How can I post-process the generated file?

• org-babel-post-tangle-hook: When the hook is executed, the current buffer (as identified by the current-buffer function) will be a buffer visiting the file of the tangled code.

  Example applications could include post-processing, compilation or evaluation of tangled code files.

Why does my Shell code chunk produce no output?

> Why is the \`diff’ command alone not exported while the piped \`diff’ via
> \`cat’ works?

The exit code is not zero since diff found differences, so Babel assumes the
script produced an error. Try this instead when you don’t know what the return
code of the last command will be or if you know that it isn’t zero even when no
error occured:

\\#+begin\_src shell :exports results :results output
diff testdiff.txt.orig testdiff.txt
:
\\#+end\_src

> Is this this a bug?

I don’t think so, although Babel could give a more enlightening message about
why it didn’t evaluate STDOUT. It gives this on your original example:

Babel evaluation exited with code 1
Code block produced no output.