Changing the file descriptor of a running process

Found this today:

http://ingvar.blog.redpill-linpro.com/2010/07/10/changing-a-process-file-descriptor-on-the-fly/

pretty cool.

#!/bin/bash 
# 
# Swap/Roll a logfile 
# 
# Usage:              <old logfile>  <new logfile> [ optional pids ] 
#         ./swap.sh /var/log/logfile /tmp/logfile  [pids] 
# 
# Author: Robert McKay <rob...@mckay.com> 
# Date:   Tue Aug 14 13:36:35 BST 2007 
#
# Update: Added usage message when needed, a fuser format fix, 
#         some whitespace cleanup, and a localization fix.
#         Ingvar Hagelund <ingvar@redpill-linpro.com>
# Date:   Sat Jul 10 02:11:49 CEST 2010

if [ "$2" = "" ]; then
 echo "Usage: $0 /path/to/oldfile /path/to/newfile [pids]
Example: $0 /var/log/somedaemon.log /var/log/newvolume/somedaemon.log 1234
Example: $0 /dev/pts/53 /dev/null 1234
"
 exit 0
fi

if gdb --version > /dev/null 2>&1; then
 true
else
 echo "Unable to find gdb."
 exit 1
fi

src=$1 
dst=$2 
shift; shift 
pids=$* 

for pid in ${pids:=$( /sbin/fuser $src | cut -d ':' -f 2 )}; 
do 
 echo "src=$src, dst=$dst"
 echo "$src has $pid using it" 
 ( 
      echo "attach $pid" 
  echo 'call open("'$dst'", 66, 0666)' 
  for ufd in $(LANG=C ls -l /proc/$pid/fd | grep $src\$ | awk ' { print $9; } '); 
      do 
   echo 'call dup2($1,'"$ufd"')'
  done 
  echo 'call close($1)'
  echo 'detach' 
  echo 'quit' 
  sleep 5
 ) | gdb -q -x -
done