I have C applications that have been running on HP-UX B.11 and Informix Dynamic Server Version 7.31.UD1 for years. I have migrated the database to the earliest Informix (V.14.2) on CentOS 7.
What are the best tools I need to use so I can continue to run the programs on Unix and access the Informix database on Linux?
It isn't clear whether you are seeking to run programs on the HP-UX box and connect to the database server on the Linux box, or whether you are seeking to migrate the programs from the HP-UX box to the Linux box.
As long as you can change the setting of the $INFORMIXSERVER environment variable and modify the $INFORMIXSQLHOSTS file (usually $INFORMIXDIR/etc/sqlhosts) to point to the new server on the Linux machine, the tools running on HP-UX box will continue to work — you don't need to install anything new there. If any of the tools have been hardwired with connection information, you may have to work a bit harder, but it should still be doable. You might have to set up a server alias for the new server that matches what was used on the HP-UX box
Any programs trying to run the server administration tools (oncheck, ontape, onparams, onspaces, onstat, etc.) will need to be run on the Linux server machine.
You will need to port the code for the existing applications from HP-UX to Linux. That means recompiling unless the programs are all Java. You need to know what APIs are in use — the process may be more contorted for ODBC than for ESQL/C, or vice versa (it depends on how well written the programs are).
If you use Java, then you'll need Informix JDBC and at least the JRE (you'll need the JDK if you need to compile the Java programs again). If you use ODBC or ESQL/C, you'll need the Informix ClientSDK, a C compiler and the general development tools (make, etc). If you use I4GL, you'll need Informix 4GL on the Linux platform too.