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Distribution: RH 6.2, Gen2, Knoppix,arch, bodhi, studio, suse, mint
that rpm -qa|grep mplayer
is a pipe. rpm -qa lists all the packages installed.
here all that text is piped into grep which searches
it for the word mplayer, then when mplayer is found,
grep prints the line that it was found on.
in general a program that can send a stream of
text or bytes is sent to the input of another program
that does something to that data.
seeing as everyone else said exactly the same thing, i might as well
rpm -qa | grep mplayer
it's also wrong... but nevermind. actually answering the question though, a program can only be used with a pipe if that is how it is written. while it could be designed for rpm to accept a file name through a pipe it is not particuarly useful, and so presumably no one bothered implementing it. personally in your example i would have said
rpm -qf `which mplayer`
but obviously that was only used as an example. you've got the right idea, it just isn't possible in that example.