How to generate self-signed SSL certificates

 Generate a root private key (rootCA.key):

openssl genrsa -out rootCA.key 2048

Generate a self-singed root certificate (rootCA.pem):

openssl req -x509 -new -nodes -key rootCA.key -days 1024 -out rootCA.pem

Create a private key for the final certificate (dovecot.key):

openssl genrsa -out dovecot.key 2048

Create a certificate sign request (dovecot.csr):

openssl req -new -key dovecot.key -out dovecot.csr

Create a certificate based on the root CA certificate and the root private key (dovecot.crt):

openssl x509 -req -in dovecot.csr -CA rootCA.pem -CAkey rootCA.key -CAcreateserial -out dovecot.crt -days 500

Copy the private key and the certificate to the /etc/dovecot/private/ directory

cp dovecot.crt /etc/dovecot/private/dovecot.crt

# cp dovecot.key /etc/dovecot/private/dovecot.key

Set the required permissions on the file:

chmod 400 /etc/dovecot/private/dovecot.crt

# chmod 400 /etc/dovecot/private/dovecot.key

Update paths to the key and the certificate in the Dovecot configuration file /etc/dovecot/dovecot.conf:

ssl_cert = < /etc/dovecot/private/dovecot.crt

ssl_key =  </etc/dovecot/private/dovecot.key

Restart the Dovecot service to apply the changes:

service dovecot restart

Systemd - systemctl - Services Management

systemctl list-units
This will show you a list of all of the units that systemd currently has active on the system.

systemctl list-units --all
This will show any unit that systemd loaded or attempted to load, regardless of its current state on the system. Some units become inactive after running, and some units that systemd attempted to load may have not been found on disk.

systemctl list-units --all --state=inactive
systemctl list-units --type=service

systemctl list-unit-files
Units are representations of resources that systemd knows about. Since systemd has not necessarily read all of the unit definitions in this view, it only presents information about the files themselves. The output has two columns: the unit file and the state.

systemctl cat atd.service
systemctl list-dependencies sshd.service
systemctl show sshd.service

sudo systemctl edit nginx.service
This will be a blank file that can be used to override or add directives to the unit definition. A directory will be created within the /etc/systemd/system directory which contains the name of the unit with .d appended. For instance, for the nginx.service, a directory called nginx.service.d will be created.

https://www.digitalocean.com/community/tutorials/how-to-use-systemctl-to-manage-systemd-services-and-units