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Opening a whole manual page just to learn what a command does is slow. whatis prints the one-line description instead, and apropos searches those descriptions.
This lab is the hands-on companion to the whatis lesson from Linux Journey (CC BY-SA 4.0).
whatis cat
whatis passwd
passwd (1) - change user password
passwd (5) - the password file
The number in brackets is the manual section.
When you don't know the command's name, search what the commands do:
apropos password
Find the command that changes the group password and save its name in answer.txt:
echo <command> > answer.txt
whatis name prints a one-line description and its section.apropos word (or man -k word) searches descriptions when you don't know the name.