set

Create or change a variable in the shell/environment with a value. Tutorial includes unset command.

unset

Delete an existing shell/environment variable. Tutorial includes set command.

fdisk

Used to create, edit, delete, or move partitions on a hard drive.

df

Shows amount of disk space being used by the machine's file systems.

du

Shows disk usage by specific files and directories and files.

rsync

Remote backup/data synchronization tool

nslookup

Troubleshooter for DNS related information.

crontab

Schedule tasks or operations to run at given times.

dmesg

Troubleshooting tool for driver and hardware issues.