Executing a command as a nologin user

Solution 1:

One way is to launch a shell for that user (explicitly specifying the shell):

sudo -u www-data bash 

This will launch a (bash) shell as the specified user. You can then execute your command(s) and logout (to return to your previous shell)

Solution 2:

As root you can use su -s /bin/sh $user — the -s option overrides the configured shell for the user.