Portable programs are 'team-players'

Systems-focused programs also need to 'fit in' with the new computing environment, to both interoperate with existing utilities, and also contribute something new.

This requires programs to make use of existing operating system supported runtime features and interfaces in a consistent manner. This makes it easier for users to quickly understand and benefit from the newly ported program.

An excellent introduction to this topic is The Art of Unix Programming, by Eric Steven Raymond, 2003:

Chapters 1 and 5 are the most relevant to the material discussed in this lecture.
In addition, some other good Chapters/Sections that are not too long or dry (in order of relevance), are:

Chpt 10 - Configuration: What Should be Configurable?; Environment Variables; Command-Line Options
Chpt 11 - Unix Interface Design Patterns: The Filter Pattern -> The ed Pattern
Chap 19 - Open Source
Chpt 16 - Reuse