the main question is 'what is the nature of the unix client' ?
Is this an informix 4gl application, an esql/c application, ODBC connection?
In any case, you will need to set the following items to identify the server you are pointing to:
HOSTNAME=server Hosting The IFMX Instance
SERVICE=service name for the IFMX tcp connection, or directly port number.
If you can install for instance the free version of IBM Informix Innovator-C on the unix box, install the whole thing (including open admin tool)
so you'll be able to use
* dbaccess to directly query the database
* use the IFMX ODBC driver on unix which is included in the CSDK
* also use the database browser in OpenAdmin tool.
Innovator-C is now available for Windows and Linux, CSDK is available for a wide range of platforms. Both can be found at this place
Or you can only download the CSDK, which includes the ODBC driver for informix.
If you give more details on the nature of your unix client, we can focus more accurately on your solution :-)