A possible solution for the onlyshell.pl program is as follows:

onlyshell.pl

#! /usr/bin/perl
 
# onlyshell.pl - read from "psout", print first line, and only lines
#  which represent shell processes.  A shell is a process in which
#  the COMMAND field (last field in "ps" output) is a
#  "csh" or "bash", which has a minus sign before it:
#
#  kjacks   21255  Sep30   0:00 -bash
#
# but not:
#
#  kjacks   24536  11:47   0:00 bash
 
use warnings;
use strict;
 
my $filename = 'psout';
 
# open $filename for reading
open PS, "<$filename"
    or die "open($filename) $!";
 
# read first line and print it
my $line = <PS>;
print $line;
 
# read remaining lines and print only those which match the shell pattern
while (<PS>)
{
    print
	if ( m/\s-(bash|csh)$/ );
}
 
# close the file
close PS;
 

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