Re: exec - interactive vs non-interactive shell

["Radoulov, Dimitre" <cichomitiko@gmail.com>]

On 02/06/2011 10:59, Peter Stephenson wrote:
> On Wed, 01 Jun 2011 23:03:23 +0100, Radoulov, Dimitre 
> <cichomitiko@gmail.com> wrote:
>> On 01/06/2011 22:42, Peter Stephenson wrote:
>>> On Wed, 01 Jun 2011 21:55:04 +0200
>>> "Radoulov, Dimitre"<cichomitiko@gmail.com>  wrote:
>>>> zsh-4.3.11[t]% cat a_script
>>>> exec cat
>>>> ok
>>>> zsh-4.3.11[t]%
>>>>
>>>>
>>>> zsh-4.3.11[t]% zsh<   a_script
>>>> ok
>>> So cat has taken over stdin, as expected.
>>>
>> ash, dash and bash, for example, behave differently (ksh and variants 
>> seem to behave like zsh).
>> Do you know if this behavior is defined by the POSIX standard?
>
> I suppose this is down to the behaviour of exec.  I didn't see anything
> in the standard about what it should do with stdin when the current
> shell was replaced with no additional redirection.
>
> I think what's happened is probably that the shells that don't show
> you anything have read ahead, so you still keep stdin from the same
> place, but you don't necessarily get the very next line.  I suspect
> this behaviour isn't defined.  (If that's right, then zsh's and ksh's
> behaviour is the philosophically correct one, but there are
> extenuating circumstances for the others.)
>
>> If the shell is interactive, there's no script name nor explicit 
>> input redirection (<) ,
>> command and standard input come from the same channel too (the 
>> terminal),
>> or I'm missing something?
>
> You're not fundamentally missing anything, no, I deliberately glossed
> over the way the shell opens the terminal.  It is based on stdin,
> but in practice the shell duplicates the file descriptor and does
> its terminal processing on a hidden one (i.e. greater than 9).
>

Thanks again!


Best regards
Dimitre