[GLLUG] read -t 1 hangs on reading from an unconnected pipe.

[Tim Woodall]

Hi Fred,

On Thu, 7 Jan 2021, Fred Youhanaie via GLLUG wrote:

> Hi Tim
>
> Happy New Year to you too.
>
> This is expected behaviour with pipes. The first process to open the pipe 
> will block until the other end opens too.
>
> You confirm with the following - the first echo does not happen until the 
> shell itself has completed the open (</tmp/pipe)
>
> 	time { echo hello ; read -t 1 ;} </tmp/pipe

Ah, thanks. This example helped me make sense of what was going on.

>
> You can try something like this and drop it in the background before the read
>
> 	sleep 999 >/tmp/pipe &
>

Neat trick. I don't actually need it now I know that the read can block
I've tweaked things so I don't get into this situation but it might be
useful in the future.

Regards,

Tim.

> Cheers,
> Fred
>
>
> On 07/01/2021 09:31, Tim Woodall via GLLUG wrote:
>> Happy new year to all.
>> 
>> 
>> I didn't expect this:
>> 
>> tim at einstein(9):~$ mkfifo /tmp/pipe
>> tim at einstein(9):~$ time read -t 1 </tmp/pipe
>> 
>> real    0m13.514s
>> user    0m0.000s
>> sys     0m0.000s
>> tim at einstein(9):~$
>> 
>> 
>> in another terminal I did:
>> tim at einstein(10):~$ echo >/tmp/pipe
>> 
>> 
>> I had expected the read to time out after one second.
>> 
>> Is there any way to make read always timeout, even if the other end of
>> the pipe isn't connected?
>> 
>> Tim.
>> 
>> 
>
>