I just began to play around mencoder, which I find a great tool.

It appeared to me that I might back up some movies with it.
As with everything, I want to do it automatically, with the less user interaction, and in the the best quality that fits on a CD.

I wonder if mencoder can do this "out of the box", or if I could write a wrapper script that would e.g. continuously monitor the encoding process (file size and percentage), and kill and restart mencoder with new settings if it sees that the resulting file would be too big.

The encoding process is supposed to run for hours, while I think that the size of the resulting file could be reliably judged after the first 5 minutes, so the time of killing and restarting mencoder could be ignored compared to the time of the whole project, while it could save me much handwork.

Do you have an idea how to monitor the mencoder output with a bash script?

mencoder does have some options that you could use to do what you ask, you might want to write a little script to automate it.

-endpos <[[hh:]mm:]ss[.ms]|size[b|kb|mb]> (see -ss and -sb option too)
Stop encoding at given time or byte position. Can be specified in many ways:

-ss <time> (see -sb option too)
Seek to given time position.

-sb <byte position> (see -ss option too)
Seek to byte position. Useful for playback from CDROM images / .VOB files with junk at the beginning.

See man mencoder for a more complete description and some examples.

Hope this helps.

that's a pretty ugly way to approach it imho... you could be there for ages. you have options:

1) http://acidrip.sf.net (next version i release will support file input)
2) use 3pass encoding to provide you with bitrates from mencoder specifially for cd sized files (see docs)
3) work it out yourself....

assuming you're gonna use a fixed bitrate audio stream, e.g. 128kbit joint stereo, video bitrate for a given film is...

Vbitrate = ( size in kbits / Length in seconds ) - audio bitrate in kbps

e.g a 90 minute film on a 700mb cd with 128 audio:

Vbitrate = ( ( 700 * 1024 * 8 ) / ( 90 * 60 ) ) - 128

Vbitrate = ( 5734400 / 5400 ) - 128

Vbitrate = 1062 - 128 = 934kbps

how easy is that? Personally i use average bitrate, not constant bitrate, which means that you'll save a couple of megs on the audio, meaning that it'll probably end up being 697Mb rather than 700, which gives a generally nicer margin for error