@Stefan Schippers Is it possible to iterate libraries in a single circuit file? I mean, if I would like to do some calculations using the models for the tt corner and then, repeat the same calculations but now using another corner (let's say ff), is it possible to do that in a single circuit file or I have to create different files for each corner (library)?

I have never been able to do that directly in ngspice. However you can run one simulation in the tt corner, then change the corner, run simulation again and using 'set appendwrite' before the write command. This way all data is appended into one file. See this thread. https://open-source-silicon.slack.com/archives/C017P3RAD42/p1633258090074700