Readbackwards usage

jayu_rao has asked for the wisdom of the Perl Monks concerning the following question:

Hi Monks, I am trying to print last 10 lines from a file but hitting a strange problem. Can you please suggest what is wrong?

more last_ten.pl
#! /usr/bin/perl -w

use strict;
use File::ReadBackwards;
use diagnostics;

open my $read_log_file, '<', '/home/jay/my_perl_programs/log.txt' or d
+ie $!;
open my $output_file, '>', '/home/jay/my_perl_programs/log_output.txt'
+ or die $!;
my @log_last_ten;
my $log_backwards = File::ReadBackwards->new($read_log_file) or die $!
+;

while (defined(my $log_lines = $log_backwards->readline)) {
   unshift @log_last_ten, $log_lines;
   last if @log_last_ten >= 10;
}

print $output_file "\n----------------------Last 10 Lines from log fil
+e------------------------------\n";
for (@log_last_ten){
print $output_file $_;
}

close $read_log_file;
close $output_file;
[download]

The error is:

$ perl last_ten.pl

Uncaught exception from user code: No such file or directory at last_ten.pl line 10. at last_ten.pl line 10

This same code works if I change the line:

open my $read_log_file, '<', '/home/jay/my_perl_programs/log.txt' or die $!;

To:

open my $read_log_file='/home/jay/my_perl_programs/log.txt';

Regards

Jay

Comment on Readbackwards usage Download Code

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Re: ReadBackwards usage
by Athanasius (Archbishop) on Mar 16, 2015 at 06:18 UTC

Hello jayu_rao,

The method File::ReadBackwards->new() takes a filename (i.e., a string) as its first (and only required) argument. But if you first say:

open my $read_log_file, '<', '/home/jay/my_perl_programs/log.txt' or d
+ie $!;
[download]

you create $read_log_file as a file handle, which is something quite different. (It’s like a special kind of pointer that Perl uses to access a file.) You can see this if you try to print it as a string:

16:13 >perl -Mstrict -wE "open(my $fh, '<', '1184_SoPW.pl') or die; sa
+y qq[>$fh<];"
>GLOB(0x7ec4f8)<

16:14 >
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If you then call File::ReadBackwards->new($fh) it will try to access a file named “GLOB(0x7ec4f8)” — and fail, because there is no file of that name.

Hope that helps,

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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Re^2: ReadBackwards usage

by jayu_rao (Sexton) on Mar 16, 2015 at 07:17 UTC

The reason is that I would have to work with a temporary file and I would not know the name of the file that is created for giving the actual string.

Jay

[reply]

Re^3: ReadBackwards usage

by Athanasius (Archbishop) on Mar 16, 2015 at 07:48 UTC

Hello jayu_rao,

Here are two possible solutions:

(1) Create the temporary file using the core module File::Temp and get the filename together with the handle:

use File::Temp 'tempfile';
...
my ($fh, $filename) = tempfile();
my  $bw = File::ReadBackwards->new($filename)
    or die "Cannot read '$filename' backwards";

while (my $log_line = $bw->readline)
{
    ...
}
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(2) Use the File::ReadBackwards TIED HANDLE INTERFACE, which accepts a filehandle:

my $filename = 'data.txt';

open(my $bw, '<', $filename)
    or die "Cannot open file '$filename' for reading";

tie *$bw, 'File::ReadBackwards', $filename
    or die "Cannot read '$filename' backwards";

while (my $log_line = <$bw>)
{
    ...
}
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(Note the asterisk in tie *$bw — this is required. Also note the two different ways of reading the next line from the file: readline vs. <$bw>. N.B.: they are not interchangeable!)

Hope that helps,

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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Re: Readbackwards usage
by NetWallah (Canon) on Mar 16, 2015 at 06:03 UTC

This same code works if I ...

Your statement :

open my $read_log_file='/home/jay/my_perl_programs/log.txt';
[download]

The error it would get if checked is:

  No such file or directory
[download]

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