I think that you need to cd to the folder in the control section first and then the d_source file can be relative not absolute. It will have to be a lower case file name though.
What filesystem is the file on?
Linux programs have to go out of their way to be case insentive and honesty I havnt met one that is.
The case sentivity comes.from the filesystem and that is why NTFS/FAT mounts end up being case insentive.
Likewise ext4 does have options these days to be case insentive
–edit–
Yes it’s ngspice
All identifiers are case insensitive - the identifier ‘npn’ is equivalent to ‘NPN’ and to ‘nPn
I have pushed a fix to ngspice git development branch pre-master-43. It will be available in ngspice-43.
Indeed Spice traditionally has been case insensitive. With file paths and names this does not make sense though, as file handling in Linux (and macOS) is case sensitive…