xclip

command line interface to X selections
xclip is a command line utility that is designed to run on any system with an
X11 implementation. It provides an interface to X selections ("the clipboard")
from the command line. It can read data from standard in or a file and place
it in an X selection for pasting into other X applications. xclip can also
print an X selection to standard out, which can then be redirected to a file
or another program.