As Linux becomes more and more prevalent, many programmers are targeting just one system: Linux. For these programmers, The Linux Programming Interface is ideal.
However, all the world is not Linux, and many of the most interesting UNIX applications need to be truly portable across systems: Linux, the BSDs, Solaris, HP-UX, AIX, and so on. For these applications, following portable programming practices and adhering to formal standards such as POSIX and the Single UNIX Specification are important.
Although I've focused on Linux in the book, I've also given close attention to formal and implementation standards, and clearly distinguished details that are Linux-specific from the features that are common across most or all UNIX implementations. This means that TLPI will be useful to programmers working on any UNIX system (but it won't provide you with too many of the nonstandard and system-specific details of those other systems). That's why we subtitled the book A Linux and UNIX System Programming Handbook.
A few statistics illustrate TLPI's emphasis on standards and portability. Of the just over half a million words in the book, here are a few word counts:
System V (excluding discussion of System V IPC), 108