ps --help - Displays a list of options that can be used with ps.
top manual page - Type man top to learn more about top and its many options.
free manual page - type man free to learn more about free and its many options.
df manual page - Type man df to learn more about the df command and its many options.
du manual page - Type man du to learn more about the du command and its many options.
lspci manual page - Type man lspci to learn more about the lspci command and its many options.
/proc/ directory - The contents of the /proc/ directory can also be used to gather more detailed system information.