Plasma installation clarification requested

[Sahlan]

Obtained a Nexus 5X on which to install Plasma OS, for porting software 
which I am also porting to Sailfish and Tizen devices.

All going well except the final step, so I am requesting some advice 
please.

I have successfully followed the instructions to unlock the phone's 
bootloader
I have checked that there is a plugdev group on my Ubuntu 18.04 box, and 
that my user name is a member of it
I have successfully fastboot formatted cache and userdata
I have downloaded pm-flashtool.git and run pm-flash when the phone is 
booted into fastboot mode
pm-flash asks me to confirm my device is a bullhead device (this is 
correct)
I type y for yes, and after a short delay the phone shows me the TWRP 
3.0.2.2 main screen, while on the Linux box I see a repeating pattern of
     adb shell
     echo 1
     test -x
     echo +n
     sleep 3

This is not what I was expecting, because no documentation, nor any 
youtube videos, tell me this is what should happen. Have I done 
something wrong, or should I now be selecting one of the options from 
the TWRP main screen? In case security is an issue, I should tell you 
that I am logged in to Ubuntu under my regular user name, but running 
the fastboot commands under sudo.

With thanks for any advice.
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