[Adoby]

Linux/Ubuntu.

You set environment variables in either ~/.profile or /etc/profile or possibly in a script used to launch calibre.

If you look you can see how other variables are set there.

However it will NOT help in keeping stuff from the cache or significantly reduce use of the root filesystem. So don't do it!

All settings and data specific to one library is stored together with the library, except the name and location of the library.

Calibre will use /tmp and the cache in your home folder, but that will have to be fixed some other way. For instance by placing /home on another drive and adding an SSD for /tmp.

There is another environment variable for setting what path calibre should use for temporary files. CALIBRE_TEMP_DIR. It might be useful to change if you are in the habit of running a lot of batch conversions.

It is general good practice to separate the root file system and /home. Use different partitions. It is also good practice to have /tmp on your fastest drive, or even in RAM using tmpfs if you have plenty of RAM installed. That also may speed up some operations in calibre.