The function to show current file's full path in mini buffer

It's the built-in function buffer-file-name that gives you the full path of your file.

The best thing to do is to have your emacs window to always show your system-name and the full path of the buffer you're currently editing :

(setq frame-title-format
      (list (format "%s %%S: %%j " (system-name))
        '(buffer-file-name "%f" (dired-directory dired-directory "%b"))))

You can also do something like this :

(defun show-file-name ()
  "Show the full path file name in the minibuffer."
  (interactive)
  (message (buffer-file-name)))

(global-set-key [C-f1] 'show-file-name) ; Or any other key you want

To borrow from Jérôme Radix's answer, if you just want to quickly see the file path of the current buffer, you can do M-: buffer-file-name.

Alternately, type (buffer-file-name) in the buffer somewhere and run C-x C-e on the closing parenthesis (this will work in any mode, not just lisp-mode).

My trick is to do a C-x C-f like to open a file, it wil prefill the minibuff with the current file path, C-g to quit. Faster than M-: buffer-file-name but far far uglier than any other methods.

The direct implementation of what you want is:

(defun copy-full-path-to-kill-ring ()
  "copy buffer's full path to kill ring"
  (interactive)
  (when buffer-file-name
    (kill-new (file-truename buffer-file-name))))

That said, I find it incredibly useful to be able to get the full path of what is in the minibuffer, and this is what I use:

(define-key minibuffer-local-completion-map "\C-r" 'resolve-sym-link)
(defun resolve-sym-link ()
  "Try to resolve symbolic links into true paths."
  (interactive)
  (beginning-of-line)
  (let* ((file (buffer-substring (point)
                                 (save-excursion (end-of-line) (point))))
         (file-dir (file-name-directory file))
         (file-true-dir (file-truename file-dir))
         (file-name (file-name-nondirectory file)))
    (delete-region (point) (save-excursion (end-of-line) (point)))
    (insert (concat file-true-dir file-name))))

And then if I want it in the clipboard, I just kill the line (C-a C-k). But we could easily copy the truename to the clipboard in the above command, just change the last line to be:

(insert (kill-new (concat file-true-dir file-name)))))

The new part is the call to 'kill-new which puts the string in the kill ring.

No need for extra function, just

M-! pwd

I have the following code already in use for a long time. It copies the full file path to the kill ring when I press the middle mouse button on the buffer name in the mode-line. It copies just the buffer name to the kill-ring when I press shift-mouse-2 on the buffer-name in the mode-line.

(defun copy-buffer-file-name (event &optional bufName)
  "Copy buffer file name to kill ring.
If no file is associated with buffer just get buffer name.
"
  (interactive "eP")
  (save-selected-window
    (message "bufName: %S" bufName)
    (select-window (posn-window (event-start event)))
    (let ((name (or (unless bufName (buffer-file-name)) (buffer-name))))
      (message "Saved file name \"%s\" in killring." name)
      (kill-new name)
      name)))

(define-key mode-line-buffer-identification-keymap [mode-line mouse-2] 'copy-buffer-file-name)
(define-key mode-line-buffer-identification-keymap [mode-line S-mouse-2] '(lambda (e) (interactive "e") (copy-buffer-file-name e 't)))

C-x C-b shows a list of buffers and the file path for each buffer where applicable.

C-x C-d, also callable via M-x list-directory, will show you the directory for your current file, and you only need to hit the "Enter" key to clear the minibuffer. Additional details are available here.

C-u C-x C-b lists buffers currently visiting files.

To do what the title says (show the current file path in the minibuffer) you can do this:

M-x buffer-file-name

To permanently show it in the mode-line, you can use this:

(setq-default mode-line-buffer-identification
              (list 'buffer-file-name
                    (propertized-buffer-identification "%12f")
                    (propertized-buffer-identification "%12b")))

Or this (color + abbrev) :

(setq-default mode-line-buffer-identification
              '((:eval
                 (list (propertize (abbreviate-file-name
                                    (expand-file-name buffer-file-name))
                                   'face 'font-lock-string-face)))))

Can I copy the path name to System(OS)'s clipboard, not the kill ring so that I can use the info with the other apps?

You can if you shell out to something like xclip (Linux), pbcopy (Mac), putclip (Cygwin).

I personally use wrapper scripts c and p for copy and paste respectively, the first reading from standard input, the latter writing to standard output. That way, this works on all my development platforms:

(shell-command (format "echo '%s' | c" buffer-file-name))

I find this more reliable and configurable than using the Emacs clipboard support. For example, my c command copies the input to all 3 clipboards on Linux (primary, secondary, clipboard), so I can paste with either Ctrl-V or middle click.

The simplest way and would be

(buffer-name)<(C-x)(C-e)> for the file name to appear in the echo area

(buffer-name)<(C-u)(C-x)(C-e)> would print the location <here>

Borrowing from Trey Jackson I came up with this:

(defun buffer-kill-path ()
  "copy buffer's full path to kill ring"
  (interactive)
  (kill-new (buffer-file-name)))

You can find more information on site

copy-buffer-file-name-as-kill from [0] does exactly what you need I think. It also has the option to copy just directory name, or just file name.

[0] http://www.emacswiki.org/emacs/download/buffer-extension.el