Re: 2.4.0 on cygwin - Bad fd number

[Karl Hammar]

Han-Wen Nienhuys <address@hidden> wrote:

> address@hidden writes:
...
> > Error invoking `latex \\nonstopmode \\input simple.tex 2>&1 1>& 
> > /dev/null '. Return value 512Error invoking `dvips  -t a4   
> > -u+ec-mftrace.map -u+lilypond.map -Ppdf simple 2>&1 1>& /dev/null '. 
> > Return value 512
> 
> Ah wait, the shell is struggling with the redirection syntax.
> 
> are there any unix gurus that know the correct syntax for redirecting
> all output to /dev/null?

Short story:

  $ ... > /dev/null 2>&1
or
  $ ... > /dev/null 2> /dev/null

Long story:

. echo writes to stdout (file descriptor 1)
. redirection "$cmd a> file" is:
  (common case: a==1 || a==2)
  close(a); open("file",O_WRONLY) /* == a */;
. to make it write to stderr (fd 2), dup2(b,a) it by "$cmd a>&b"
. you can omit the "1" in 1>... below, because it is the default number

  $ (echo output; echo error 1>&2 )
  output
  error
  $

  Example A
=============
Example below works since:
1 close(1); open("a",O_WRONLY) /* == 1 */;
2 fork() and exec() subshell /* remember file descriptor table is inherited */
3 run echo sequentially

  $ (echo output; echo error 1>&2 ) 1> a
  error
  $ cat a
  output
  $

  Example B
=============
  $ (echo output; echo error 1>&2 ) 2> b
  output
  $ cat b
  error
  $

  Example C
=============
close(1); open("c",O_WRONLY) /* == 1 */; dup2(1,2);
  $ (echo output; echo error 1>&2 ) 1> c 2>&1
  $ cat c
  output
  error
  $

  Example D
=============
This one is equivalent to the above, but less to type
it is a special syntax to redirect both stdout and stderr
(you could also use " >& d")
  $ (echo output; echo error 1>&2 ) &> d
  $ cat d
  output
  error
  $
The drawback is that not all shells accepts it,
e.g. ash treats it like
. a "(echo output; echo error 1>&2 ) &" command
. followed by a commandless redirection "> d", which empties the file
  $ ash
  $ (echo output; echo error 1>&2 ) &> d
  $ output
  error

press return to get a new prompt
  [1] + Done                       (echo output; echo error 1>&2)
  $ cat d
  $


  Example E
=============
Redirection is order dependant, this one does:
dup2(1,2); close(1); open("e",O_WRONLY) /* == 1 */;
  $ (echo output; echo error 1>&2 )  2>&1 1> e
  error
  $ cat e
  output
  $

  Example F
=============
And yes, you can bypass it (/dev/tty is "current" tty)
  $ (echo output; (echo wall > /dev/tty; echo error) 1>&2 )  &> f
  wall
  $ cat f
  output
  error
  $

  Example G
=============
So:
  $ (echo output; echo error 1>&2 )  2>&1 1> g
  error
  $ cat g
  output
  $
will become:
 dup2(1,2); /* all's well, stderr will go to current stdout (/dev/tty) */
 close(1); open("g",O_WRONLY) /* == 1 */;

  Example H
=============
And lastly, don't use 1>&,
bash accepts it and treats it like >&, but
ash says it's a syntax error
  $ (echo output; echo error 1>&2 )  2>&1 1>& h
  $ cat h
  output
  error
  $ ash
  $ (echo output; echo error 1>&2 )  2>&1 1>& h
  ash: Syntax error: Bad fd number
  $

Regards
/Karl