#!/usr/bin/perl
# This script takes a command-line arg of a source directory
# that will be passed to rsync, and generates a set of excludes
# that will exclude all mount points from the list.  This is
# useful if you have "bind" mounts since the --one-file-system
# option won't notice the transition to a different spot on
# the same disk.  For example:
#
# mnt-excl /dir | rsync --exclude-from=- ... /dir /dest/
# mnt-excl /dir/ | rsync --exclude-from=- ... /dir/ /dest/
# ssh host mnt-excl /dir | rsync --exclude-from=- ... host:/dir /dest/
#
# Imagine that /dir/foo is a mount point: the first invocation of
# mnt-excl would have output /dir/foo, while the second would have
# output /foo (which are the properly anchored excludes).
#
# NOTE:  This script expects /proc/mounts to exist, but could be
# easily adapted to read /etc/mtab or similar.
#
# ADDENDUM:  The addition of the --filter option (which has support for
# absolute-anchored excludes) can make this screen unneeded in some
# scenarios.  If you don't need delete protection on the receiving side
# (or if the destination path is identical to the source path), then you
# can exclude some absolute paths from the transfer based on the mount
# dirs.  For instance:
#
# awk '{print $2}' /proc/mounts | grep -v '^/$' | \
#   rsync -avf 'merge,/- -' /dir host:/dest/

use strict;
use warnings;
use Cwd 'abs_path';

my $file = '/proc/mounts';
my $dir = shift || '/';
my $trailing_slash = $dir =~ m{./$} ? '/' : '';
$dir = abs_path($dir) . $trailing_slash;
$dir =~ s{([^/]*)$}{};
my $trailing = $1;
$trailing = '' if $trailing eq '.' || !-d "$dir$trailing";
$trailing .= '/' if $trailing ne '';

open(IN, $file) or die "Unable to open $file: $!\n";
while (<IN>) {
    $_ = (split)[1];
    next unless s{^\Q$dir$trailing\E}{}o && $_ ne '';
    print "- /$trailing$_\n";
}
close IN;