Search all the Linux man pages for a particular command or text

Q. Under KDE I get nice facility to search all man pages for particular command or phrase. I am login to remote server over ssh. How do I search all the man pages for a particular command at Linux shell prompt?

A. You need to use man command with -K option or use apropos command.

apropos command

The apropos command searches a set of database files containing short descriptions of system commands for keywords and displays the result on the standard output.
Syntax:
apropos oneword
apropos "string or phrase"

Search command / functions related to compare operation:
$ apropos compare

Search command to remove a file:
$ apropos "remove file"
Output:

rm                   (1)  - remove files or directorie

man -K

-K option use to search for the specified string in all man pages.
Syntax:
man -K oneword
man -K "string or phrase"

Search all man pages for fopen word:
$ man -K "fopen"
Output:

/usr/share/man/en/man3/fclose.3.gz? [ynq]   

Type y to open/display man page, n to continue search, q to Quit search.

This is probably very slow! It helps to specify a section (1-7).
$ man 3 -K "open"

Above commands also works with UNIX and *BSD oses.