Answer to your second question, try Fn key and right arrow key, it should give you the option of the brightness of your keyboard ie: off, 25%, u[p to 100%. Hope this helps?

I don't know which laptop you have, but the XPS13 9333 allows you to control the keyboard backlight with Fn+F6 and using Fn keys to control the backlight is not what I'm looking for.

I'm asking for a way to control it without using any key and smbios-token-ctl allows me to do that, but as I said I don't have the complete control of the backlight.

The thing is that having the keyboard backlight constantly on is sometimes a waste of power and when you are in a really bright environment you can't really tell whether the backlight is on or off. If I'm able to change the brightness of the keyboard from the command line, I could easily solve this problem with a script.

The auto backlight would actually be fine if there was a way to have it back without rebooting the machine. Because of the electrical noises this laptop makes, I have to change the backlight from time to time and after that I'm stuck with the manual mode.

One thing I noticed is that when I use Windows the keyboard backlight is never automatic, so I'm wondering if this has anything to do with the unexpected behavior of smbios-token-ctl described in the first post or it's really wanted.

Answer to your second question, try Fn key and right arrow key, it should give you the option of the brightness of your keyboard ie: off, 25%, u[p to 100%.

Hope this helps?

This new XPS 13 does not respond at all to the use of the Fn-F6 keys -- nothing happens, not the keyboard backlighting or anything else.  Fn-F4 and F5 do adjust screen brightness, but nothing I've found seems to bring up keyboard lighting, ambient or otherwise, on or off battery.

Suggestions for next steps?  (I've had this ultrabook for two days, the keyboard backlighting has never come on.)

I would suggest taking it back to where you bought it from as it does sound like an actual problem with the ultrabook, I don't think it's anything that you have done...

UPDATE:  

You were absolutely right about it being a hardware issue. Since I had purchased it online from Dell with premium warranty, they arranged (after a lot of discussion) for a tech to come to my workplace and replace the keyboard.

 I watched as he disassembled the entire machine, in order to get to the keyboard. Out came the battery pack, the fan, motherboard, HDD, and so on, along with about 3 dozen small screws.  Lots of disconnecting of stuff.  Pretty alarming to us non-hardware folks.

Took probably an hour altogether. It may be that the replaced keyboard wasn't defective after all, but that the small light-cable just wasn't plugged in -- we didn't notice if that was the case during initial disassembly, but after the new keyboard still didn't light up, the tech quickly spotted this small connector and after it was plugged in, keyboard lit up fine.

Machine has been working perfectly this past 36 hours and if my luck holds I'll never need to try to deaal with Dell's long-distance support folks again.  It was quite a difficult process with lots of my calling, my initiation of online chat, failure to set up repair appointment times correctly, annoying effects on my work schedule and so on. Despite many promises the folks overseas never did return any of my calls to check on the success of the process.

It's unfortunate that a brand new machine came already with a defect.  But a bright point (and an important one) is that the tech seemed very competent.

So, is it possible to have a way to control the keyboard backlight from the command line? That would be great.

When I'm in a bright ambient is sometimes hard to tell whether the keyboard backlit is on or not and I'd really like to use the light sensor to automatically set the desired level in a defined way.

With a sysfs interface to control the backlit, all the problem would be solved.

I tried to see how much power the keyboard backlight requires, this is more or less what I got using powertop [1]:
6.55 W - max brightness
5.75 W - low brighntess
5.25 W - off

As you can see, there's more than 1 W of difference between off and max brightness, that's a lot.

[EDIT]
What I mean with these numbers is that keeping the backlight on when not required consumes a lot of battery for no reason. This is a problem in bright ambients as you really don't think about turning the backlight off or you just can't really see whether is on or off.

smbios-token-ctl is the only solution, but it's not working perfectly as I can only set the keyboard to its max or turn it off. As shown here above, there's an important power difference between low and max, so I would like to set the brightness to low sometimes.
[/EDIT]

[1] No X session running, WiFi and BT off and constant screen backlight level.

Also, what about this automatic keyboard illumination? Is it a BIOS bug? Why isn't there a way to set it from the Fn keys? It seems a rather old bug: https://bugs.launchpad.net/dell-sputnik/+bug/1056386

I have the new XPS 2015.

My keyboard backlight wasn't working for a long time.

I kept reading to use Fn+F6, which is incorrect.

Turns out it's actually Alt+F10

The F10 key has a little lighted key image on it.

There appear to be three setting, Bright, Very Bright and Off.

Hope this helps others who have accidentally disabled their backlighting.

I'm pretty sure this is in some manual somewhere, but it's not intuitive.

Thanks! I was going nuts pressing Fn + F10 as I figured that was the lighted key image but would never have thought to press Alt.... 

It's Fn+F10 (or just F10, depending on which mode you have your function keys in), not Alt+F10. Notice the icon on the key. It used to be on F6 on previous generations.

So is it possible to change the brightness *level*, perhaps using cmd?

My brightness key (F10) toggles between Bright (100%), Almost As Bright (80%), and Off (0%), which is great. but what I'd really love is for it to toggle between 100%, 70%, 30% and 0%.

Any ideas?