#!/bin/sh

## In order for this script to work, the following must be installed (except
## for pv, these should all be in your base systems):
##
## On the local host       : ssh expr pv
## On the source host      : ssh du cut tail
## On the destination host : ssh du cut touch cat
##
## Exit status is 0 on success, 2 if the destination file is the same size
## as the source file.  If one of the commands fails, it'll probably exit
## with a 1.  The external commands might exit 2 as well; if you have verbose
## on, then you'll see a message on stderr from this script if it's exiting 2
## because the files were the same size (so if you have an exitstatus of 2 and
## no message, you know it came from an external command).

## Hostnames of the two servers, as you would pass them to the SSH client
source_host="srchost"
destination_host="desthost"

## These filenames are going straight to the SSH client, so relative paths
## are relative to your home directory on each remote system.
##
## There's no reason the file in question has to be a .gz file, I'm just using
## that extension as an example; any file will work the same way.
##
## You can leave the destination filename empty and the source filename will
## be used (including any path)
source_filename="testfile.gz"
#destination_filename=""

## Set this to >0 to have the script print some diagnostics to stderr
verbose=1

###############################################################################
###############################################################################

## Retrieve the source file's size
source_file_size=$(ssh "${source_host}" "du --bytes ${source_filename} | cut --fields 1")

## Use source filename for destination of we don't already have one
[ -z "${destination_filename}" ] && destination_filename="${source_filename}"

## Retrieve the destination file's size
destination_file_size=$(ssh "${destination_host}" "du --bytes ${destination_filename} 2>/dev/null | cut --fields 1")
[ -z "${destination_file_size}" ] && destination_file_size=0

if [ ${destination_file_size} -eq ${source_file_size} ]; then
    ## Files are the same size, we must be done; exit with a unique status
    [ ${verbose} -ge 1 ] && echo "File sizes are the same; looks like we're done." >&2
   exit 2
fi

if [ ${destination_file_size} -gt 0 ]; then
    byte_offset=$(expr ${destination_file_size} + 1)
    transfer_size=$(expr ${source_file_size} - ${destination_file_size})
else
    byte_offset=0
    transfer_size=${source_file_size}
fi

if [ ${verbose} -ge 1 ]; then
    ## Print diagnostics to stderr
    echo "Source                : ${source_host}:${source_filename}"                       >&2
    echo "Destination           : ${destination_host}:${destination_filename}"             >&2
    echo "Source file size      : ${source_file_size}"                                     >&2
    echo "Destination file size : ${destination_file_size}"                                >&2
    echo "Byte offset           : ${byte_offset}"                                          >&2
    echo "Transfer size         : ${transfer_size}"                                        >&2
    echo ""                                                                                >&2
    echo "###############################################################################" >&2
    echo ""                                                                                >&2
fi

## Release the hounds!
ssh "${source_host}" "tail -c +${byte_offset} ${source_filename}" | pv -s ${transfer_size} | ssh "${destination_host}" "touch ${destination_filename}; cat - >> ${destination_filename}"

## EOF
########