When I write the code in Windows, this code can load the font file just fine:

ImageFont.truetype(filename='msyhbd.ttf', size=30);

I guess the font location is registered in Windows registry. But when I move the code to Ubuntu, and copy the font file over to /usr/share/fonts/, the code cannot locate the font:

 self.font = core.getfont(font, size, index, encoding)
 IOError: cannot open resource

How can I get PIL to find the ttf file without specifying the absolute path?

According to the PIL documentation, only Windows font directory is searched:

On Windows, if the given file name does not exist, the loader also looks in Windows fonts directory.

http://effbot.org/imagingbook/imagefont.htm

So you need to write your own code to search for the full path on Linux.

However, Pillow, the PIL fork, currently has a PR to search a Linux directory. It's not exactly clear yet which directories to search for all Linux variants, but you can see the code here and perhaps contribute to the PR:

https://github.com/python-pillow/Pillow/pull/682