rsync3

#!/bin/bash

# Remote-to-remote invocation of rsync, with the help of SSH's -R.
# Use like 'rsync', but you're allowed to specify a 'remote:' on both ends.
#
# Copyright (C) 2023  Bearstech
#
# 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 3 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 <https://www.gnu.org/licenses/>.

# Pick a listening port on the source target, let's hope it's available.
# (only binds on 127.0.0.1, should not have security/firewalling issues)
port=45997

# Turn $@ into a list because it's easier to pick the last elements this way
# Also intercept our own flag(s).
opt=()
opt_nocheck=
for a in "$@"; do
  case "$a" in
    # The SSH client will tell 'Warning: Permanently added <hostkey>' but it's a lie
    "-Y") opt_nocheck=" -o StrictHostKeyChecking=no -oUserKnownHostsFile=/dev/null";;
       *) opt+=("$a");;
  esac
done

if [ ${#opt[@]} -lt 2 ]; then
  cat <<EOF >&2
Remote-to-remote rsync invocation. Only works with the SSH transport on both
ends. The trafic to/from both ends is relayed through this host, like with
'scp -3'. The source server must (obviously) trust the destination SSH server
hostkey.

Usage: rsync3 [-Y] [rsync args] source:/path... destination:[/path2...}

  -Y  skip destination's SSH hostkey checking on source server

EOF
  exit 1
fi 

# Separate host and paths in targets
src="${opt[-2]}"; src_host="${src%%:*}"; src_path="${src#*:}"
dst="${opt[-1]}"; dst_host="${dst%%:*}"; dst_path="${dst#*:}"
if [ "$src" = "$src_host" ] || [ "$dst" = "$dst_host" ]; then
  echo "Error: rsync3 must be invoked with both remote source and destination" >&2
  exit 1
fi

# Since the destination is interpreted on the src_host, we must pre-resolve the
# user-login here if it was implicit. Otherwise it's not what the user meant.
dst_user="${dst_host%%@*}"
dst_host="${dst_host#*@}"
if [ "$dst_user" = "$dst_host" ]; then
  dst_user="$(whoami)"
fi

# Build modified rsync args, re-quote them in order whitespace in args survive
# through SSH/shell passing.
opt[-2]="$src_path"
opt[-1]="$dst_user@127.0.0.1:$dst_path"
rsync="rsync -e 'ssh -p $port$opt_nocheck'"
for a in "${opt[@]}"; do
  rsync="$rsync $(printf %q "$a")"
done

exec ssh -A -R 127.0.0.1:$port:"$dst_host":22 "$src_host" "$rsync"