Basic linux command line(1)

--> date
:displays the current time and date
Example:

--> cal
:displays a calendar of the current month
Example:

--> df
:displays current amount of free space on your disk drives
Example:

--> free
:display the amount of free memory
Example:

--> exit
:end a terminal session
Example:

--> pwd
:display the current working directory
Example:

--> ls
:display list the files and directories in the current working directory
Example:

--> cd
:change directory
Example:

Reference
E-book :The Linux command line [electronic resource]  a complete introduction  by William E. Shotts