Re: Unexpected behavior of grep-use-null-device in non-interactive mode

[Eli Zaretskii <eliz@gnu.org>]

> From: <david.chappaz@free.fr>
> Date: Mon, 22 Feb 2021 15:00:24 -0800
> 
> Why is /dev/null added to the command at step 3 ?
> 
> Is this a bug ? Or perhaps my expectations are not quite right ?

The function that auto-detects null-device support is only called when
you invoke the command interactively.  And the doc string actually
says so:

  (defcustom grep-use-null-device 'auto-detect
    "If t, append the value of `null-device' to `grep' commands.
  This is done to ensure that the output of grep includes the filename of
  any match in the case where only a single file is searched, and is not
  necessary if the grep program used supports the `-H' option.

  In interactive usage, the actual value of this variable is set up
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  by `grep-compute-defaults'
  ^^^^^^^^^^^^^^^^^^^^^^^^^

Why do you need to call 'grep' noninteractively, and still want it to
show the hits as if it were an interactive call?