Output from bash command not storing in array

Question

my below code is very simple... all im doing is grepping a file using an IP address REGEX, and any IP addresses that are found that match my REGEX, I want to store them in @array2.

i know my REGEX works, because ive tested it on a linux command line and it works exactly how I want, but when its integrated into the script, it just returns blank. there should be 700 IP's stored in the array.

#!/usr/bin/perl
use warnings;
use strict;

my @array2 = `grep -Eo "\"\b[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\"" test1.txt`;
print @array2;

I would say newbies don't mix perl and shell as it doesn't work for them, and experienced ones simply know better. — mpapec
– mpapec, Commented Aug 27, 2014 at 20:16

Miller · Accepted Answer · 2014-08-27 20:27:42Z

3

Backticks `` behave like a double quoted string by default.

Therefore you need to escape your backslashes:

my @array2 = `grep -Eo "\\"\\b[0-9]{1,3}\\.[0-9]{1,3}\\.[0-9]{1,3}\\.[0-9]{1,3}\\"" test1.txt`;

Alternatively, you can use a single quoted version of qx to avoid any interpolation:

my @array2 = qx'grep -Eo "\"\b[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\"" test1.txt';

However, the method I'd recommend is to not shell out at all, but instead do this logic in perl:

my @array2 = do {
    open my $fh, '<', 'test1.txt' or die "Can't open file: $!";
    grep /\b[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\b/, <$fh>;
};

edited Aug 27, 2014 at 20:27

answered Aug 27, 2014 at 20:22

Miller

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Comments

mpapec · Accepted Answer · 2014-08-27 20:31:07Z

2

I really wouldn't mix bash and perl. It's just asking for pain. Perl can do it all natively.

Something like:

open (my $input_fh, "<", "test.txt" ) or die $!;
my @results = grep ( /\b[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}/, <$input_fh> );

This does however, require slurping the file into memory, which isn't optimal - I'd generally use a while loop, instead.

edited Aug 27, 2014 at 20:31

mpapec

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answered Aug 27, 2014 at 20:22

Sobrique

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Comments

tripleee · Accepted Answer · 2014-08-27 20:16:21Z

1

The text inside the backticks undergoes double-quotish substitution. You will need to double your backslashes.

Running grep from inside Perl is dubious, anyway; just slurp in the text file and use Perl to find the matches.

answered Aug 27, 2014 at 20:16

tripleee

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1 Comment

Sobrique Over a year ago

Perl does have a grep builtin, which'll do the trick. I'd tend to use a while loop instead though, because it's less memory expensive.

Jeremy Melanson · Accepted Answer · 2014-08-28 03:14:37Z

0

The easiest way to retrieve the output from an external command is to use open():

open(FH, 'grep -Eo \"\b[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\" test1.txt'."|")  
my @array2=<FH>;
close (FH);

..though I think Sobrique's idea is the best answer here.

answered Aug 28, 2014 at 3:14

Jeremy Melanson

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