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.