bash redirection

[Kurt Granroth]

Craig White wrote:
> I am so weak on shell scripting - and I do try and I wear out the info
> pages (way too many on bash)...
> 
> how do I redirect output to input...
> 
> I am trying to pipe output of mplayer into ffmpeg and I can't get my
> head around it since ffmpeg needs an input file...
> 
> ffmpeg -i some_file_somewhere outputfile_somewhere 
> (or off the X11 display which doesn't seem useful at all.)
> 
> ideally, I would like to do something like this...
> 
> mplayer dvd://$TITLE -chapter "$X-$X" -dumpstream -dumpfile
> "$X-$NAME.vob" | \
> ffmpeg -i "$X-$NAME.vob" -s 360x240 -ab 128 "$X-$NAME.mp4"
> 
> but even better, I would love to not write the intermediate.vob file at
> all.
> 
> I just want to rip my music DVD's to my iPod (by the way...the 360x240
> at 128 bps audio from high quality DVD gives really great quality iPod
> video/audio for a very small footprint).

Okay, I'm going to first talk in general and then for your specific case.

In general, you only use pipe | when you are outputting to stdout and
passing it as stdin to the next program.  So using a pipe makes no sense
when you are using an intermediate file as input and output.

Most utilities that require an input file accept '-' as stdin and quite
a few accept it as the notation for stdout as well.  So in theory you
could do something like:

mplayer dvd:// -o - | ffmpeg -i - myfile.mp4

In practice, that won't work.  That's because while ffmpeg *does* honor
the - as stdin, I don't know if mplayer can actually dump to stdout at all.

That's all for the general case.  In your specific case, I wouldn't
recommend using mplayer and ffmpeg at all.  Instead, use mencoder.
mencoder ships with mplayer, usually, and it is designed to transcode
video streams on the fly.  Here is nearly *exactly* what you want to do:

http://gentoo-wiki.com/HOWTO_Rip_DVD_mencoder

And if you want a script that does it all for you:

http://diveintomark.org/archives/2007/06/07/howto-batch-encode-video-for-ipod-under-linux-2007-edition

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