removing unwanted *typo* systemd instance'd template units

 Systemd has this great feature : 

file: foo@.service 

contains the definition, as usual, with "%i"

=> will replace anything in the service file 

for example:

systemctl start foo@bar.service

systemctl start foo@resto.service

=> will start  service foo@bar  and foo@resto

but if resto does not make a valid service, it fails and can't be easily removed from the list of apparently formerly existing services.

(cf. for example https://bbs.archlinux.org/viewtopic.php?id=286912&p=1 )

As you can see with the command : 

 systemctl list-units --type=service | grep 'foo'

  UNIT                                                                                      LOAD   ACTIVE SUB     DESCRIPTION

  foo@alpha.service                                                               loaded active running Foo for protocol alpha

● foo@lish.service                                                              masked failed failed  foo@lish.service

=> Solution (on ubuntu, but no reason it does not work elsewhere) 

 systemctl mask foo@resto.service #this creates a symlink / replaces the symlink with one pointing to /dev/null, and marks the service as failed => no restart will happen again

 systemctl reset-failed # cleans failed services