get last few lines of file stored in variable

Question

How could I get the last few lines of a file that is stored in a variable? On linux I would use the tail command if it was in a file.

1) How can I do this in perl if the data is in a file?
2) How can I do this if the content of the file is in a variable?

Answer 1

To read the end of a file, seek near the end of the file and begin reading. For example,

open my $fh, '<', $file;
seek $fh, -1000, 2;
my @lines = <$fh>;
close $fh;

print "Last 5 lines of $file are: ", @lines[-5 .. -1];

Depending on what is in the file or how many lines you want to look at, you may want to use a different magic number than -1000 above.

You could do something similar with a variable, either

open my $fh, '<', \$the_variable;
seek $fh, -1000, 2;

or just

open my $fh, '<', \substr($the_variable, -1000);

will give you an I/O handle that produces the last 1000 characters in $the_variable.

Answer 2

The File::ReadBackwards module on the CPAN is probably what you want. You can use it thus. This will print the last three lines in the file:

use File::ReadBackwards
my $bw = File::ReadBackwards->new("some_file");
print reverse map { $bw->readline() } (1 .. 3);

Internally, it seek()s to near the end of the file and looks for line endings, so it should be fairly efficient with memory, even with very big files.