how to redirect in bash+crontab

[Oleg Goldshmidt]

On Tue, Jul 24, 2012 at 7:51 PM, Nadav Har'El <nyh at math.technion.ac.il>wrote:

> On Tue, Jul 24, 2012, Oleg Goldshmidt wrote about "Re: how to redirect in
> bash+crontab":
> > Isn't it the same as redirecting stderr to a real file and finding out
> that
> > you don't have permissions to write to it?
>
> No, this cannot happen. write(2) cannot fail because of permission
> problems, only the open(2) can.


But you cannot write(2) before you open(2), can you?

Once a process has a file descriptor
> it can write it even if it could never open the original file itself.
>

Which s true for file descriptors (e.g., dup()), but to redirect to a file
you do need to open(2) the file, methinks.


> > > But I think it's important that this caveat is documented in proc(5),
> >
> > And in bash(1), as I pointed out in my reply to Amos.
>
> Holy shit!
>
> I just noticed that bash(1) claims that ">/dev/stderr" doesn't actually
> open a file called /dev/stderr - but just does dup(2). Either this is
> not true, or you simply weren't using Bash - perhaps your crontab uses
> dash or something else.
>

Whatever crontab uses is irrelevant since all there is in crontab is the
name of a script that is explicitly #!/bin/bash (and al the scripts that
are called are explicitly bash as well).

Strictly speaking, "man bash" does not explicitly say it does not open()
/dev/stderr (it says it dup()s but does not say this is the only thing it
does). Nevertheless I think the documentation's intent is exactly that - no
open(), or at least ignoring mode during open(), consistent with the
/dev/fd/* behaviour as described by Stevens and everywhere else.

I would be curious to know how recent versions of
SuSE/debian/ubuntu/*BSD/Mac OS X/whatever implement /dev/stderr.

-- 
Oleg Goldshmidt | pub at goldshmidt.org <oleg at goldshmidt.org>
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