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UNIX fragmentation and the lack of a single conforming standard affected the development of portable applications.
To address this issue, AT&T created System V Interface Definition (SVID).
Later, X/Open (now The Open Group), created the X/Open Portability Guide (XPG).
e.g., UNIX95, UNIX98, UNIX03 (depending on the version of the specification)
POSIX, the Portable Operating System Interface for Computer Environments, was developed at the behest of the Institution of Electrical and Electronics Engineers (IEEE).
POSIX is a set of standard interfaces based on the UNIX operating system. POSIX compliance ensures that a set of programs developed on one machine can be moved to another without recoding.
POSIX.1
Specifies the C application program interface.
POSIX.2
Deals with the shell and utilities.
POSIX refers to operating systems in general, but was based on UNIX.
SUSV3 is the unification of two standards; XPG and POSIX.
a.k.a. IEEE 1003.1:2001
We will make references to POSIX, but they should be interpreted to mean the SUSV3 as well.
Once software has been developed on any POSIX-compliant UNIX system, it can be easily ported to another POSIX-compliant UNIX machine with minimum modifications. ("wrtie once, adopt everywhere" approach)
Link to SUSV3: http://www.unix.org/unix03.html