Fwd: exec - interactive vs non-interactive shell

Fwd: exec - interactive vs non-interactive shell

[Radoulov, Dimitre]

Hi all,
could anybody explain what happens here?


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
zsh-4.3.11[t]% zsh -i<  a_script
zsh-4.3.11[t]%


No output with -i.


bash does the opposite:

zsh-4.3.11[t]% bash<  a_script
zsh-4.3.11[t]% bash -i<  a_script
4.2.8(1)-release$ exec cat
ok
zsh-4.3.11[t]%



Best regards
Dimitre

Re: exec - interactive vs non-interactive shell

[Peter Stephenson]

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.

> zsh-4.3.11[t]% zsh -i<  a_script
> zsh-4.3.11[t]%
> 
> 
> No output with -i.

You're confusing the effects of "zsh -i <a_script" with "zsh -i
a_script".  The first one starts up zsh as an interactive shell, with a
line editor, but with standard input (not commands, which are
interactive) coming from a_script.  You'll find that prompt on the line
after is inside the new interactive shell.  Try typing at this point:

read
print $REPLY

You should see "exec ok".

The rules for how zsh finds the commands and standard input are roughly:

- Standard input is only redirected if you explicitly use a "<".
(So "zsh a_script" leaves stdin where it is and cat hangs waiting for
input.)  It's independent of interaction with the terminal for reading
commands (though there's the more complicated issue of how the terminal
gets set up in the first place).

- If there's a script name, use that for commands, regardless of whether
the shell is interactive or not.  (So "zsh -i a_script" behaves
basically like without the -i, although you could have interactive shell
commands such as vared in the script.)

- If there's no script name and the shell is interactive, use the
terminal for commands.

- If there's no script name and the shell is not interactive, fall back
to reading commands from standard input.  This is the only case (without
making special arrangements) where commands and standard input come from
the same channel.

-- 
Peter Stephenson <p.w.stephenson@ntlworld.com>
Web page now at http://homepage.ntlworld.com/p.w.stephenson/

Re: exec - interactive vs non-interactive shell

[Radoulov, Dimitre]

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.
>


Yes,
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?


>> zsh-4.3.11[t]% zsh -i<   a_script
>> zsh-4.3.11[t]%
>>
>>
>> No output with -i.
> You're confusing the effects of "zsh -i<a_script" with "zsh -i
> a_script".  The first one starts up zsh as an interactive shell, with a
> line editor, but with standard input (not commands, which are
> interactive) coming from a_script.  You'll find that prompt on the line
> after is inside the new interactive shell.  Try typing at this point:
>
> read
> print $REPLY
>
> You should see "exec ok".
>
> The rules for how zsh finds the commands and standard input are roughly:
>
> - Standard input is only redirected if you explicitly use a "<".
> (So "zsh a_script" leaves stdin where it is and cat hangs waiting for
> input.)  It's independent of interaction with the terminal for reading
> commands (though there's the more complicated issue of how the terminal
> gets set up in the first place).
>
> - If there's a script name, use that for commands, regardless of whether
> the shell is interactive or not.  (So "zsh -i a_script" behaves
> basically like without the -i, although you could have interactive shell
> commands such as vared in the script.)
>
> - If there's no script name and the shell is interactive, use the
> terminal for commands.
>
> - If there's no script name and the shell is not interactive, fall back
> to reading commands from standard input.  This is the only case (without
> making special arrangements) where commands and standard input come from
> the same channel.
>

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?



Thank you for the explanation!


Regards
Dimitre

Re: exec - interactive vs non-interactive shell

[Peter Stephenson]

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).

pws



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Re: exec - interactive vs non-interactive shell

[Radoulov, Dimitre]

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