Name
       replacekey - replace placeholder in a FITS header

Synopsis
       replacekey <in> <card> [placeholder]

Description
       replacekey  is  aimed  at replacing placeholder cards in a
       FITS file's header by cards provided by the  user  on  the
       command-line.  The  idea  is to decouple the process doing
       the actual image processing on the data from  the  process
       that  needs  to  update the FITS header. To do this, it is
       enough to place "blank" cards  (or  placeholders)  in  the
       files produced by the image processing routine, and update
       the output header with this utility.  The  default  place-
       holder is a comment card as:
       COMMENT PLACEHOLDER

       You can change the name of the placeholder by providing it
       (in quotes) on the command-line  as  third  argument.  You
       need  to  provide the name of the input FITS file you want
       to modify, the value to place in the header,  and  option-
       ally   this   placeholder   name.  You  can  actually  use
       replacekey to do  plain  search  and  replace  in  a  FITS
       header,  by providing any character string as placeholder.
       The first card matching the string  you  provide  will  be
       replaced.

       Examples:

       You  want  to  place  a  card  containing  VALUE=32 in the
       header, in  place  of  the  default  placeholder  (COMMENT
       PLACEHOLDER), in the file named myfile.fits:
       % replacekey myfile.fits "VALUE  = 32"

       If  your  do  not care about the HISTORY cards you have in
       the header, and want to replace the first one by VALUE=32,
       you would do:
       % replacekey myfile.fits "VALUE  = 32" "HISTORY"

       If  you want to replace the previous VALUE=32 by VALUE=64,
       you would simply do:
       % replacekey myfile.fits "VALUE  = 64" "VALUE"

       Notice that the character string you give as second  argu-
       ment will be placed verbatim in the header. If you provide
       more than 80 characters, they will  be  truncated  to  the
       first  80  characters.  If  you  provide  less characters,
       blanks will be added up to character  80.  You  should  be
       careful when adding cards, that they respect the FITS for-
       mat. Nothing prevents you from doing:
       % replacekey myfile.fits "VALUE= 1 2 3 4 /" "HISTORY"

       which will replace the first occurence of a  HISTORY  card
       by  the  line  you provided, but the replacing line is not
       FITS-compliant. No tests are performed internally to check
       the compliance of the FITS line you are adding.

Options
       None

See Also
       hierarch28  to  convert  HIERARCH  ESO keywords to regular
       8-char keywords.

Files
       Files shall all comply with FITS format

Bugs
       Modifications are so far only possible in the main header,
       not in the extensions.