Re: Tangling flow control

[Philip Hudson <phil.hudson@iname.com>]

On 9 November 2016 at 17:54, Charles C. Berry <ccberry@ucsd.edu> wrote:
> On Tue, 8 Nov 2016, Philip Hudson wrote:
>
> [snip]
>>
>> How do you do "looping" flow control?
>>
>> For context, what I'm trying to write is a single Org file from which
>> I can tangle out a number of =~/.ssh/config= files, one for each of
>> several hosts on a LAN. Within this file I need to repeatedly place a
>> template =BEGIN_SRC ssh-config= block, each time with a few words and
>> numbers changed. Do you do this anywhere? If so, how have you
>> implemented it?
>
> It sounds like what you want is a template for the src block and another src
> block that does substitutions in that template using a table of values
> inside a loop.
>
> Just to get you started, with this template:
>
> #+NAME: template
> #+BEGIN_SRC org
>   ,#+BEGIN_SRC shell :tangle %to-file
>   ls -lt %filename
>   ,#+END_SRC
> #+END_SRC
>
> and this helper src-block
>
> #+NAME: get-body
> #+BEGIN_SRC emacs-lisp :var src-block-name="c-code"
>   (save-excursion
>     (org-babel-goto-named-src-block
>      src-block-name)
>     (cadr (org-babel-get-src-block-info)))
>
> #+END_SRC
>
>
> running
>
> #+header: :wrap src org :var tmpl=get-body("template")
> #+BEGIN_SRC emacs-lisp
>   (org-fill-template tmpl
>     '(("to-file" . "abc.sh")("filename" . "my-dir")))
> #+END_SRC
>
> yields
>
> #+RESULTS:
> #+BEGIN_src org
> ,#+BEGIN_SRC shell :tangle abc.sh
> ls -lt my-dir
> ,#+END_SRC
> #+END_src
>
>
> To revise this for your application, you need to provide a table of the
> associated values for the "to-file" and "filename" keys in the alist, read
> that table using a :var header, loop thru the table reconstructing the alist
> each time creating src blocks, and send the output to an org tempfile.  Then
> you tangle the tempfile.
>
> Alternatively, you simply write the script files directly without bothering
> to write to an org tempfile.

Thanks Chuck. I think I've got that straight. I hadn't come across
`org-fill-template' before. I think I know how to loop thru a table,
though I haven't done it myself before; I've seen how to "get" a table
as a list.

What I'm not clear about is why the template nests a shell-script
block inside an Org block. Should the outer Org block not have a
%-escaped placeholder for a :tangle target, an intermediate Org file?

-- 
Phil Hudson                  http://hudson-it.ddns.net
Pretty Good Privacy (PGP) ID: 0x887DCA63