the output of a perl script which calls "sudo lsof | grep username | wc -l" differs in cron vs manually

Question

We are having issues with a perl script we run for file count:

#!/usr/bin/perl
    $stat=`sudo lsof | grep username | wc -l`;
    $date=`date '+ %Y/%m/%d-%H:%M:%S'`;
    $exit=`echo $?`;
    if ( $exit == 0 ) {
            print "$date\n Statistic: $stat\n";
            exit 0;
            }

We are able to run this script manually and get the output fine:

# perl test.pl
 2020/07/30-19:17:10
 Statistic: 580

Running as a cronjob */5 * * * * perl /filepath/test.pl >> /filepath/test.txt, it outputs 0:

 2020/07/30-19:20:01
 Statistic: 0

This is running on a RHEL4 machine and we want to run it as a cronjob to append a file to get statistical data.

Answer 1

Improving the script might work:

#!/usr/bin/perl
use POSIX qw(strftime);
$user = shift; $outputfile = shift;
$lsof=`lsof 2> /dev/null`;
exit $? unless($? == 0);
$count=0;
foreach(split /\n/, $lsof) {
        $count++ if(/$user/);
}
open($fh, ">>$outputfile") or die "Can't append to $outputfile";
print $fh strftime(" %Y/%m/%d-%H:%M:%S\n Statistic: $count\n", localtime);
close $fh;

Which would become this in cron

*/5 * * * * root perl /filepath/test.pl username /filepath/test.txt

Things that were wrong in your case:

• The username field was missing in cron (I use root so we don't have to elevate privileges with sudo)
• Even if there was a username, sudo would fail because of a missing password
• echo $? will be the returncode of the echo-command, all other commands are ignored
• Not exactly wrong, but advised against: too much shell code in your perlscript
• Might not be wrong but I think you don't want it: All the warnings of lsof weren't filtered away

If it still doesn't work try switching lsof by the full path (most likely /usr/bin/lsof)

Edit: Another problem: your grep and my regexp will also find lines that contain username when this string is not meant as a username.
This code is even better:

#!/usr/bin/perl
use POSIX qw(strftime);
$user = shift; $outputfile = shift;
$count=split(/n/,`lsof -u $user 2> /dev/null`);
exit $? unless($?==0);
open($fh, ">>$outputfile") or die "Can't append to $outputfile";
print $fh strftime(" %Y/%m/%d-%H:%M:%S\n Statistic: $count\n", localtime);
close $fh;