Byte InsightChecking on available filespace
You can check on available file space using df -h or du To get a list in ascending order use: du -ah | sort -rn you can also | head to get top few files only.
File Permissions
What the numbers mean: 4 = Read 2 = Write 1 = Execute e.g. 7 = Read, Write and Execute. Works on Users, Groups and Everyone (left to right)
User Management
Managing users on Unix: Add user: adduser [username] userdel [username] passwd [username]
Setting File and Folder Permissions
I find these two commands really useful when I want to ensure that I have folder and file permissions correct for a website.