@Calvin Yan Also consider that in ngspice, inside a

.control / .endc

block you can use environment variables. Example:

.control

...

...

write $FILENAME

.endc

if environment variable

FILENAME

is defined before starting ngspice:

export FILENAME=circuit.raw

the above write command will write into

circuit.raw

(@Tim Edwards did you know this?). Unfortunately environment variables expansion does NOT work outside a .control block, so for example we can not use

$PDK_ROOT

in a

.lib

statement (sigh).

Unfortunately complaining does not help.

@holger vogt Adding env variables recognition anywhere in a netlist is probably not an easy task to do without nasty side effects. In many netlist specifications $ is a inline comment character (so anything following $ in a line will be ignored. So I don't know if this is an easy task to do. It would be nice to allow in a netlist the following:

.lib $PDK_ROOT/$PDK/libs.tech/combined tt

This will make the netlist portable (no absolute pathnames) But I know the $ has another meaning (as an in-line comment character), so I recognize it is not trivial.

No, I did not know that environment variables will be substituted inside a control block. Interesting. Not sure I would make use of it; I try to avoid environment variables except where they actually describe the environment, which is how they are intended to be used.

$PDK_ROOT

is a true environment variable, because it describes where the PDK is in the user's environment and makes files portable.

$FILENAME

, by contrast, is specific to the simulation. It's basically being used as an end-run around the fact that ngspice's interpreter doesn't accept arguments passed from the command line. But I noticed from the ngspice documentation that variables can be created and passed directly to the program from the command line with the

-D

argument. It's a bit oblique (and hard to find in the documentation), but seems reasonable.

There is a commit on ngspice git branch pre-master-43 which says: Search path for .lib and .include has been enhanced to read complete or partial paths from an environmental variable. The env variable name has to start with '$' as the first character after the .inc or .lib token. It is read until it hits a directory separator '/' or '\\', or if none is found, until the end of the line. In this case the filename has to be included in the env variable. If the env variable is read successfully, a ngspice variable with the same name (without leading '$') and contents is defined for use in a .control section.

@Holger Vogt Thank you! this makes netlists portable across systems (given proper environment setup). I have verified pre-master-43 and it works, including the ngspice variables creation.