"apt-mark showmanual" will show you all packages that you have manually installed
"apt-mark showauto" will show you all packages that are automatically installed
"apt-mark showinstall" will show you all the packages that are installed
How is this useful? It tells you what packages are installed in your system.
So you can go "apt-mark showinstall | grep somelibrary" to see if you have a particular library installed that's a dependency.
1U BareMetal pfsense opnsense Router Firewall DNS Server 6x 10GB Ethernet Ports
$149.00
Australia Linux Server- 32 GB RAM + 500 HDD + 6 Core processor
$66.88
Open Eye OE-MTX08 8TB Micro Server With Linux (Boots To Login) No Accessories
$282.92
IBM 9006-22C 12 Bay LFF SAS3 12G Dual 22 Core 2.6GHz Power9 P9 Linux Server
$2799.99
PFSENSE 15" Depth Server Supermicro X11SSH-F E3-1240 V5 V6 - 128GB NVME 32GB RAM
$359.00
PFSENSE 15" Depth Server Router Firewall Supermicro X11SSH-F E3-1240 V5 32GB RAM
$382.00
Linux Server Administration CBT Training Videos
$5.25
The Complete Redhat Linux 5.2 Operating System Deluxe Secure Server New Sealed
$25.00
POGO Linux WEBWARE 1150 Rack-Mount Server Pentium 4 2.8GHz 512MB - No Drives
$179.99
1U Open Source Router Firewall X10SLH-N6-ST031 E3-1270 V3 6x 10GB Ethernet 16GB
$419.00