The clipboard that we all use
To copy any output from any command, simply pipe the command output into xclip.
Example:
echo "stuy linux is cool" | xclip -selection c
Note: For most users, it is recommended to use the -selection c flag, as this copies to the clip board (X11 selection area) that most desktop environments allow you to access via CONTROL+C and CONTROL+V. This clipboard is called "XA_CLIPBOARD"
A different clipboard
It is also possible to copy any output to the "PRIMARY" X11 selection area (which, ironically, is NOT what is used when users use CONTROL+C and CONTROL+V on most distros). Simply use xclip without specyfing the given X11 selection area.
echo "stuy linux is cool" | xclip
To paste from this X11 selection area, use the middle mouse click (on mice) or triple finger click (on trackpads).
Pasting IMAGES from the terminal
xclip -selection c -t image/png input_file.png
Note: That command is for PNG files. For other files, change the image/TYPE to be whatever the file type is (ex: image/jpg).