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

[Paul Gover]

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-* 

*acer /boot #* uname -v 

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-*  

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-*  


(I know "file vmlinuz" can pull the necessary info, but "file" isn't in busybox.  I'll listen to 
alternatives.)




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