Information about commands

echo $PATH	Display your PATH
export PATH=$PATH:/usr/sbin	Add /usr/sbin/to your PATH

which commandname
Determine if and where commandname is in your PATH

whereis commandname
Determine if commandname is available in the system PATH

find /usr -name filename
Search and display all files named filename in the directory /usr

man commandname	Read the manual page about the command commandname
apropos keyword	Perform a simple search of manual page titles for keyword