Where to place user created systemd unit files
The recommended place is /etc/systemd/system/nginx.service
Then issue the command :
systemctl enable nginx
And finally
systemctl start nginx
Usually, in Ubuntu, the package provided unit files go in /lib/systemd/system/ directory e.g. /lib/systemd/system/nginx.service whereas the user provided or any modification to the package provided unit file(s) go in /etc/systemd/system/ directory.
Now, to override, you can:
Name the unit file as the package provided one e.g. to override
/lib/systemd/system/nginx.service, you can create the file/etc/systemd/system/nginx.serviceand put the directives in thereOr you can create a drop-in snippet ending in
.confin a directory that is named after the original unit file with.dappended to the directory name e.g. you can create a file named/etc/systemd/system/nginx.service.d/override.confand put statements in there
Now, you can leverage the systemctl edit command to do all the manual stuffs for you i.e. it will create the necessary override file for you (at first it will create a temporary file (with intermediate directory(ies) for drop-in snippets), and upon saving it will commit and rename() the temp file to desired final file), and you just need to edit the file to put in your overrides.
For example, to create an override snippet for ngnx.service (/lib/systemd/system/nginx.service) mentioned above:
systemctl edit nginx.service
By default, it will use the file /etc/systemd/system/nginx.service.d/override.conf. If --full is used then the full replacement file /etc/systemd/system/nginx.service will be used (the original content from /lib/systemd/system/nginx.service will be copied to this file).
You can also use --runtime to create the snippet in /run/systemd/system/ (at first drop-in snippet, then whole so --runtime and --full are not mutually-exclusive) that will be temporary of course.
You can obviously choose the editor to use, the order of precedence is:
$SYSTEMD_EDITOR > $EDITOR > $VISUAL > editor > nano > vim > vi