Resolution:
On UNIX, quoted strings with embedded blanks cannot be passed as command arguments to UNIX shell scripts by means of the Custom::execute, Custom::executeForNumber and Custom::executeForString methods. The Java Virtual Machine (JVM) parses these strings as blank delimited tokens and passes them as separate arguments.
To work around this, utilize a shell script as the executable. Pass all arguments that do not contain spaces first. Then use the "shift" shell command and a wildcard argument variable, "$*", to pass the space containing argument at the end.