Are there any open source, command line, subtitle converters - preferably for Linux? Subtitles perl swiss army knife scroll to the end of the page. Here you can find more options. I never tried this, but reading the man, it should work.
The open source program Subtitle Edit has a command line converter and is available for both Windows and Linux. What is it you want to convert exactly?
If it is between subtitle formats ass it depends on which formats you are talking about. Those which are bitmap based will require OCR to convert to text format and generally always require user input for confirming srt accuracy of the OCR.
If it is all text formats then Girl fucked by masseuse or Aegisub may be of use.