du

List total size of current directory (in KB)
df -s

List total size of current directory (in MB+)
df -sh

List size of all directories in current directory
du -sh *

Who is using the port?

Find which process is using tcp port

Linux
netstat -anp | grep LISTEN

AIX
lsof -i:portnumber
The above command will list the processes and pids.
grep using the pid to know the config
ex: ps -ef | grep httpd | grep pid

Windows
netstat -an |find /i "listening"
netstat -an |find /i "9443"

netstat -no
netstat -ao
netstat -a

SSH

Problem
Address 1xx.xx.xx.xx maps to hostname, but this does not map back to the address - POSSIBLE BREAK-IN ATTEMPT!

Solution
check /etc/hosts file and make sure ip address is in the first

tar

tar commands:
Create a tar file
tar -cvf out.tar dirname/*

Extract a tar file
tar -xvf dirname/*

Extract a tar file inside .tgz
gunzip file.tgz
tar -xvf file.tar
  or
gzip -dc file.tgz | tar xf -

Create a tar excluding a sub-directory
tar -cvf output.tar --exclude sub-dir-name *

Problem
tar: directory checksum error

Solution
Native tar is located at /usr/bin/tar.
Try using GNU tar located at /usr/local/bin/tar