To keep track of which file you are reading use the $ARGV variable. See perlop manual page for more information on the powerful <> 'diamond' operator.

Assuming you are dealing with files that can comfortably sit in memory I suggest a simple hash of hash to keep track of which line was seen by which file. 

 
use strict;
use warnings;
my $file_count=@ARGV; 
my %seen;
while(<>) {
    # File $ARGV saw line $_ at least once by file $ARGV
    $seen{$_}{$ARGV}++;  
}

while( my ($line, $by_whom) = each %seen) { 
    if ( $file_count == keys %$by_whom) {
        # Every file saw this line at least once
        print $line 
    }
}

[download]

If you want this as a one-liner you can use the -n flag to wrap a diamond operator around the central loop. This leads to the slightly obfuscated one liner with plenty of scope for golfing

 
perl -ne 'BEGIN{$count = @ARGV}; $seen{$_}{$ARGV}=1; END{while( ($line
+, $by_whom) = each %seen){ print $line if $count == keys %$by_whom}} 
+'

[download]

In reply to Re: find common lines in many files by amcglinchy
in thread find common lines in many files by Anonymous Monk

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