Re: How to run shell command with stream input, to get string output

[Jean Louis <bugs@gnu.support>]

* Marcin Borkowski <mbork@mbork.pl> [2019-07-01 09:22]:
> 
> On 2019-07-01, at 00:32, Jean Louis <bugs@gnu.support> wrote:
> 
> > Hello,
> >
> > I would like to know how to make in Emacs Lisp the
> > equivalent function to what I have here below in
> > Common Lisp.
> >
> > I wish to feed a string as stream to a command and
> > to get the string output.
> 
> How about `shell-command-to-string'?

That one runs only shell command, without the
input.

I am looking to something equivalent to shell

cat | markdown
Hello
=====

## Hello
CTRL-D here
<h1>Hello</h1>

<h2>Hello</h2>

I have to process thousands of files, I would not
like writing to hard disk the feed data for
processing to spare the hard disk.

As I cannot find solution yet in emacs lisp how to
feed some string as input to the shell command, I
am using now the virtual memory in /run/user/$UID
as this way the hard disk is spared of writing
files.

It would be ideal if I do not write anything to
file system.

(defun command-stream-in-out (command string &rest args)
  (let* ((uid (number-to-string (user-uid)))
	 (xdg-runtime-dir (getenv "XDG_RUNTIME_DIR"))
	 (runtime-dir (concat "/run/user/" uid))
	 (runtime-dir (if xdg-runtime-dir xdg-runtime-dir runtime-dir))
	 (infile (concat (slash-add runtime-dir) "markdown-input")))
    (string-to-file-force string infile)
    (with-temp-buffer
      (apply 'call-process command infile (current-buffer) nil args)
      (buffer-string))))

Then I can do something like this:

(defun markdown (string)
  (command-stream-in-out "markdown" string))

(defun pandoc-markdown (string)
  (command-stream-in-out "pandoc" string "-f" "markdown" "-t" "html"))

(markdown "Hello\n=============")

"<h1>Hello</h1>
"

(pandoc-markdown "Hello\n=============")

"<h1 id=\"hello\">Hello</h1>
"

I have solved my problem. But if somebody knows
how to feed the string to command as its input,
without writing to file system, let me know.

Jean