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POSIX ( // POZ-iks), an acronym for "Portable Operating System Interface", is a family of standards specified by the IEEE for maintaining compatibility between operating systems. POSIX defines the application programming interface (API), along with command line shells and utility interfaces, for software compatibility with variants of Unix and other operating systems.
The POSIX specifications for Unix-like operating system environments originally consisted of a single document for the core programming interface, but eventually grew to 19 separate documents (for example, POSIX.1, POSIX.2 etc) . The standardized user command line and scripting interface were based on the Korn shell. Many user-level programs, services, and utilities including awk, echo, ed were also standardized, along with required program-level services including basic I/O (file, terminal, and network) services. POSIX also defines a standard threading library API which is supported by most modern operating systems. Nowadays, most of POSIX parts are combined into a single standard, IEEE Std 1003.1-2008, also known as POSIX.1-2008.
As of 2009[update], POSIX documentation is divided in two parts:
Before 1997, POSIX comprised several standards:
After 1997, the Austin Group developed the POSIX revisions. The specifications are known under the name Single UNIX Specification, before they become a POSIX standard when formally approved by the ISO.
POSIX.1-2001 or IEEE Std 1003.1-2001 equates to the Single UNIX Specification version 3
This standard consisted of:
This standard consists of:
POSIX mandates 512-byte block sizes for the df and du utilities, reflecting the default size of blocks on disks. When Richard Stallman and the GNU team were implementing POSIX for the GNU operating system, they objected to this on the grounds that most people think in terms of 1024 byte (or 1 KiB) blocks. The environmental variable POSIXLY_CORRECT was introduced to force the standards-compliant behaviour. The variable POSIX_ME_HARDER was also discussed and was implemented in a few packages before being obsoleted by POSIXLY_CORRECT.
Depending upon the degree of compliance with the standards, one can classify operating systems as fully or partly POSIX compatible. Certified products can be found at the IEEE's website.
The following operating systems conform (i.e., are 100% compliant) to one or more of the various POSIX standards.
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The following, while not officially certified as POSIX compatible, conform in large part:
Mostly POSIX compliant environments for OS/2:
Partially POSIX compliant environments for DOS include:
The following are not officially certified as POSIX compatible, but they conform in large part to the standards by implementing POSIX support via some sort of compatibility feature, usually translation libraries, or a layer atop the kernel. Without these features, they are usually noncompliant.
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