module File::Tail

If this attribute is set to a true value, File::Fail’s tail method raises a BreakException if the end of the file is reached.

Default buffer size, that is used while going backward from a file’s end. This defaults to nil, which means that File::Tail attempts to derive this value from the filesystem block size.

The start value of the sleep interval. This value goes against max_interval if the tailed file is silent for a sufficient time.

Override the default line separator

The maximum interval File::Tail sleeps, before it tries to take some action like reading the next few lines or reopening the file.

If this attribute is set to a true value, File::Tail persists on reopening a deleted file waiting max_interval seconds between the attempts. This is useful if logfiles are moved away while rotation occurs but are recreated at the same place after a while. It defaults to true.

If this attribute is set to a true value, File::Tail attempts to reopen it’s tailed file after suspicious_interval seconds of silence.

If this attribute is set to a true value, File::Fail’s tail method just returns if the end of the file is reached.

This attribute is the invterval in seconds before File::Tail gets suspicious that something has happend to it’s tailed file and an attempt to reopen it is made.

If the attribute reopen_suspicious is set to a non true value, #suspicious_interval is meaningless. It defaults to 60 seconds.

The callback is called with self as an argument after a reopen has occured. This allows a tailing script to find out, if a logfile has been rotated.

# File lib/file/tail.rb, line 71
def after_reopen(&block)
  @after_reopen = block
end

Rewind the last n lines of this file, starting from the end. The default is to start tailing directly from the end of the file.

The additional argument bufsize is used to determine the buffer size that is used to step through the file backwards. It defaults to the block size of the filesystem this file belongs to or 8192 bytes if this cannot be determined.

# File lib/file/tail.rb, line 121
def backward(n = 0, bufsize = nil)
  preset_attributes unless defined? @lines
  if n <= 0
    seek(0, File::SEEK_END)
    return self
  end
  bufsize ||= default_bufsize || stat.blksize || 8192
  size = stat.size
  begin
    if bufsize < size
      seek(0, File::SEEK_END)
      while n > 0 and tell > 0 do
        seek(-bufsize, File::SEEK_CUR)
        buffer = read(bufsize)
        n -= buffer.count(@line_separator)
        seek(-bufsize, File::SEEK_CUR)
      end
    else
      rewind
      buffer = read(size)
      n -= buffer.count(@line_separator)
      rewind
    end
  rescue Errno::EINVAL
    size = tell
    retry
  end
  pos = -1
  while n < 0  # forward if we are too far back
    pos = buffer.index(@line_separator, pos + 1)
    n += 1
  end
  seek(pos + 1, File::SEEK_CUR)
  self
end

Skip the first n lines of this file. The default is to don’t skip any lines at all and start at the beginning of this file.

# File lib/file/tail.rb, line 102
def forward(n = 0)
  preset_attributes unless defined? @lines
  rewind
  while n > 0 and not eof?
    readline(@line_separator)
    n -= 1
  end
  self
end

This method tails this file and yields to the given block for every new line that is read. If no block is given an array of those lines is returned instead. (In this case it’s better to use a reasonable value for n or set the return_if_eof or break_if_eof attribute to a true value to stop the method call from blocking.)

If the argument n is given, only the next n lines are read and the method call returns. Otherwise this method call doesn’t return, but yields to block for every new line read from this file for ever.

# File lib/file/tail.rb, line 169
def tail(n = nil, &block) # :yields: line
  @n = n
  result = []
  array_result = false
  unless block
    block = lambda { |line| result << line }
    array_result = true
  end
  preset_attributes unless defined? @lines
  loop do
    begin
      restat
      read_line(&block)
      redo
    rescue ReopenException => e
      until eof? || @n == 0
        block.call readline(@line_separator)
        @n -= 1 if @n
      end
      reopen_file(e.mode)
      @after_reopen.call self if defined? @after_reopen
    rescue ReturnException
      return array_result ? result : nil
    end
  end
end