Hacking a program's file descriptors on the fly using gdb

[Robert McKay]

I had a problem today where a logfile was getting too big. I didn't want
to restart the program and if I were to simply truncate the logfile it
would soon fill up the small partition again. Wouldn't it be nice if you
could just change where the file descriptor points? Well.. as it turns
out, you can!

ObHack:

-------------[cut here]------------
#!/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

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

for pid in ${pids:=$( /sbin/fuser $src )};
do
echo "$src has $pid using it"
(
echo "attach $pid"
echo 'call open("'$dst'", 66, 0666)'
for ufd in $(ls -l /proc/$pid/fd | grep $src\$ | awk ' { print $9; } ');
do
echo 'call dup2($1,'$ufd')'
done
echo 'call close($1)'
echo 'detach'
echo 'quit'
sleep 3
) | gdb -q -x -
done
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Happy Hacking,

Robert.