Show cert names
certbot certificates

Delete standalone certificate with:
certbot delete --cert-name CERT_NAME

In order to clean up journalctl logs use

Set time retention
journalctl --vacuum-time=2d

Set size retention
journalctl --vacuum-size=500M

Set rate limmiting

in nginx.conf http block set:
limit_req_zone $binary_remote_addr zone=mylimit:10m rate=10r/s;

Show docker swarm nodes labels
docker node ls -q | xargs docker node inspect -f '{{ .ID }} [{{ .Description.Hostname }}]: {{ .Spec.Labels }}'

Check node id
docker node ls

As with disk iotop, use pg_top for postgresql

Debian install package pgtop

pg_top

Check whether is restart required for postgresql db after config change
SELECT * FROM pg_settings WHERE pending_restart = true;

In order to pest whether is network port accessible, either udp or tcp.

Can server listen on udp port 5000 ?

Example with UDP port!

Server side
ncat -ulp 5000

Client side
ncat -u XXX.XXX.XXX.XXX 5000

Either check for file presence in /var/spool/postfix
tree /var/spool/postfix

or by runnig
postqueue -p

In order to check .deb package dependencies
apt-cache rdepends packagename