xclipsync is a trivial little script for synchronizing the X11 "clipboard" selection (the one you use with Ctrl-C/Ctrl-V, as opposed to highlight/middleclick) between two X11 servers.
For example, if you use Xnest or Xephyr (nested X11 servers inside your main one), you can use xclipsync to keep their clipboards tied together. I use this with ChromeOS+crostini, so that I can use a "full" X11 session with my favourite window manager.
Some people suggest using the "synergy" daemon for this, but unfortunately it seems to have many longstanding bugs related to clipboard synchronization. In particular, it only seems to sync the clipboard once when one X server acquires the selection; it doesn't seem to sync again until you make a selection on the other X server, and so on, alternating back and forth. Also, statically transfers the clipboard data as soon as it changes owners, which is not quite correct: if you transfer the clipboard to rxvt, for example, then rxvt can change the content of the clipboard numerous times without sending any notifications. It would be better to retrieve the clipboard data from the remote screen only at paste time, which is what xclipsync does.
How do I use it?
First, install tcl/tk (yes! In 2018!) and xclip. Then just put something like this in your .xsession:
if [ "$DISPLAY" != ":0" ]; then xclipsync & fi
This will sync your nested X server's clipboard with the clipboard on :0. You can create any number of X servers synced this way, and they will all sync smoothly with :0 and therefore with each other.
How does it work?
xclipsync is a shell script that just alternates between two steps:
Acquire clipboard ownership on server A and, when someone on server A tries to paste the clipboard, send them the data from server B. If someone else acquires the clipboard on server A (ie. by copying text), go to step 2.
The actual work of step 1 and 2 are done by a silly tcl/tk script called xclipfrom. Why tcl/tk? Because it was the easiest way to listen for clipboard events. xclip almost does everything we want, but you have to provide it with the clipboard text immediately when acquiring the clipboard, and that's not quite valid: the clipboard owner could change the clipboard content as often as it wants, without sending any notifications. With tcl/tk, we can acquire clipboard ownership, and handle the request each time someone wants to paste.
Other people write scripts which just fetch the clipboard content periodically from each node and then see if it has changed, but that creates race conditions (especially when syncing multiple screens) and requires you to pick a delay between syncs, which means you have to choose between wasting CPU time and waiting before you can paste. xclipsync is instantaneous but doesn't waste time polling when you're not using the clipboard.
Room for improvement
This set of scripts really sucks! There's plenty of room to improve. Patches are welcome. Suggestions:
Remove hardcoded CLIPBOARD and let people sync the PRIMARY (ie. middleclick) selection as well. I tested it already, and it works fine, but we'll have to add command line options to make it work.
Better error handling if xclip is missing, etc.
Send log messages to stderr instead of stdout.
Don't hardcode :0. Let the caller decide which screen to sync with.
Convert from tcl/tk to something less obsolete, like C or python.
Modify xclip itself to add its one missing feature: be able to run a command to retrieve text, rather than reading stdin at startup.