Backup-Recent Shell Script
==========================

Offsite backups can be very useful for disaster recovery. However,
backing up all data to the cloud can be slow and expensive. A compromise
is to backup only recently modified data to the cloud. Many ISPs/WebHosts
provide small amounts of online storage, which can be useful
for recent-file backup. For example, DreamHost accounts come with 50GB
of free backup storage -- not enough for a complete backup, but plenty
of space to hold many months of recent work.

How it Works
------------

Although rsync is very powerful, it doesn't have an option to filter
files based on age. To work around this, the script creates a scratch
shadow copy of the user's home directory containing only recently
modified files.

Of course, a naive shadow copy of recent files would consume large
amounts of disk and time. For efficiency, the shadow copy is built with
hard-linked copies, which means only a partial directory structure is
truly duplicated. This is done by combining find's modification-time
filter (-mtime) and cpio's pass-thru hard-link mode (-p and -l). The
basic core of the script are the commands:

    find $HOME -mtime -30d -print | cpio -dpl $SHADOW_HOME
    rsync -avz --delete --omit-dir-times $SHODOW_HOME/ $BACKUP_ACCOUNT:recent-changes

Much of the script bulk is added filtering to eliminate temporary or
derivative files and directories from the process.