When running in a terminal, use the `xclip' utility for copying from and pasting to the X clipboard.

clipboard.el

;; Terminal (with XClip): Use CLIPBOARD for explicit cuts. Don't use PRIMARY.
(when (and (not window-system)
           (getenv "DISPLAY")
           (executable-find "xclip"))
  ;; On explicit yank, copy to CLIPBOARD.
  (defun xclip-copy-to-clipboard (text)
    "Copy TEXT to X's clipboard, using the `xclip` utility."
    (let* ((process-connection-type nil)
           (proc (start-process "xclip" nil "xclip" "-selection" "clipboard")))
      (process-send-string proc text)
      (process-send-eof proc)))
  (setq interprogram-cut-function 'xclip-copy-to-clipboard)

  ;; When pasting, check the CLIPBOARD and if there's something new, insert it into the kill-ring.
  (defun xclip-paste-from-clipboard ()
    "If another program provided text for pasting, return it (as a string).
If the last paste was from Emacs or is identical to the last one from Emacs, return nil.
Uses the `xclip` utility."
    (let ((clipboard-contents (shell-command-to-string "xclip -o -selection clipboard")))
      (if (not (string= (car kill-ring) clipboard-contents))
          clipboard-contents
        nil)))
  (setq interprogram-paste-function 'xclip-paste-from-clipboard))