Since I deal with many SOAP web services for work, I also have to deal with lots of XML files. In the application server log they usually appear on one line, which makes the reading unnecessarily painful. Fortunately, Emacs coupled with the handy xmllint comes to the rescue.

First, I installed xmllint with: sudo apt-get install libxml2-utils

Now I could format my XML buffers with: C-x h C-u M-| xmllint -format - RET

However, this is Emacs, so there is always a more productive way to solve problems. Why should I have to input the options for xmllint or remember the necessary key bindings all the times?

(defun mu-xml-format ()
  "Format an XML buffer with `xmllint'."
  (shell-command-on-region (point-min) (point-max)
                           "xmllint -format -"
                           (current-buffer) t
                           "*Xmllint Error Buffer*" t))

Since I only need this utility in XML buffers, I bound it for nxml-mode-map.

(use-package nxml-mode                  ; XML editing
  :mode "\\.xml\\'"
  :bind (:map nxml-mode-map
              ("C-c m f" . mu-xml-format))