Re: [9fans] rc output redirection

[hugo rivera <uair00@gmail.com>]

2009/4/3, Russ Cox <rsc@swtch.com>:
> > But when you do something like
>  > cmd1 | cmd2 |[2] cmd3
>  > you get cmd1's stdout piped to cmd2's stdin; but my confusion begins
>  > here: is it cmd1's or cmd2's stderr that gets redirected to cmd3's
>
> > stdin? maybe both? my guess is that ...
>
>  why guess?
>
>  % {echo cmd1 >[1=2]} | {echo cmd2 >[1=2]} | sed 's/^/cmd3: /'
>  cmd1
>  cmd2
>  % {echo cmd1 >[1=2]} | {echo cmd2 >[1=2]} |[2] sed 's/^/cmd3: /'
>  cmd3: cmd1
>  cmd3: cmd2
>  %
>
>  not what i expected, but there you have it.
>  and it makes sense: cmd1 and cmd2 are both
>  writing to the same stderr, so redirecting it
>  redirects both.
>
>
>  > how can I redirect cmd1's stderr to cmd3's stdin instead
>  > of cmd2's? or both?
>
>
> if you don't care about cmd2's stderr, then you can get rid of it
>
>  % {echo cmd1 >[1=2]} | {echo cmd2 >[2]/dev/null} |[2] sed 's/^/cmd3: /'
>  cmd2
>  cmd3: cmd1
>
> %
>
>  > It's easy to think of more complex cases.
>
>
> it's easy to think of elephants.
>
>  rc does its general-purpose job very well.
>  if you have special-purpose needs,
>  as lucio suggests, write a c program.
>
>
>  russ
>
>

ok, thanks

--
Hugo