How to expand Perl variable in BASH context

Question

Alright, so the question heading might be a bit confusing. I have replicated my actual problem in below code:

I am creating files with two different IP addresses as a prefix and then I want to list the files only with a particular ip address prefix:

I am creating files with "10.10.10.10" and "11.11.11.11" as prefix and then I want to list files only with the "10.10.10.10" prefix.

Here is my code:

#!/usr/bin/perl -w
use strict;

my $ipaddr1 = '10.10.10.10';
for ( my $counter = 1; $counter < 10; $counter++)
{
    my $filename = $ipaddr1."_".$counter.".txt";
    open (my $fh, ">", $filename) or die "$! \n";
}

my $ipaddr2 = '11.11.11.11';
for ( my $counter = 1; $counter < 10; $counter++)
{
    my $filename = $ipaddr2."_".$counter.".txt";
    open (my $fh, ">", $filename) or die "$! \n";
}
print "Done with creating hte files. \n";

print "Lets print the files with only 10.10.10.10 prefix \n";

for my $var ( `ls -1 $ipaddr1_*` )  ### <<< this is the probelm
{
    print $var;
}

Output:

Global symbol "$ipaddr1_" requires explicit package name at test.pl line 21.
Execution of test.pl aborted due to compilation errors.

So, I fully understand the error that it is not able to interpret the perl variable while executing a bash command.

Question:

how to achieve this? I do not want to pass explicit IP address to the ls -1 command but instead want to pass the variable which holds the respective IP address.

Thanks.

I suggest using another approach, File::Glob. Explicitly or via the angle-bracket operator, <> — larsen
– larsen, Commented Mar 7, 2013 at 10:34

Ilmari Karonen · Accepted Answer · 2013-03-07 10:56:33Z

3

The problem is that Perl interprets the underscore _ as part of the variable name. The most straightforward solution is to add disambiguating brackets so that this won't happen:

`ls -1 ${ipaddr1}_*`

answered Mar 7, 2013 at 10:56

Ilmari Karonen

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Comments

amon · Accepted Answer · 2013-03-07 13:05:08Z

The ${ipaddr1}_ syntax given by Ilmari Karonen is the correct solution. There are a few other things that might be improved:

Don't start a new process for something Perl can already do:
```
for my file (glob "${ipaddr1}_*") {
  print "$file\n";
}
```
This is also more portable :)
Be aware that open my $fh, ">", $filename will clobber the file. If you only want to create non-existing files but don't want to clobber the content, use the >> append mode instead.

Make your life easy and your code short! Use loops and ranges:

for my $ipaddr ("10.10.10.10", "11.11.11.11") {
  for my $counter (1 .. 9) {
    my $filename = sprintf "%s_%d.txt", $ipaddr, $counter;
    open my $fh, ">", $filename or die "Can't open $filename: $!";
  }
}

slayedbylucifer · Accepted Answer · 2013-03-07 10:19:48Z

1

Solved:

my $cmd = "ls -1 ". $ipaddr1."_*.txt";
for my $var ( `$cmd` )
{
    print $var;
}

answered Mar 7, 2013 at 10:19

slayedbylucifer

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