VM/CMS is IBM's other mainframe operating system. Historically speaking, it is Unix's uncle: the common ancestor is the CTSS system, developed at MIT around 1963 and running on the IBM 7094 mainframe. The group that developed CTSS then went on to write Multics, the immediate ancestor of Unix. IBM established a group in Cambridge to write a timesharing system for the IBM 360/40, a modified 360 with (for the first time on an IBM system) a paging MMU.^[39] The MIT and IBM programmers continued to interact for many years thereafter, and the new system got a user interface that was very CTSS-like, complete with a shell named EXEC and a large supply of utilities analogous to those used on Multics and later on Unix.

In another sense, VM/CMS and Unix are funhouse mirror images of one another. The unifying idea of the system, provided by the VM component, is virtual machines, each of which looks exactly like the underlying physical machine. They are preemptively multitasked, and run either the single-user operating system CMS or a complete multitasking operating system (typically MVS, Linux, or another instance of VM itself). Virtual machines correspond to Unix processes, daemons, and emulators, and communication between them is accomplished by connecting the virtual card punch of one machine to the virtual card reader of another. In addition, a layered tools environment called CMS Pipelines is provided within CMS, directly modeled on Unix's pipes but architecturally extended to support multiple inputs and outputs.

When communication between them has not been explicitly set up, virtual machines are completely isolated from each other. The operating system has the same high reliability, scalability, and security as MVS, and has far greater flexibility and is much easier to use. In addition, the kernel-like portions of CMS do not need to be trusted by the VM component, which is maintained completely separately.

Although CMS is record-oriented, the records are essentially equivalent to the lines that Unix textual tools use. Databases are much better integrated into CMS Pipelines than is typically the case on Unix, where most databases are quite separate from the operating system. In recent years, CMS has been augmented to fully support the Single Unix Specification.

The UI style of CMS is interactive and conversational, very unlike MVS but like VMS and Unix. A full-screen editor called XEDIT is heavily used.

VM/CMS predates the client/server distinction, and is nowadays used almost entirely as a server operating system with emulated IBM terminals. Before Windows came to dominate the desktop so completely, VM/CMS provided word-processing services and email both internally to IBM and between mainframe customer sites — indeed, many VM systems were installed exclusively to run those applications because of VM's ready scalability to tens of thousands of users.

A scripting language called Rexx supports programming in a style not unlike shell, awk, Perl or Python. Consequently, casual programming (especially by system administrators) is very important on VM/CMS. Free cycles permitting, admins often prefer to run production MVS in a virtual machine rather than directly on the bare iron, so that CMS is also available and its flexibility can be taken advantage of. (There are CMS tools that permit access to MVS file systems.)

There are even striking parallels between the history of VM/CMS within IBM and Unix within Digital Equipment Corporation (which made the hardware that Unix first ran on). It took IBM years to understand the strategic importance of its unofficial timesharing system, and during that time a community of VM/CMS programmers arose that was closely analogous in behavior to the early Unix community. They shared ideas, shared discoveries about the system, and above all shared source code for utilities. No matter how often IBM tried to declare VM/CMS dead, the community — which included IBM's own MVS system developers! — insisted on keeping it alive. VM/CMS even went through the same cycle of de facto open source to closed source back to open source, though not as thoroughly as Unix did.

What VM/CMS lacks, however, is any real analog to C. Both VM and CMS were written in assembler and have remained so implemented. The nearest equivalent to C was various cut-down versions of PL/I that IBM used for systems programming, but did not share with its customers. Therefore, the operating system remains trapped on its original architectural line, though it has grown and expanded as the 360 architecture became the 370 series, the XA series, and finally the current z/Series.

Since the year 2000, IBM has been promoting VM/CMS on mainframes to an unprecedented degree — as ways to host thousands of virtual Linux machines at once.