From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Charles C. Berry" <ccberry@ucsd.edu>
Subject: Re: Bug: Feature Request: add 'org-babel-before-execute-hook' [8.3.4
 (8.3.4-99-ga8e4a3-elpa @ /Users/macbookair/.emacs.d/elpa/org-20160704/)]
Date: Fri, 8 Jul 2016 09:09:55 -0700
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To: Jiajie Chen <jiegec@qq.com>
Cc: emacs-orgmode@gnu.org

On Fri, 8 Jul 2016, Jiajie Chen wrote:
[snip]

> Now there exists 'org-babel-after-execute-hook'. I want to implement
> this functionality: Check whether the path specified in `:file' exists
> in file system and throw an error if that does not exists. If there is
> `org-babel-before-execute-hook', we can do that instead of using
> advice. I love symmetry :)
>

Not a bug, of course.

You can check on things like file existence in a number of ways.

First, you can put emacs-lisp in header args, for example:

This evaluates when there is a file called "elisp" in the default 
directory:

#+header: :eval (or (file-exists-p "elisp") "no")
#+BEGIN_SRC emacs-lisp :eval (file-exists-p "elisp")
"got it!"
#+END_SRC


and this does not (when there is no 'eeeelisp'):

#+header: :eval (or (file-exists-p "eeeelisp") "no")
#+BEGIN_SRC emacs-lisp
"got it!"
#+END_SRC

I've illustrated here with `:eval', but `:file' will also take such an 
elisp snippet.


Alternatively, you can define a function for `org-confirm-babel-evaluate' 
that will block evaluation.

Chuck