A brief comparison of z/OS and UNIX

What would we find if we compared z/OS® and UNIX®? In many cases, we'd find that quite a few concepts would be mutually understandable to users of either operating system, despite the differences in terminology.

For experienced UNIX users, Mapping UNIX to z/OS terms and concepts provides a small sampling of familiar computing terms and concepts. As a new user of z/OS, many of the z/OS terms will sound unfamiliar to you. As you work through this information center, however, the z/OS meanings will be explained and you will find that many elements of UNIX have analogs in z/OS.

A major difference for UNIX users moving to z/OS is the idea that the user is just one of many other users. In moving from a UNIX system to the z/OS environment, users typically ask questions such as "Can I have the root password because I need to do..." or "Would you change this or that and restart the system?" It is important for new z/OS users to understand that potentially thousands of other users are active on the same system, and so the scope of user actions and system restarts in z/OS and z/OS UNIX are carefully controlled to avoid negatively affecting other users and applications.

Under z/OS, there does not exist a single root password or root user. User IDs are external to z/OS UNIX System Services. User IDs are maintained in a security database that is shared with both UNIX and non-UNIX functions in the z/OS system, and possibly even shared with other z/OS systems. Typically, some user IDs have root authority, but these remain individual user IDs with individual passwords. Also, some user IDs do not normally have root authority, but can switch to "root" when circumstances require it.

Both z/OS and UNIX provide APIs to allow in-memory data to be shared between processes. In z/OS, a user can access another user's address spaces directly through cross-memory services. Similarly, UNIX has the concept of Shared Memory functions, and these can be used on UNIX without special authority.

z/OS cross-memory services, however, require the issuing program to have special authority, controlled by the authorized program facility (APF). This method allows efficient and secure access to data owned by others, data owned by the user but stored in another address space for convenience, and for rapid and secure communication with services like transaction managers and database managers.

Table 1. Mapping UNIX to z/OS terms and concepts
Term or concept	UNIX	z/OS
Start the operating system	Boot the system	IPL (initial program load) the system.
Virtual storage given to each user of the system	Users get whatever virtual storage they need to reference, within the limits of the hardware and operating system.	Users each get an address space, a range of addresses extending to 2 GB (or even 16 EB) of virtual storage, though some of this storage contains system code that is common for all users.
Data storage	Files	Data sets (sometimes called files)
Data format	Byte orientation; organization of the data is provided by the application	Record orientation; often an 80-byte record, reflecting the traditional punched card image
System configuration data	The /etc file system controls characteristics.	Parameters in PARMLIB control how the system IPLs and how address spaces behave.
Scripting languages	Shell scripts, Perl, awk, and other languages	CLISTS (command lists) and REXX™ execs
Smallest element that performs work	A thread. The kernel supports multiple threads.	A task or a service request block (SRB). The z/OS base control program (BCP) supports multiple tasks and SRBs.
A long-running unit of work	A daemon	A started task or a long-running job; often this is a subsystem of z/OS.
Order in which the system searches for programs to run	Programs are loaded from the file system according to the user's PATH environmental variable (a list of directories to be searched).	The system searches the following libraries for the program to be loaded: TASKLIB, STEPLIB, JOBLIB, LPALST, and the linklist.
Using the system interactively	Users log in to systems and execute shell sessions in the shell environment. They can issue the rlogin or telnet commands to connect to the system. Each user can have many login sessions open at once.	Users log on to the system through TSO/E and its panel-driven interface, ISPF. A user ID is limited to having only one TSO/E logon session active at a time. Users can also log in to a z/OS UNIX shell environment using telnet, rlogin, or ssh.
Editing data or code	Many editors exist, such as vi, ed, sed, and emacs.	ISPF editor
Source and destination for input and output data	stdin and stdout	SYSIN and SYSOUT SYSUT1 and SYSUT2 are used for utilities. SYSTSIN and SYSTSPRT are used for TSO/E users.
Managing programs	The ps shell command allows users to view processes and threads, and kill jobs with the kill command.	SDSF allows users to view and terminate their jobs.