Test if a directory exists in z/OS Unix System Services

  • Post category:Utilities
  • Reading time:1 mins read

Snippet of shell script code to test if a directory exists in z/OS Unix System Services.

if test -d "$DEST"                       
    # It is a directory #                
    echo "Directory found ..."           
    # does not exist                     
    echo "Directory does not exist"      

The Unix parts of z/OS

In the previous DBAOTM post I have introduced you to the z/OS operating system, the flagship operating system for the mainframe. In this post I will introduce you into the Unix side that z/OS has been equipped with over the past two decades.

Since the 1990s IBM has added Unix functionality to z/OS. The first extension was z/OS Unix System Services – z/OS Unix in short – and recently we IBM have added z/OS Container Extensions.

z/OS Unix

The Unix part of z/OS is a runtime environment that is an integral part of the z/OS operating system. z/OS Unix is fully Unix (Posix) compliant. z/OS Unix provides an interactive command interface that is called a shell in Unix terminology.

IBM has developed this part in the 1990s to make it easier to port applications from other platforms to z/OS. Many off-the-self and open source middleware and application packages that are available on z/OS make use of z/OS Unix. Examples are IBM’s own WebSphere Application Server and IBM Integration Bus, the banking product BASE24-eps from ACI, and open source tools like PHP and Git.

z/OS Unix has regular files the same as other Unix systems. In the z/OS Unix shell you can use normal Unix commands like ls, cd, more and many more of the standard Unix commands. You can set up a telnet or SSH session with z/OS Unix and do many more things you can also do with other Unix environments.

z/OS Container Extensions

A very recent development on z/OS (it came with z/OS 2.4, end 2019) is the possibility to run Linux Docker Containers on z/OS.

Docker containers are a hardware independent and lightweight manner to run many applications in a virtualized configuration. The Docker technology has been available on x86 platforms for a long time. With Docker containers you get a virtualization solution that does not need a complete virtual machine with an operating system running in it for every application. Instead your application is running in a small container that provides only a limited set of virtualization facilities. Just the things that the application needs. You can run many Docker containers – so applications – on a single real operating system image.

The interesting thing is that in a conceptual way Docker is a quite like z/OS as we have seen it in section Address Spaces are processes. On a z/OS operating system you can run many applications in “Address Spaces”. With Docker you run many container processes on a single real operating system image.

I will talk a bit more about Docker in section Linux in z/OS Container Extensions.

z/OS Address Spaces versus Docker containers

All Unix variants

A small elaboration, as you may already get confused with the Unix on the mainframe. I mentioned Linux for the mainframe, now I talk about z/OS Unix and Linux in Containers.

It is important to understand the difference between z/OS Unix, z/OS Container Extensions and Linux for Z.

z/OS Unix and z/OS Container Extensions are an integral part of z/OS. You get these options with z/OS.

z/OS Unix applications use the Unix flavour that z/OS provides, which is not Linux.

In z/OS Container Extensions you get the option to run applications with the Linux flavour of Unix, and run these in a containerized setup.

Linux for Z is an operating system that is in no way related to z/OS. Applications running on Linux for Z use the Linux flavour, and in an LPAR or Virtual Machine of its own.

I have tried to put all the Unix variants on z/OS in the picture below. You see z/OS Unix, as part of a z/OS operating systems, you see a z/OS container process running a Linux Docker container, and separate from z/OS there is an LPAR running Linux for Z.

The Unix flavours of z/OS and Linux for Z

What’s next

Now we have seen the basics of z/OS, we can turn to the more specialized and specific parts. In the next post I will discuss the unique clustering feature of z/OS, called parallel sysplex.