bug#69929: Can grep -q report matches in incomplete lines?

[Niels Möller]

Hi,

I'm having grep -q read input from a pipe. I would like grep to exit
successfully as soon as a match occurs, without requiring the line to be
terminated by newline or EOF (unless the grep pattern includes '$', that
is).

E.g., if I run 

  (printf foo ; sleep 30) | grep -q foo

I want grep to exit successfully right away. Currently, grep waits until
it gets EOF on the input, 30 seconds later. I'm testing with GNU grep
3.8, running on Debian GNU/Linux.

Another less friendly example:

  (printf foo ; cat /dev/zero | tr '\0' x) | grep -q foo

It would be nice if that exited quickly. And without any excessive
memory usage, with or without match.

My actual usecase is for a test booting a machine in qemu. I pipe qemu's
output (including the console of the guest machine) to grep, and I want
to check that the machine gets to the login prompt. But there's usually
no trailing newline after that prompt.

Regards,
/Niels


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