#!/bin/sh

# propagate_hard_links
#
# This script works in conjunction with the backup directory structure
# created by the rsync_snapshot series of scripts.  It looks in the
# "previous-backup" directory for files, and propogates all files to the
# "current-backup" directory as hard links.
#
# This script is intended for use with "piggyback" backups, where only
# changed files are saved in the backup directories, and so a manual
# means of keeping hard links to previous backups is required.
#
# Copyright 2006 Jason Healy, Suffield Academy
#

BASE=$1
shift

# names of the backup symlinks
PREV="previous-backup"
CURR="current-backup"

# path check
if [ -z "$BASE" ]; then
    echo "Usage: propagate_hard_links <path>"
    echo ""
    echo "  Where <path> is a path to a snapshot directory containing"
    echo "  '$PREV' and '$CURR' links."
    echo ""
    exit 1
fi

# find links
if [ ! -L "$BASE/$PREV" ]; then
    echo "Unable to find previous backup link '$PREV' in '$BASE'"
    exit 1
fi

if [ ! -L "$BASE/$CURR" ]; then
    echo "Unable to find current backup link '$CURR' in '$BASE'"
    exit 1
fi

# dereference symlinks
# (you can use `ls -l "$link" | sed -e 's/.*-> //'` instead of readlink)
prev=$(readlink "$BASE/$PREV")
case "$prev" in
    /*)
	# absolute link; no changes required
	;;
    *)
	# relative link; must prepend full path
	prev="$BASE/$prev"
	;;
esac

curr=$(readlink "$BASE/$CURR")
case "$curr" in
    /*)
	# absolute link; no changes required
	;;
    *)
	# relative link; must prepend full path
	curr="$BASE/$curr"
	;;
esac

# final sanity check
if [ ! -d "$prev" ]; then
    echo "Unable to find previous target dir '$prev'"
    exit 1
fi

if [ ! -d "$curr" ]; then
    echo "Unable to find current target dir '$curr'"
    exit 1
fi


# now we're ready to move into the previous directory and propagate the
# links forward into the current directory
cd "$prev"

# pax is an archiver.  -r -w means to read and write (e.g., make a copy)
# -l means to hard-link to existing files, and -u means to not replace
# newer files with the same name
pax -r -w -l -u . "$curr"