You have a file (say syslog), it is open by a process (say logger) and logger is writing to it.

A second process (perl) comes along and renames the file to syslog.bak and opens a new file called syslog it then proceeds to read lines from syslog.bak and selectively copy some of them to syslog, Process logger is still writing to syslog.bak. The perl script finishes processing, closes syslog.

What have you achieved? The log file that logger is writing to still contains all the lines it had to start with. You have a new file with a subselection of those lines, and have possibly archived the rest somewhere, but the file logger is writing to is still as big as it was originally, and growing. You have no way to remove those lines you archived from the original logfile, and no way to persuade logger to use the new shortened version of the file without stopping and starting logger?

Below are the results I got when I tried this on my (Win32) system. Maybe the results are different on *nix, but the fact that the logger process continues to use the unchanged, unshortened version of the file isn't?

C:\test>type logger.pl
#! perl -sw
use strict;
use POSIX 'strftime';
$|++;
#! Sat Aug 31 23:13:46 CDT 2002

while (1) {
        sleep 1;
        print strftime( '%a %b %d %T %Z %Y',localtime), "@{[times]}", 
+$/;
}

C:\test>logger
Mon Oct 07  BST 20020.18 0.04 0 0
Mon Oct 07  BST 20020.18 0.04 0 0
Mon Oct 07  BST 20020.18 0.04 0 0
Mon Oct 07  BST 20020.18 0.04 0 0
Mon Oct 07  BST 20020.18 0.04 0 0
^C
C:\test>^Clogger >log

# different session
C:\test>dir log
 Volume in drive C has no label.
 Volume Serial Number is 8C87-E163

 Directory of C:\test

02/10/07  09:59p                   980 log
               1 File(s)            980 bytes
                             25,827,328 bytes free

C:\test>dir log*
 Volume in drive C has no label.
 Volume Serial Number is 8C87-E163

 Directory of C:\test

02/10/07  09:59p                 1,120 log
02/10/07  07:17p                   186 logger.pl
               2 File(s)          1,306 bytes
                             25,827,328 bytes free

C:\test>perl -wni.bak -e "print if /7/;" log
Can't do inplace edit on log: File exists.

C:\test>perl -wni -e "print if /7/;" log
Can't do inplace edit without backup.

C:\test>
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