How to do "ls /tmp > /dev/null" in Guile?

How to do "ls /tmp > /dev/null" in Guile?

[Alex Kost]

Hello, in the guile REPL I evaluated the following:

  (with-output-to-port (%make-void-port "w")
    (lambda () (display "foo") (newline)))

and I got no output as expected.  Then I tried the following:

  (with-output-to-port (%make-void-port "w")
    (lambda () (system* "ls" "/tmp")))

but there was an output from "ls" command.  So my question is: how to
get rid of this output?

-- 
Thanks,
Alex

Re: How to do "ls /tmp > /dev/null" in Guile?

[Marko Rauhamaa]

Alex Kost <alezost@gmail.com>:

> Hello, in the guile REPL I evaluated the following:
>
>   (with-output-to-port (%make-void-port "w")
>     (lambda () (display "foo") (newline)))
>
> and I got no output as expected.  Then I tried the following:
>
>   (with-output-to-port (%make-void-port "w")
>     (lambda () (system* "ls" "/tmp")))
>
> but there was an output from "ls" command.  So my question is: how to
> get rid of this output?

When you use ports (which the operating system knows nothing about),
Guile needs to actively jockey the data between them.

    (use-modules (ice-9 popen))
    (with-output-to-port (%make-void-port "w")
      (lambda ()
         (let ((output (open-input-pipe "ls /tmp")))
           (let loop ()
             (let ((c (read-char output)))
               (if (not (eof-object? c))
                   (begin
                     (write-char c)
                     (loop)))))
           (close-input-port output))))


Marko

Re: How to do "ls /tmp > /dev/null" in Guile?

[Alex Kost]

Marko Rauhamaa (2016-03-19 14:02 +0300) wrote:

> Alex Kost <alezost@gmail.com>:
>
>> Hello, in the guile REPL I evaluated the following:
>>
>>   (with-output-to-port (%make-void-port "w")
>>     (lambda () (display "foo") (newline)))
>>
>> and I got no output as expected.  Then I tried the following:
>>
>>   (with-output-to-port (%make-void-port "w")
>>     (lambda () (system* "ls" "/tmp")))
>>
>> but there was an output from "ls" command.  So my question is: how to
>> get rid of this output?
>
> When you use ports (which the operating system knows nothing about),
> Guile needs to actively jockey the data between them.
>
>     (use-modules (ice-9 popen))
>     (with-output-to-port (%make-void-port "w")
>       (lambda ()
>          (let ((output (open-input-pipe "ls /tmp")))
>            (let loop ()
>              (let ((c (read-char output)))
>                (if (not (eof-object? c))
>                    (begin
>                      (write-char c)
>                      (loop)))))
>            (close-input-port output))))

Ah, thanks!  I get it.  But I also want to check an exit status of the
running command (sorry, that I didn't mention it).  So I would like to
have the following procedure:

(define (system-no-output* . args)
  "Like 'system*' but suppress the output of the command indicated by ARGS."
  ???)

Or even better (it would be a perfect solution for me) the following macro:

(define-syntax-rule (with-no-process-output body ...)
  "Run BODY and suppress all output of the executed sub-processes."
  ???)

So that I could do something like this:

(with-no-process-output
  (let ((status1 (system* "ls" "/tmp"))
        (status2 (system* "ls" "/foo")))
    (format #t "Very useful info: ~a, ~a~%" status1 status2)))

and there would be no standard/error output from both "ls" calls.  Is it
possible?

-- 
Alex

Re: How to do "ls /tmp > /dev/null" in Guile?

[Marko Rauhamaa]

Alex Kost <alezost@gmail.com>:

> Ah, thanks!  I get it.  But I also want to check an exit status of the
> running command (sorry, that I didn't mention it).  So I would like to
> have the following procedure:
>
> (define (system-no-output* . args)
>   "Like 'system*' but suppress the output of the command indicated by ARGS."
>   ???)
>
> Or even better (it would be a perfect solution for me) the following macro:
>
> (define-syntax-rule (with-no-process-output body ...)
>   "Run BODY and suppress all output of the executed sub-processes."
>   ???)

Replace (close-input-port) with (close-pipe); that should give you the
exit status. Also you don't need to copy the data to a dummy port if you
only want to ignore it.

> and there would be no standard/error output from both "ls" calls. Is
> it possible?

The diagnostic output (stderr) is a different story. You want to be sure
to want to ignore it. To implement that properly, you should go
lower-level with operating system calls (fork, exec, waitpid). Simply
open "/dev/null" for writing and dup the file descriptor into the slots
1 and 2 of the child process.


Marko

Re: How to do "ls /tmp > /dev/null" in Guile?

[Alex Kost]

Marko Rauhamaa (2016-03-20 12:23 +0300) wrote:

> Alex Kost <alezost@gmail.com>:
>
>> Ah, thanks!  I get it.  But I also want to check an exit status of the
>> running command (sorry, that I didn't mention it).  So I would like to
>> have the following procedure:
>>
>> (define (system-no-output* . args)
>>   "Like 'system*' but suppress the output of the command indicated by ARGS."
>>   ???)
>>
>> Or even better (it would be a perfect solution for me) the following macro:
>>
>> (define-syntax-rule (with-no-process-output body ...)
>>   "Run BODY and suppress all output of the executed sub-processes."
>>   ???)
>
> Replace (close-input-port) with (close-pipe); that should give you the
> exit status. Also you don't need to copy the data to a dummy port if you
> only want to ignore it.

(close-pipe) is what I needed, thanks!  For the record here is the procedure:

(define (system-no-output* . args)
  "Like 'system*' but suppress the output of the command indicated by ARGS."
  (let ((port (apply open-pipe* OPEN_READ args)))
    (read-string port)
    (close-pipe port)))

>> and there would be no standard/error output from both "ls" calls. Is
>> it possible?
>
> The diagnostic output (stderr) is a different story. You want to be sure
> to want to ignore it. To implement that properly, you should go
> lower-level with operating system calls (fork, exec, waitpid). Simply
> open "/dev/null" for writing and dup the file descriptor into the slots
> 1 and 2 of the child process.

Thanks for the info!  There are things to think about.

-- 
Alex