UNIX System Services
|OS family||UNIX - POSIX|
|Source model||Closed source|
|Initial release||March 30, 2001 (V1R1, announced October, 2000)|
|Latest release||Release 2.1 (V2R1) / September 30, 2013|
|Marketing target||Enterprise / Mainframes|
|Available in||English and most other languages|
|Official website||UNIX System Services|
|History of IBM mainframe operating systems|
UNIX System Services (USS) is a required, included component of z/OS. USS is a certified UNIX operating system implementation (XPG4 UNIX® 95) optimized for mainframe architecture. It is the first UNIX 95 to not be derived from the AT&T source code. Through integration with the rest of z/OS, additional Time Sharing Option (TSO) commands are available alongside the usual UNIX services, making it possible to process UNIX files using ISPF. Extensions in JCL make it possible to use these files in batch processing.
UNIX System Services allows UNIX applications from other platforms to run on IBM System z mainframes running z/OS. In many cases only a recompile is necessary, although additional effort may be advisable for z/OS integration (such as SMP/E installation support). While USS supports ASCII and Unicode, and there's no technical requirement to modify ASCII and Unicode UNIX applications, many z/OS users often prefer EBCDIC support in their applications including those running in USS. Consequently USS provides application and administrator services for converting to/from EBCDIC. Programs running under USS have full, secure access to the other internal functions of z/OS. Database access (DB2 via Call Attach) is one example of how USS can access services found elsewhere in z/OS. Naturally such programs cannot be ported to non-mainframe platforms without rewriting if they use these z/OS-specific services. Conversely, if a program adheres to UNIX® standards such as POSIX and ANSI C, it will be easier to move them between different UNIX® operating systems including z/OS UNIX®.
The file systems for USS (the older HFS and the now preferred zFS), which support UNIX-style long filenames, appear as special VSAM datasets to the rest of z/OS. Numerous core z/OS subsystems and applications rely on UNIX System Services, including Java. USS also provides a shell environment, OMVS.
Many critical z/OS functions and services depend on UNIX System Services and would never be able to function otherwise, including the z/OS Management Facility, XML parsing and generation services, OpenSSH, the IBM HTTP Server for z/OS, the z/OS SDK for Java, and some z/OS PKI services as examples.
IBM continues to enhance UNIX System Services. In 2007, IBM announced z/OS 1.9 which includes several new USS features, including partial alignment with the newer Single UNIX Specification Version 3 (UNIX 03). Typically every release of z/OS includes enhancements to USS.