man - an interface to the on-line reference manuals
Show reference manual of ls command:
man ls
Show reference manual of ls command (case sensitive):
man -I ls
Show manual page from specific sections:
man 3 printf
Show all manual pages with the same common name:
man -f printf
Show all man pages for 'printf':
man -a printf
Show all man pages for 'printf', considering regular expression:
man -k printf
Show the location of manual pages:
man -w printf