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

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

[david.chappaz]

Hi,

 

I'm using emacs 27.1.

 

Here is a simple experiment in interactive mode:

 

1/ Start emacs with --no-init

2/ I can check that grep-use-null-device is set to 'auto-detect'

3/ Run interactively M-x grep, something. The command that's echoed in the
grep buffer is:

grep --color -nH --null -e something

4/ I can check that grep-use-null-device has been set to nil

 

So far so good. Now consider the non-interactive variant:

 

1/ Start emacs with --no-init

2/ I can check that grep-use-null-device is set to 'auto-detect'

3/ Now I execute non- interactively (grep "grep --color -nH --null -e
something") . The command that's echoed in the grep buffer is:

grep --color -nH --null -e something /dev/null

4/ I can check that grep-use-null-device has been set to nil

5/ If I execute non- interactively the same (grep "grep --color -nH --null
-e something"), the command that's echoed in the grep buffer now is:

grep --color -nH --null -e something

 

Why is /dev/null added to the command at step 3 ?

Is this a bug ? Or perhaps my expectations are not quite right ?

 

Thanks !

David

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

[Eli Zaretskii]

> 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?

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

[david.chappaz]

 

> The function that auto-detects null-device support is only called when

> you invoke the command interactively.  And the doc string actually

> says so:

 

Thanks for pointing that out, it makes sense !

 

In my non-interactive variant though, 'grep' is not called interactively at
any point.

*         The result from step 3 doesn't contradict the fact that
auto-detection for the null-device support only takes place when you invoke
the command interactively.

*         However, the transition from step 3 to step 4 shows that
grep-use-null-device is changed (to nil) from 'auto-detect', so some kind of
auto-detection seems to have taken place

*         Furthermore, results from step 5 (in contrast to the results from
step 3) clearly show that auto-detection has taken place, despite the
absence of any interactive invocation.

 

In a few words, if I read the documentation correctly, I would expect
grep-use-null-device to be changed from 'auto-detect' on the first
*interactive* invocation, but never before that. Am I right ?

 

> Why do you need to call 'grep' noninteractively, and still want it to

> show the hits as if it were an interactive call?

 

Although I love emacs, I'm very inexperienced in elisp and there is no doubt
many people would scream when looking at my very obscure elisp functions.

In this particular case, I'm building a wrapper around 'grep', very much
inspired by this:

 

https://ideoplex.com/2014/08/03/emacs-windows-and-ack/

 

The only difference is that my own wrapper has some interactive steps at the
beginning, e.g. to ask for the pattern to search, to ask for the folder
where to search from, etc... Eventually it builds a command and calls 'grep'
non-interactively.

 

David

 

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

[Eli Zaretskii]

> From: <david.chappaz@free.fr>
> Date: Tue, 23 Feb 2021 18:47:45 -0800
> 
> In a few words, if I read the documentation correctly, I would expect
> grep-use-null-device to be changed from 'auto-detect' on the first
> *interactive* invocation, but never before that. Am I right ?

If you think there's a bug here, please use "M-x report-emacs-bug RET"
to report it with all the details.

Thanks.