Re: tf - tail a file and output separator line when inactive

Often I find myself doing tail -f file | grep --line-buffered "kernel" and your program has given me the inspiration to automate that. This is my version. It features taint mode and File::Tail:

#!/usr/bin/perl -T

use strict;
use warnings;
use File::Tail;

die "Not enough arguments\n" unless @ARGV;
my ($file, $regex) = @ARGV;
$regex = '.' unless defined $regex && length $regex;

my $tail = File::Tail->new (
    name        => $file,
    maxinterval => 5,
    adjustafter => 10,
);

$SIG{ALRM} = sub { printf "\n%s\n", '='x80; };

while (defined ($_ = $tail->read)) {
    print && alarm 2 if /$regex/;
}
[download]

Let me anticipate that someone is going to reply talking about code interpolation in the regex:

# ~/bin/tf2.pl file '(??{print "Hello, world!\n"})'
Eval-group not allowed at runtime, use re 'eval' in regex m/(??{print 
+"Hello, world!\n"})/ at /root/bin/tf2.pl line 20.
[download]

Update: Minor change according to imp's comment.

--
David Serrano

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Re^2: tf - tail a file and output separator line when inactive by imp (Priest) on Jul 18, 2006 at 17:46 UTC
Nice addition. I would only change one thing: `while (defined ($_ = $tail->read)) { if (/$regex/) { print; alarm 2; } }` [download] That will prevent separator lines after reading a line that was filtered.	[reply] [d/l]
Re^2: tf - tail a file and output separator line when inactive by ikegami (Patriarch) on Jul 18, 2006 at 17:09 UTC
`use re 'eval';` would remove the error, but using it would defy taint checking.	[reply] [d/l]
Re^3: tf - tail a file and output separator line when inactive by Hue-Bond (Priest) on Jul 18, 2006 at 17:38 UTC
My point is precisely that there's no way of running arbitrary code even when the regex isn't being checked or cleaned. Of course, the user running this will often be root so this is only an issue if the script is somehow made setuid. I only pointed it out to avoid the typical subthread about "Your usage of `/$regex/` is insecure!". -- David Serrano	[reply]
Re^4: tf - tail a file and output separator line when inactive by ikegami (Patriarch) on Jul 18, 2006 at 17:51 UTC
Sorry, I had misunderstood. Had I understood your point, I would have said: Indeed, without `use re 'eval';`, regexps won't knowingly run Perl code in interpolated variables. However, it's still possible to write very expensive regepxs and (I think) to crash Perl. I don't know if a crash can be exploited to execute native code.	[reply] [d/l]
Re^3: tf - tail a file and output separator line when inactive by Aristotle (Chancellor) on Jul 18, 2006 at 18:31 UTC
using it would defy taint checking. You mean “defeat” – unless you want to say that the code will doggedly refuse to be taint-checked. `:-)` Makeshifts last the longest.	[reply]