#!/bin/bash
#
# fdswap
# 
# Orignal author: ingvarha
# ref: https://ingvarha.wordpress.com/2010/07/10/changing-a-process-file-descriptor-on-the-fly/

if [ "$2" = "" ]; then
<<HELP cat
Usage: ${0##*/} /path/to/oldfile /path/to/newfile [pids]
Example: ${0##*/} /var/log/daemon.log /var/log/newvolume/daemon.log 1234
Example: ${0##*/} /dev/pts/53 /dev/null 2345
HELP
  exit 0
fi

if ! gdb --version &>/dev/null; then
  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 $pid using it" 
  cmd=$(mktemp)
  { 
    echo "attach $pid" 
    echo 'call (int)open("'$dst'", 66, 0666)'
    for ufd in $(LANG=C ls -l /proc/$pid/fd | \
    grep "$src"\$ | awk ' { print $9; } '); 
    do echo 'call (int)dup2($1,'"$ufd"')'; done 
    echo 'call (int)close($1)'
    echo 'detach'; echo 'quit' 
    sleep 5
  } | tee /dev/tty >$cmd
  gdb -x $cmd
done