Not limited to Ubuntu Server on Raspberry Pi, but I use these commands on my Pi 4.
Current CPU Frequency : sudo cat /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_cur_freq
Maximum CPU Frequency : sudo cat /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_max_freq
Get CPU Temperature : cat /sys/class/thermal/thermal_zone*/temp
Active ssh sessions : ss -p | grep -i ssh
SSH tunnel to a port : sudo -u <username> ssh -fN -R 80:localhost:80 <remoteserver>
Network Consumption : sudo vnstat -i -m eth0
Kill close_wait proc. : sudo ss --tcp state CLOSE-WAIT --kill
Check Port connections: sudo netstat -pan | egrep "tcp.*:8080.*"
List systemctl timers : systemctl list-timers
Memory Usage(RAM) : sudo free -h
Vegeta Load : echo "GET https://singhangad.in/" | vegeta attack -duration=0s | tee result.bin | vegeta report
Kill TCP process : sudo fuser -k 443/tcp
List port and process : sudo netstat -plotn
Show process as tree : pstree -p
Create symbolic link. : sudo ln -sf ../sites-available/print.singhangad.in ../sites-enabled/
File encryption with OpenSSL (AES-256-CBC):
Encrypt file : echo "password" | openssl enc -aes-256-cbc -in archive.tgz -out archive.tgz.enc -pbkdf2 -pass stdin
Decrypt file : openssl enc -aes-256-cbc -d -in archive.tgz.enc -out archive.tgz -pbkdf2 -pass pass:password