      fields (key) ... [-x N]

      Read from stdin a list of file names.  For each file name, print in
      sequence the value of the given FITS header keywords.  Any number of
      keys may be present.  By default, the key/values are taken from
      primary header unit.  If the -x is provided the Nth extension's
      header unit is used instead, with 0 being the first extension after
      the PHU.