To complement @rkhayrov’s answer, there exists a command for that already: xclip. Or more exactly, there’s a patch to xclip which was added to xclip later on in 2010, but hasn’t been released yet that does that. So, assuming your OS like Debian ships with the subversion head of xclip (2019 edit: version 0.13 with those changes was eventually released in 2016 (and pulled into Debian in January 2019)):
To list the targets for the CLIPBOARD selection:
$ xclip -selection clipboard -o -t TARGETS
TIMESTAMP
TARGETS
MULTIPLE
SAVE_TARGETS
text/html
text/_moz_htmlcontext
text/_moz_htmlinfo
UTF8_STRING
COMPOUND_TEXT
TEXT
STRING
text/x-moz-url-priv
To select a particular target:
$ xclip -selection clipboard -o -t text/html
<a href="https://stackoverflow.com/users/200540/rkhayrov" title="3017 reputation" class="comment-user">rkhayrov</a>
$ xclip -selection clipboard -o -t UTF8_STRING
rkhayrov
$ xclip -selection clipboard -o -t TIMESTAMP
684176350
And xclip can also set and own a selection (-i instead of -o).