How do I open an array of files in Perl?

Question

In perl, I read in files from a directory, and I want to open them all simultaneously (but line by line) so that I can perform a function that uses all of their nth lines together (e.g. concatenation).

my $text = `ls | grep ".txt"`;
my @temps = split(/\n/,$text);
my @files;
for my $i (0..$#temps) {
  my $file;
  open($file,"<",$temps[$i]);
  push(@files,$file);
}
my $concat;
for my $i (0..$#files) {
  my @blah = <$files[$i]>;
  $concat.=$blah;
}
print $concat;

I just a bunch of errors, use of uninitialized value, and GLOB(..) errors. So how can I make this work?

Answer 1

A lot of issues. Starting with call to "ls | grep" :)

Let's start with some code:

First, let's get list of files:

my @files = glob( '*.txt' );

But it would be better to test if the given name relates to file or directory:

my @files = grep { -f } glob( '*.txt' );

Now, let's open these files to read them:

my @fhs = map { open my $fh, '<', $_; $fh } @files;

But, we need a way to handle errors - in my opinion the best way is to add:

use autodie;

At the beginning of script (and installation of autodie, if you don't have it yet). Alternatively you can:

use Fatal qw( open );

Now, that we have it, let's get the first line (as you showed in your example) from all of the inputs, and concatenate it:

my $concatenated = '';

for my $fh ( @fhs ) {
    my $line = <$fh>;
    $concatenated .= $line;
}

Which is perfectly fine, and readable, but still can be shortened, while maintaining (in my opinion) readability, to:

my $concatenated = join '', map { scalar <$_> } @fhs;

Effect is the same - $concatenated contains first lines from all files.

So, whole program would look like this:

#!/usr/bin/perl
use strict;
use warnings;
use autodie;
# use Fatal qw( open ); # uncomment if you don't have autodie

my @files        = grep { -f } glob( '*.txt' );
my @fhs          = map { open my $fh, '<', $_; $fh } @files;
my $concatenated = join '', map { scalar <$_> } @fhs;

Now, it might be that you want to concatenate not just first lines, but all of them. In this situation, instead of $concatenated = ... code, you'd need something like this:

my $concatenated = '';

while (my $fh = shift @fhs) {
    my $line = <$fh>;
    if ( defined $line ) {
        push @fhs, $fh;
        $concatenated .= $line;
    } else {
        close $fh;
    }
}