Re: A better way to run shell cmd?

[Daniel Hartwig <mandyke@gmail.com>]

On 13 June 2012 14:55, Nala Ginrut <nalaginrut@gmail.com> wrote:
> hi folks! I'm on my trip and inconvenient to meet you guys on IRC.
> Things gonna be normal next month.
>
> Anyway, there's a problem for you.
> I'm trying to write a simple wrapper for "sed" with our popen module:
> --------------code------------
> (use-modules (ice-9 popen) (rnrs))
> (define (sed pattern str)
>  (let ((p (open-input-output-pipe (string-append "sed " pattern))))
>    (display str p)
>    (get-string-all p)))
> ----------------end-------------
>
> I expect it run like this:
> -----------------------
> (sed "s:a:b:g" "abcabc")
> ==> "bbcbbc"
> -----------------------
>
> But it halts that I have to interrupt.
> Is it accepted?
>

The sed program may be expecting a newline, or waiting for EOF or a
full buffer before producing it's output.  Some users have had trouble
with this in the past, because you can not send EOF to one end of the
pipe without closing the other.

One way around this to use run-with-pipe from the guile-lib module (os
process).  This returns two separate pipe objects for input and
output, thus you can close/EOF one and continue to use the other.
Many filter programs do not flush their output until their input EOFs,
which is why your solution using 'echo -n' works.

See this thread for a short discussion:

http://lists.gnu.org/archive/html/guile-user/2012-02/msg00066.html

Updating guile to support closing one end of the pipe is long off on my TODO.