Rebuild man's Database of Commands

man -u
Occasionally you'll try to use man to find out information about a command and man reports that there is no page for that command. Before giving up, try again with the -u option (or --update), which forces man to rebuild the database of commands and man pages it uses. It's often a good first step if you think things aren't quite as they should be.

$ man ls
No manual entry for ls
$ man -u ls
LS(1) User Commands LS(1)
NAME
ls - list directory contents
SYNOPSIS
ls [OPTION]... [FILE]...
[Listing condensed due to length]