Get a list of running processes
ps aux
Filter through the processes replace "<keyword>"
ps aux| grep <keyword>
Search for 2 or more processes
ps aux| egrep '<keyword1>|<keyword2>|<keyword3>'
Show running processes using top
top -c
Or use htop
To check if a port is occupied, and by which process
lsof -i :80
or
ss -patun| grep -w 80
Read about SAR for understanding cpu avg over time etc. Install atop to monitor historic events
To check disk usage Overall disk usage
df -Th
Top 10 files
du -ach --max-depth=1 / | sort -rh | head -n10
Change password
passwd username
Reset MySQL root password https://linuxize.com/post/how-to-reset-a-mysql-root-password/
Reboot a server
reboot