Of these environment variables only FIFO_PATH is required by the SIMPL toolkit during runtime. FIFO_PATH must point to a valid directory with create permissions for the userID running the SIMPL application. Many SIMPL users call this subdirectory fifo but there is no requirement that it be so named.
SIMPL_HOME is strictly only required during the building of the SIMPL library. However, many SIMPL users elect to define SIMPL_HOME in their startup profile because it conveniently allows them to ammend their PATH environment variable to add:
Several useful SIMPL commands are located in $SIMPL_HOME/bin and the SIMPL testing framework commands are located in $SIMPL_HOME/scripts. Having these in the PATH environment variable saves lots of typing and makes SIMPL startup scripts more compact.
Many SIMPL developers who are using either the Tcl/Tk-SIMPL or Python-SIMPL extension utilize the SIMPL_HOME environment variable from within their code to point the interpreter to the location of the relevant shared library. In these instances SIMPL_HOME like FIFO_PATH is required to be set at runtime.
The TEST_HOME environment variable is project specific. Many SIMPL users are working on multiple projects simultaneously so this is typically set manually on each working console.
As noted, many SIMPL users elect to set FIFO_PATH and SIMPL_HOME in their startup profile for the Linux system they are using. This is not the only way, however. Sometimes it makes sense to export these variables directly in the startup script for the application. In this case these variables will only have the scope of that startup script.