobbes.ncsa.uiuc.edu/