Can't get "grep -F" to work like real grep

[Quentin Rameau]

> <Appologies: re-sent as the previous version got corrupted by "rich text" 
> fields in the email>

Hi Paul,

> I hope I'm doing something mindnumbingly stupid; grep -F output is not as 
> expected.  Please can anyone spot the problem?  Thanks.
> 
>  (Background: I'm writing a script to manage kernel images in /boot, and want 
> to prevent the user deleting the kernel currently being executed.)
> 
>  This is sys-apps/busybox-1.30.1 on Gentoo,
> compared with sys-apps/busybox-1.30.1
> all running on an amd64 box.
> 
>  Running in /boot, where:
> 
> acer /boot # ls vmlinuz-* 
> vmlinuz-5.1.16-gentoorx570  vmlinuz-5.3.12-gentoorx570.new
> 
> acer /boot # uname -v 
> #6 SMP Sun Sep 29 00:17:32 BST 2019
> 
> Linux grep produces:
> acer /boot # grep -Fl "#6 SMP Sun Sep 29 00:17:32 BST 2019" vmlinuz-* 
> vmlinuz-5.1.16-gentoorx570
> 
> whereas busybox grep produces no matches.  Removing the -F fixes it:
> acer /boot # busybox grep -l "#6 SMP Sun Sep 29 00:17:32 BST 2019" vmlinuz-*  
> vmlinuz-5.1.16-gentoorx570
> 
> In case it was some weird issue with globbing or UTF-8 etc.,
> I tried:
> acer /boot # uname -v > ver 
> acer /boot # busybox grep -Flf ver vmlinuz-*
> 
> Still no output, but:
> acer /boot # busybox grep -lf ver vmlinuz-*  
> vmlinuz-5.1.16-gentoorx570
> 
>  
> (I know "file vmlinuz" can pull the necessary info, but "file" isn't in busybox.  
> I'll listen to alternatives.)

The grep utility is specified to work on text files.
I suppose your vmlinuz-5.1.16-gentoorx570 file is a compressed binary
Linux kernel, so not text.

From that, you can't really expect a text tool to work on binary data.