[syslinux] How to use --once? Does it work?

[Ady]

> Hi Folks,
> 
> I've got what I hope is a clueless newbie question for you. I couldn't
> find much about it in the last 8 or so months or achives, or anywhere
> else, so I'm asking before settling down to read the fine source code
> ;-)
> 
> I'm working on a system derived from ARCH linux, running what they
> call version 6.02-4 of the syslinux package (which would be your
> version 6.02 plus their/our patches).  I've got a syslinux.cfg
 
ArchLinux currently is already using 6.03-pre12 or newer, so I would 
suggest for you to update.
 
> with several named menu options, and the right things happen if I set
> the DEFAULT option in syslinux.cfg, or select one of the options at
> the console during boot. 
> 
> What I want to do is reboot into a particular menu option once, and if
> that fails, reboot back on the usual default, without requiring anyone
> to interact with the console. 
> 
> This appears to be the purpose of --once.
> 
> The best example of its usage I could find was at
> https://bbs.archlinux.org/viewtopic.php?id=140049 comment #4
> 
> I modified this to be suitable to a linux system.  Unfortunately, the
> results did not work. Perhaps I don't understand the correct syntax?
> Or perhaps I'm hitting a bug? 
> 
> In my syslinux.cfg I have several menu options, simplified here. 
> 
> DEFAULT up
> 
> LABEL up
>    MENU LABEL Generic
>    LINUX ../vmlinuz-linux
>    APPEND root=/disk/a
>    INITRD ../pxe-linux.img
> 
> Label updebug
>    MENU LABEL Debug
>    LINUX ../vmlinuz-linux
>    APPEND root=/disk/a debug
>    INITRD ../pxe-linux.img
> 
> 
> So I do $ sudo extlinux --once updebug /boot/syslinux 
> 
> The file /boot/debug/ldlinux.sys changes, and part of the change is 2
> copies of the characters u p d e b u g
 
I would guess you are not using "/boot/debug/" but actually 
"/boot/syslinux/" as installation path, right? Where is your 
ldlinux.sys really located?
 
> 
> But when I reboot, it comes up from the first option. And if I sit at
> the console, I see that option showing as gthe default, just as if I
> hadn't been using --once to change it. 
> 
> However, the modifications _are_ removed from /boot/debug/ldlinux.sys
> So part of the desired effect of --once is happening on boot up
> 
> Perhaps I don't understand what is meant by "command" in the man page:
> 
>       -o, --once=command
>               Execute a command once upon boot.
 
The command should (better) be label in your cfg file. FWIW, labels 
shall not include space characters and shall be unique.

According to your report (and you are not the first one), there seems 
to be some partial success but the desired result is not achieved. 
Have you tried using the "=" sign and the "--install" option too? For 
instance:

 $ sudo extlinux --once=updebug --install /boot/syslinux 

where "updebug" is a unique label in your syslinux.cfg file and 
"/boot/syslinux/" is the mounted point where SYSLINUX will be 
re-installed.
 
> 
> Any help would be greatly appreciated.
> 
> Thank-you.
> 
> --
> Arlie
> 
Regards,
Ady.