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.