FAQ 5.4 How do I delete the last N lines from a file?

Discussion in 'Perl Misc' started by PerlFAQ Server, Jan 31, 2011.

  1. This is an excerpt from the latest version perlfaq5.pod, which
    comes with the standard Perl distribution. These postings aim to
    reduce the number of repeated questions as well as allow the community
    to review and update the answers. The latest version of the complete
    perlfaq is at http://faq.perl.org .

    --------------------------------------------------------------------

    5.4: How do I delete the last N lines from a file?

    (contributed by brian d foy)

    The easiest conceptual solution is to count the lines in the file then
    start at the beginning and print the number of lines (minus the last N)
    to a new file.

    Most often, the real question is how you can delete the last N lines
    without making more than one pass over the file, or how to do it without
    a lot of copying. The easy concept is the hard reality when you might
    have millions of lines in your file.

    One trick is to use "File::ReadBackwards", which starts at the end of
    the file. That module provides an object that wraps the real filehandle
    to make it easy for you to move around the file. Once you get to the
    spot you need, you can get the actual filehandle and work with it as
    normal. In this case, you get the file position at the end of the last
    line you want to keep and truncate the file to that point:

    use File::ReadBackwards;

    my $filename = 'test.txt';
    my $Lines_to_truncate = 2;

    my $bw = File::ReadBackwards->new( $filename )
    or die "Could not read backwards in [$filename]: $!";

    my $lines_from_end = 0;
    until( $bw->eof or $lines_from_end == $Lines_to_truncate )
    {
    print "Got: ", $bw->readline;
    $lines_from_end++;
    }

    truncate( $filename, $bw->tell );

    The "File::ReadBackwards" module also has the advantage of setting the
    input record separator to a regular expression.

    You can also use the "Tie::File" module which lets you access the lines
    through a tied array. You can use normal array operations to modify your
    file, including setting the last index and using "splice".



    --------------------------------------------------------------------

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    so please include as much information as possible and relevant in any
    corrections. The perlfaq-workers also don't have access to every
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    corrections to examples that do not work on particular platforms.
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    If you'd like to help maintain the perlfaq, see the details in
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    PerlFAQ Server, Jan 31, 2011
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