How to communicate with Keithley devices on linux (without NI VISA)?
There is a project called Linux GPIB that allows you to control any GPIB instrument. 

Unfortunately, not all GPIB interfaces (the board or USB adapter that connects your PC to the GPIB bus) are supported.  The supported hardware list is here.

The interface is similar to the NI-GPIB library, which is a slightly lower-level (and less general-purpose) interface than VISA.