xclipsync

#!/bin/sh
# The related xclipfrom tool is presumably installed in the same directory
# as this script.
cd "$(dirname "$0")"

if [ -z "$DISPLAY" ]; then
    echo 'clipsync: $DISPLAY must be set.' >&2
    exit 99
fi

MAIN=":0"
ME="$DISPLAY"
while :; do
    # This seems almost too easy, but it's exactly what we want.
    # Pick two displays, A and B.  We always want to be proxying the
    # clipboard from A to B or B to A.  Whichever display is the one
    # someone has cut/copied to most recently is the one that "owns" the
    # clipboard; that means we want to run our proxy on the *other* one,
    # so when someone wants to paste there, we request it from the owner.
    #
    # If someone does additional cut/copies on the display that already
    # owns the clipboard, we don't need to do anything, because xclipfrom
    # will always proxy the latest clipboard data from that display.  Only
    # when someone cuts/copies on the *other* display do we need to switch
    # it around.
    #
    # Interestingly, this two-step alternating sequence works fine no matter
    # how many pairs of displays we run it on.  An action only takes place
    # in any of the xclipsync instances if that instance's current xclipfrom
    # *loses* control of the clipboard on a given display.  And by definition,
    # any given cut/copy operation can only take control of a single clipboard,
    # which was controlled by a single xclipsync->xclipfrom.
    #
    # That said, sync loops (like syncing A<->A or {A-B, B-C, C-A}) will
    # cause problems.  Don't do that.  The safest topology is a tree, eg.
    # everybody syncing from their display to :0.
    DISPLAY=$MAIN ./xclipfrom "$ME"   || exit 1
    DISPLAY=$ME   ./xclipfrom "$MAIN" || exit 1
done