If you want only the ISBN numbers:
Code:
calibredb list --fields isbn --separator , > library.csv
You actually get the Calibre database entry number, too, along with the ISBN number, each in their own column. It'll be in a file called
library.csv in whatever folder you're in when you run it. (Where you are is listed on the command-line, in front of where you type the commands. Usually.)
m a r