phpman > perldoc > File::ReadBackwards(3pm)

NAME
    File::ReadBackwards.pm -- Read a file backwards by lines.

SYNOPSIS
        use File::ReadBackwards ;

        # Object interface

        $bw = File::ReadBackwards->new( 'log_file' ) or
                            die "can't read 'log_file' $!" ;

        while( defined( $log_line = $bw->readline ) ) {
                print $log_line ;
        }

        # ... or the alternative way of reading

        until ( $bw->eof ) {
                print $bw->readline ;
        }

        # Tied Handle Interface

        tie *BW, 'File::ReadBackwards', 'log_file' or
                            die "can't read 'log_file' $!" ;

        while( <BW> ) {
                print ;
        }

DESCRIPTION
    This module reads a file backwards line by line. It is simple to use, memory efficient and fast.
    It supports both an object and a tied handle interface.

    It is intended for processing log and other similar text files which typically have their newest
    entries appended to them. By default files are assumed to be plain text and have a line ending
    appropriate to the OS. But you can set the input record separator string on a per file basis.

OBJECT INTERFACE
    These are the methods in "File::ReadBackwards"' object interface:

  new( $file, [$rec_sep], [$sep_is_regex] )
    "new" takes as arguments a filename, an optional record separator and an optional flag that
    marks the record separator as a regular expression. It either returns the object on a successful
    open or undef upon failure. $! is set to the error code if any.

  readline
    "readline" takes no arguments and it returns the previous line in the file or undef when there
    are no more lines in the file. If the file is a non-seekable file (e.g. a pipe), then undef is
    returned.

  getline
    "getline" is an alias for the readline method. It is here for compatibility with the IO::*
    classes which has a getline method.

  eof
    "eof" takes no arguments and it returns true when readline() has iterated through the whole
    file.

  close
    "close" takes no arguments and it closes the handle

  tell
    "tell" takes no arguments and it returns the current filehandle position. This value may be used
    to seek() back to this position using a normal file handle.

  get_handle
    "get_handle" takes no arguments and it returns the internal Perl filehandle used by the
    File::ReadBackwards object. This handle may be used to read the file forward. Its seek position
    will be set to the position that is returned by the tell() method. Note that interleaving
    forward and reverse reads may produce unpredictable results. The only use supported at present
    is to read a file backward to a certain point, then use 'handle' to extract the handle, and read
    forward from that point.

TIED HANDLE INTERFACE
  tie( *HANDLE, 'File::ReadBackwards', $file, [$rec_sep], [$sep_is_regex] )
    The TIEHANDLE, READLINE, EOF, CLOSE and TELL methods are aliased to the new, readline, eof,
    close and tell methods respectively so refer to them for their arguments and API. Once you have
    tied a handle to File::ReadBackwards the only I/O operation permissible is <> which will read
    the previous line. You can call eof() and close() on the tied handle as well. All other tied
    handle operations will generate an unknown method error. Do not seek, write or perform any other
    unsupported operations on the tied handle.

LINE AND RECORD ENDINGS
    Since this module needs to use low level I/O for efficiency, it can't portably seek and do block
    I/O without managing line ending conversions. This module supports the default record separators
    of normal line ending strings used by the OS. You can also set the separator on a per file
    basis.

    The record separator is a regular expression by default, which differs from the behavior of $/.

    Only if the record separator is not specified and it defaults to CR/LF (e.g, VMS, redmondware)
    will it will be converted to a single newline. Unix and MacOS files systems use only a single
    character for line endings and the lines are left unchanged. This means that for native text
    files, you should be able to process their lines backwards without any problems with line
    endings. If you specify a record separator, no conversions will be done and you will get the
    records as if you read them in binary mode.

DESIGN
    It works by reading a large (8kb) block of data from the end of the file. It then splits them on
    the record separator and stores a list of records in the object. Each call to readline returns
    the top record of the list and if the list is empty it refills it by reading the previous block
    from the file and splitting it. When the beginning of the file is reached and there are no more
    lines, undef is returned. All boundary conditions are handled correctly i.e. if there is a
    trailing partial line (no newline) it will be the first line returned and lines larger than the
    read buffer size are handled properly.

NOTES
    There is no support for list context in either the object or tied interfaces. If you want to
    slurp all of the lines into an array in backwards order (and you don't care about memory usage)
    just do:

            @back_lines = reverse <FH>.

    This module is only intended to read one line at a time from the end of a file to the beginning.

AUTHOR
    Original author: Uri Guttman, "uri AT stemsystems.com"

    Current maintainer: Graham Ollis "plicease AT cpan.org"

COPYRIGHT
    Copyright (C) 2000-2021 by Uri Guttman. All rights reserved. This program is free software; you
    can redistribute it and/or modify it under the same terms as Perl itself.