Editing UNIX System Services (section)

== Overview ==
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 z/OS UNIX 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 z/OS UNIX. Consequently, z/OS UNIX provides application and administrator services for converting to/from EBCDIC. Programs running under z/OS UNIX have full, secure access to the other internal functions of z/OS. Database access ([[IBM Db2|Db2]] via Call Attach) is one example of how z/OS UNIX 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 it between different UNIX operating systems including z/OS UNIX.

The file systems for z/OS UNIX (the older [[Hierarchical File System (IBM MVS)|HFS]]<ref>{{cite web |url=http://www.ti-leipzig.de/os390/book/openmvs/hfs.pdf |title=Hierarchical File System |website=ti-leipzig.de|access-date=2016-05-12 |url-status=dead |archive-url=https://web.archive.org/web/20040906071912/http://www.ti-leipzig.de/os390/book/openmvs/hfs.pdf |archive-date=2004-09-06 }}</ref><ref>{{cite web | url=http://www.redbooks.ibm.com/redbooks/pdfs/sg245482.pdf | title=Hierarchical File System Usage Guide | accessdate=2023-04-23 | date=September 2000 | publisher=IBM }}</ref> with DFSMS/MVS Version 1 Release 2<ref>{{cite web
 |     title = IBM DFSMS/MVS Version 1 Release 2
 |        id = LTR ZP94-0202
 |      date = March 1, 1994
 |       url = https://www.ibm.com/common/ssi/ShowDoc.wss?docURL=/common/ssi/rep_ca/2/877/ENUSZP94-0202/index.html&request_locale=en
 |      type = Announcement Letter
 | website = IBM
 }}
</ref> and the now preferred [[ZFS (z/OS file system)|zFS]]), which support UNIX-style [[long filename]]s, appear as [[Data set (IBM mainframe)|datasets]]{{efn|HFS has a special dataset type; zFS uses a [[VSAM]] Linear Data Set as a container}} to the rest of z/OS. Numerous core z/OS subsystems and applications rely on UNIX System Services, 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 (programming language)|Java]], and some z/OS PKI services as examples.  z/OS UNIX also provides a [[Shell (computing)|shell]] environment, OMVS.

z/OS UNIX's predecessor was an operating system component called [[MVS#MVS.2FESA|OpenEdition MVS]], first implemented in [[MVS/ESA]] SP 4.3 and enhanced in [[MVS/ESA]] 5.1. OpenEdition MVS initially only supported the [[POSIX]] standards, but X/OPEN, the predecessor to The Open Group, certified MVS/ESA SP V5.1.

IBM continues to enhance UNIX System Services. Typically every release of z/OS includes enhancements to z/OS UNIX.