Re: How can I trim top of file w/o disrupting other writers?

On 07/05/2009, at 9:35 AM, bradjpeek <[e-mail]> wrote:

> 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.
>

Why can't you simply open the file itself for read write then replace  
the contents? That's worked fine for me before


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> 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
>