Re: [Chicken-users] Controlling output of processes

[Jim Ursetto]

What you want to do in effect is redirect stderr to stdout so that  
output comes out in the right order.  The simplest way using a UNIX  
shell as you know is:
(system "ls -l 2>&1")

We can avoid relying on the shell via something like:

(define command "ls")
(define args '("-l"))
(process-wait
 (process-fork
  (lambda ()
    (duplicate-fileno fileno/stdout fileno/stderr)
    (process-execute command args))))

Furthermore if we want to write the command's combined stdout/stderr  
to a file we can redirect our own stdout first (like `ls -l > test.txt  
2>&1`)

(define fd (file-open "test.txt" (+ open/wronly open/trunc open/creat)))
(process-wait
 (process-fork
  (lambda ()
    (duplicate-fileno fd fileno/stdout)
    (duplicate-fileno fileno/stdout fileno/stderr)
    (process-execute command args))))
(file-close fd)
(print "done")

This assumes that your exec'ed command actually flushes its output  
after every write.  If it does not then its output will likely be  
jumbled when writing to the file, due to stdio buffering.

This method doesn't use pipes or ports, instead acting as a pass- 
through.  (process) could do it but it doesn't provide the capability  
to redirect stderr to stdout.  I wonder if such an addition would be  
useful.

On Mar 12, 2010, at 10:14 AM, Lasse Kliemann wrote:

> How can I start a process and control where it sends its stdout
> or stderr? I know that I can do:
>
> (receive
>   (p-stdout p-stdin p-pid p-stderr)
>   (process* command args)
>   (...))
>
> This gives input and output ports connected to the process. But
> how can I express, e.g.: "everything available on port p-stdout
> should go to (current-output-port)"? Or similar statements
> involving p-stdout and/or p-stderr? I could use a loop that reads
> from those ports and writes to (current-output-port) until it
> sees EOF. But this feels plain wrong. In particular, when
> forwarding both p-stdout and p-stderr, this would not represent
> the order in which the process generated the output (but it would
> first show all its stdout and then all its stderr, or vice
> versa).
>
> Can you give me some suggestions?
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