From: bradjpeek <bradjpeek@...>
Date: 2009-05-07T08:35:04+09:00
Subject: How can I trim top of file w/o disrupting other writers?

As a way to learn ruby, I wrote a method to trim the first "n" lines
from a log file.   I had planned to use it on a series of log files
that are constantly growing.   However, some of the programs that are
appending to these logs don't tolerate the somewhat brute-force method
I'm using, which is:

1) Write the lines I want to keep to a temp file
2) Rename the temp file to the original log file name

This works fine for some log files, but others stop receiving log data
after I run my program.   I suspect it is because they have the file
open and don't notice that the inode (or some other pointer to the log
file) has been pulled out from underneath them.

I realize this is more of a Linux/UNIX question, but I'd still like to
implement the solution as a ruby method if possible.

For what it's worth, my current method is (please excuse the lack of
ruby-ness)

# Trims a file down to last "n" number of lines.
# If save_orig == TRUE then the trimmed lines are
# appended to a file suffixed with _saved.  Otherwise
# the trimmed lines are discarded.
#
def trim_file (file_name, nbr_lines_to_keep = 5000, save_orig = FALSE)

  # The file to be trimmed must be writable by the process owner
  #
  if File.writable?(file_name)

    # The file to be trimmed must either be owned by the process owner
    # (i.e. same account that is running this program) or the process
    # owner needs to be root (or running as root via sudo).
    #
    if (File.stat(file_name).owned?) || (Process.euid == 0)

       # get current uid and gid of the file (in case it
       # isn't same as default)
       #
       f_uid = File.stat(file_name).uid
       f_gid = File.stat(file_name).gid

       all_lines = IO.readlines(file_name)
       nbr_lines = all_lines.size
       if nbr_lines > nbr_lines_to_keep

         start_line  = nbr_lines - nbr_lines_to_keep
         tmpfilename = file_name + "_temptrimfile"

         tmpfile_nst = File.new(tmpfilename, "w")
         tmpfile_nst.puts(all_lines[start_line..nbr_lines])
         tmpfile_nst.close

         if save_orig == TRUE
             savefile_nst = File.new(file_name + "_saved", "a")
             savefile_nst.puts(all_lines[0..start_line-1])
             savefile_nst.close
             File.chown(f_uid, f_gid, file_name + "_saved")
         end

         File.rename(tmpfilename, file_name)
         # This script *could* be running as root or via sudo
         # in which case we need to preserve the original uid
         # and gid of the file.  Otherwise the trimmed and/or
         # saved file would be owned by root
         File.chown(f_uid, f_gid, file_name)
         print "#{file_name} => trimmed to #{nbr_lines_to_keep} lines.
"
         print "First #{start_line} lines "
         print "appended to #{file_name}_saved\n" if save_orig == TRUE
         print "discarded\n"                      if save_orig ==
FALSE
       else
         print "File #{file_name} not trimmed.  Nbr lines (#
{nbr_lines}) "
         print "not greater than #{nbr_lines_to_keep}.\n"
       end
    else
      puts "File #{file_name} not trimmed.  You are not file owner or
root"
    end
  else
    puts "File #{file_name} not trimmed.  Not writable or doesn't
exist."
  end
end             # end method trim_file