Problem about disk size and gnome

I am an average Linux user used to Debian Linux and I am not system admin or anything like that, just home user and from yesterday I am new to FreeBSD. Yesterday I installed FREEBSD from USB and my disk is 400 GB but it shows 383 GB. Only I don't know why and only this is my operating system on harddisk. Maybe some drive is not erased properly during FreeBSD UFS installation process. Another problem is Gnome I installed but it has no shutdown button like I had in Linux Debian before. Is there a way of checking all disk usage like I had on Debian Linux called "disk". Thank you in advance for your valuable advises.
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You can use the following commands to check the free space on your disk: df -h filesystem. Filesystem being "/" or "home", or whichever filesystem you want to check. The "h" puts the output in "human readable" format, based on powers of 1024.