$ man apropos
  
  apropos searches a set of database files containing
  short descriptions of system commands for keywords and
  displays the result on the standard output.

It answers the question "I wonder how to do [this English word]."

For example: "I wonder how I copy a file."

  $ apropos copy

Dumps out all of the matching entries from the man pages for the word "copy". If you just wanted general commands, as opposed to library functions, and assuming your system organizes man page sections in the typical manner, you can filter it down with:

  $ apropos copy | grep "(1)"
  cp                   (1)  - copy files and directories 
  cpio                 (1)  - copy files to and from archives

Instead of using `grep "(1)"`, use the `-s` switch:

    $ apropos -s1 copy
    bsdcpio (1)          - copy files to and from archives
    cp (1)               - copy files and directories
    dd (1)               - convert and copy a file
    ....

I think it's better to use grep. Knowing the options to apropos has very low value. Knowing grep has lots and lots of value.

I agree. I have no idea how to use `find`. I always do `find . | grep whatever`, since I know the basics of grep.

Learning find is a worthy investment.

    find ./ -name "*.log" -size +2G -ctime +3 | xargs -P 8 gzip

Will find all log files over 2 gigs and older than 3 days. Then it will gzip them split across 8 parallel gzip processes.

This is why `man` came before `apropos`. :P

http://www.unix.com/man-page/Linux/1/apropos/

It searches for a keyword in the manpages and tells you what page contains the text.

    $ apropos printf
    fmtcheck(3)              - sanitizes user-supplied printf(3)-style format string
    format(ntcl)             - Format a string in the style of sprintf
    fwkpfv(1)                - FireWire kprintf viewer
    fwprintf(3), swprintf(3), vfwprintf(3), vswprintf(3), vwprintf(3), wprintf(3) - formatted wide character output conversion
    printf(1)                - formatted output
    printf(3), fprintf(3), sprintf(3), snprintf(3), asprintf(3), dprintf(3), vprintf(3), vfprintf(3), vsprintf(3), vsnprintf(3), vasprintf(3), vdprintf(3) - formatted output conversion
    wprintf_l(3), fwprintf_l(3), swprintf_l(3), vwprintf_l(3), vfwprintf_l(3), vswprintf_l(3) - formatted wide character output conversion

It lets you find other commands, basically. http://en.wikipedia.org/wiki/Apropos_(Unix)