redirected output is buffered, how do you flush it?

[Tom Christiansen]

>From the keyboard of rhoward at msd.gatech.edu (Robert L. Howard):
:I have a script that after several pipe stops outputs a line
:of information.  When run from a tty it outputs a line every
:few seconds to several minutes.  Wrapped around the commands
:is an infinite loop.  I can specify a trap command that will
:close up things gracefully (print summary info etc.).
:
:The problem comes in when I run:
:
:% script > some_file
:
:and then kill it some number of minutes later.  The total output
:of the script is still in some buffer somewhere and doesn't make
:it to the file.  Is there some command I can put in the 'trap' to
:force it to flush the buffers?  Or is there a recommended way to
:kill the job (other than ^C) that will force the buffers to flush?

This is a general problem that comes up often, and I don't know any
way of doing it unless you can get the program doing the writes to 
flush its buffers now and then.  If awk only had a way to force flushes,
then it wouldn't be so rough; sadly, it doesn't.

:Here is the script if you are wondering what I am talking about...
:
:------------------------------------------------------------------
:#! /bin/sh
:#
:pstat=/usr/etc/pstat
:
:
:trap <Some-command_here> 1 2 3 14 15
:
:echo "Starting at `date`"
:echo "Interval is $1 seconds."
:echo ""
:
:while true
:do
:    $pstat -T | sed -e 's/\//\ /g'
:    sleep $1
:done | awk '
:/files/	{ if ($1 > files) {
:	files = $1
:	printf ("max files\t%5d out of %5d, or %6.2f%\n", \
:		files, $2, 100*files/$2) }
:    }' -

Here's a fairly direct translation of your program into perl, 
which does have a way to set buffering: if $| is non-zero, then
output will be flushed after each print.  It even looks a lot
like your script.  

    #!/usr/bin/perl
    $pstat='/usr/etc/pstat';
    $nap = shift; 		# i like to name my args
    $| = 1; 			# auto-flush at end of print statements
    print "Starting at ", `date`;
    print "Interval is $nap seconds.\n\n";
    while (1) {
	`$pstat -T` =~ /(\d+)\/(\d+) files/; # $1 and $2 get set here
	if ($1 > $files) {
	    $files = $1;
	    printf ("max files\t%5d out of %5d, or %6.2f%%\n", 
		    $files, $2, 100*$files/$2);
	}
	sleep $nap;
    }

--tom
--
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