No current Linux system meets POSIX requirements for OS or utilities behavior. On a POSIX conforming system, you never have to give a --posix option to make a utility behave as specified by the POSIX standard. On a POSIX conforming system, the command:

Code:

echo -n abc

will write the characters -n abc and a trailing <newline> character to standard output.

On a POSIX conforming system, each thread in a process shares a single process ID; on a Linux system each thread gets its own process ID.

There are hundreds of places where Linux systems do not conform to POSIX standard requirements. Fortunately, for a lot of the stuff you run into in daily run-of-the-mill programming, they are quite similar. But, if you try write portable code that will work on any POSIX system (and all UNIX branded systems are POSIX conforming systems and have to also meet additional requirements) there is no guarantee that it will run on a Linux system. (Of course no test suite is perfect, so a UNIX or POSIX branded system may have bugs that haven't been caught yet; but vendors of these branded systems once notified that a conformance bug is present have a contractual obligation to fix it within 6 months or lose their right to use the brand.)

As the POSIX standards evolve, they are picking up some new features from Linux systems. And, over time, many GNU Utilities are coming closer to meeting POSIX requirements. But, for the foreseeable future, Linux systems are most definitely not POSIX conforming systems and cannot be branded as UNIX systems or POSIX systems even if one of the Linux distro vendors was willing to pay the certification costs and fill out all of the paperwork involved.

There is also a standard for Linux systems (the Linux Standard Base AKA LSB), but the last I heard, no Linux system has ever conformed to any version of the LSB either.