Gist of http://ankaerith.livejournal.com/147007.html (some background here http://s3sync.net/forum/index2214.html?topic=251.0) by Matthew Staver

s3rep.sh

#!/bin/sh
#
#  S3rep.sh version 1.0
#  Copyright 2009 Matthew Staver
#
#    This program is free software: you can redistribute it and/or modify
#    it under the terms of the GNU General Public License as published by
#    the Free Software Foundation, either version 2 of the License, or
#    (at your option) any later version.
#
#    This program is distributed in the hope that it will be useful,
#    but WITHOUT ANY WARRANTY; without even the implied warranty of
#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#    GNU General Public License for more details.
#
#    You should have received a copy of the GNU General Public License
#    along with this program.  If not, see <http://www.gnu.org/licenses/>

pid="$$"
# Set type to first argument.
type="$1"
# Set path to the rest of the arguments.
path="${*:2:$#}"

# Default incron mask, if you never plan to use deletion=1 and your
# directories are fairly static you can set this to:
#
# mask="IN_CREATE,IN_CLOSE_WRITE,IN_MOVED_TO"
#
# This may slightly reduce system load, but your incrontab file will
# not be cleaned of watched directories that are moved or deleted.
# I highly recommend you leave this with the default setting.
mask="IN_CREATE,IN_DELETE,IN_CLOSE_WRITE,IN_MOVED_TO,IN_MOVED_FROM"

# The location of your spool/queue directory.
queue="/var/spool/s3rep"
lockfile=/var/run/s3rep.pid
# The location of s3sync
s3path="/root/s3sync"
# The location of your incrontab file
incrontab="/var/spool/incron/root"
# The s3bucket you want to use
bucket="yourbucket"
# Set this to 0 to turn off syslog logging
syslog=1
# Set this to 1 to enable deletion from S3 of removed files/directories
deletion=0

function log () {
   [ $syslog -eq 1 ] && logger -t "s3rep[$pid]" "$@" || cat > /dev/null
}

case "$type" in
   'IN_CREATE')
      exit 0
   ;;
   'IN_CREATE,IN_ISDIR' | 'IN_MOVED_TO,IN_ISDIR' | 'IN_CLOSE_WRITE' | 'IN_MOVED_TO' | '-a')
      # If a file or directory is created, edited, or moved into a watched directory
      # symlink the it in the queue directory. If no uploads are currently running,
      # call this script with the UPLOAD option to start processing the queue directory.
      symname="$pid`date +%N`"
      ln -s "$path" "$queue/$symname"
      log "Queued: '$path' in '$queue/$symname'"
      [ ! -e $lockfile ] && $0 UPLOAD &
   ;;
   'IN_DELETE' | 'IN_MOVED_FROM')
      # If a file is deleted or moved out of a watched directory: Check to
      # see if deletion is enabled in the script.  If so, delete it from S3.
      [ $deletion -eq 1 ] && $s3path/s3cmd.rb --ssl --verbose delete $bucket:"$path" 2>&1 | log
   ;;
   'IN_DELETE,IN_ISDIR' | 'IN_MOVED_FROM,IN_ISDIR' | '-d')
      # If a directory is deleted or removed from a watched directory: Remove
      # the directory from the watched list in the incrontab file.  Check
      # to see if deletion is enabled in the script and if so, remove all files
      # from within the watched directory.
      sed -i "\|^${path// /\\\ }|d" $incrontab
      log "Removing '$path' and all sub-folders from watch list '$incrontab'"
      [ $deletion -eq 1 ] && $s3path/s3cmd.rb --ssl --verbose deleteall $bucket:"$path" 2>&1 | log
   ;;
   'UPLOAD')
      # Check for lockfile and being uploads if it doesn't exist.
      if ( set -o noclobber; echo "$$" > "$lockfile") 2> /dev/null;
       then
         # Trap exits and clean up lockfile.
         trap 'rm -f "$lockfile"; exit $?' INT TERM EXIT
         # Pop oldest symlink, store it, and unlink it.
         symlink=$(ls -rt1 $queue | sed -n -e '1,1p')
         linkpath="$(readlink "$queue/$symlink")"
         unlink "$queue/$symlink"
         # If it is a directory s3sync the files inside.
         if [ -d "$linkpath" ];
          then
            log "Syncing '$linkpath' and all sub-folders to S3 bucket: '$bucket'"
            $s3path/s3sync.rb --ssl -r --verbose "$linkpath/" $bucket:"$linkpath" 2>&1 | log
            # Recurse directories, stream edit them to escape any spaces with \,
            # append each directory to the incrontab file. NB: For some reason
            # incron does not pass trailing spaces in triggered events.  I've
            # added a trailing \ to the incrontab and this appears to resolve this.
            # I assume it causes incron to pass the trailing space and the \, and
            # bash ignores the \.
            find "$linkpath" -type d |sed -e 's| |\\ |g' -e "s|\$| $mask $0 \$% \$@/\$#\\\|" >>$incrontab
            log "Added '$linkpath' and all sub-folders to watch list '$incrontab'"
          else
            # Upload single file vis s3cmd put.
            [ -e "$linkpath" ] && $s3path/s3cmd.rb --ssl --verbose put $bucket:"$linkpath" "$linkpath"  2>&1 | log
         fi
         # Clean up lockfile
         rm -f "$lockfile"
         trap - INT TERM EXIT
         # Continue processing queue directory if symlinks remain.
         if [ "$(ls $queue|wc -l)" -gt "0" ]; then
            exec $0 UPLOAD
         fi
      else
         log "Upload $(cat $lockfile) in progress.  Failed to acquire lockfile: $lockfile."
      fi
   ;;
   *)
      echo "Usage: s3rep.sh [OPTION] DIRECTORY"
      echo
      echo "Options:"
      echo "  -a      add DIRECTORY and sub-folders to incrontab and sync to S3"
      echo "          NOTE: Do not include a trailing / after DIRECTORY"
      echo "  -d      remove DIRECTORY and all sub-folders from incrontab and"
      echo "          if deletion is enabled, remove them from S3"
      echo
      exit 1
esac
exit 0