What are apropos, whatis, and man used for?
All three read the same manual database, but at different zoom levels: man shows the full page, whatis shows the one-line summary, and apropos searches by keyword when you don't even know the command's name.
They form a natural workflow for "I need help" depending on how much you already know:
| Command | Use it when… | Example |
|---|---|---|
apropos |
you don't know the command name | apropos password |
whatis |
you know the name, want a one-liner | whatis clear |
man |
you need the full details | man ls |
So you might discover a command with apropos, confirm it's the right one with whatis, then learn its options with man — a funnel from broad to deep.
apropos password # search summaries for "password"
# passwd (1) - change user password
# passwd (5) - the password file
whatis clear # clear (1) - clear the terminal screen
man 5 passwd # full page for passwd in section 5 (the file format)
(man -k is literally the same as apropos.)
Gotcha: apropos/whatis read a pre-built index, not the live pages. On a fresh system the index may be empty, so they return "nothing appropriate." Fix it by building the index once as root: mandb. If apropos ever surprises you by finding nothing, that's the usual cause.