Inotify

inotify is a way for userspace programs to intercept kernel-based filesystem events.

Instant chown watch example

#!/bin/bash
# This script watches my home directory like a cybernetic hawk
DIR=/home/kyle
OG=kyle:users

# Normal chown will actually spawn more inotify events.
# This means we have to only chown when we actually need to 
function selective-chown {
   FILE=$1
   CURRENTOG=`stat -c %U:%G "$FILE"`
   if [ "$OG" != "$CURRENTOG" ]; then
	echo "   actually chowning $FILE"
   	chown $OG "$FILE"
   fi 
}

# Scripts cant read inotifywaits output easily, and we need to be careful with spaces
# I *think* the only two syscalls we care about are attrib and create
inotifywait -rm -e attrib -e create --format=%w%f $DIR \
| while read file; do
       selective-chown "$file"
       echo Running selective-chown on $file
done