Clipboard – How to Get a Graphic on the Clipboard to the Disk?

Well, there are a few different clipboards in X :) The one xclip copies text into by default is "selection buffer" - usually you just select some text with your mouse and then can paste it with middle mouse button. This buffer is separate from the one from which you can paste with Ctrl-Shift-V.

Try

echo "hello" | xclip -selection clipboard

also, see

man xclip

for more details about xclip

You can use xsel. To install it run this command in a terminal:

sudo apt install xsel

Now, when you want to clear your clipboard, enter

xsel -bc

For more information about this command enter man xsel

Source.