As part of setting up an interactive environment, the OMVS command must start another process, the shell, to interpret and run shell commands. When you specify the NOSHAREAS parameter, the shell is started using fork() and exec. However, when you specify the SHAREAS parameter, the OMVS command starts the shell using attach_exec. This function starts the shell process running in the address space of the OMVS command. Other applications can also use attach_exec to start a process without the overhead of an additional address space.
Because SMF data is collected on an address space basis, process identification (such as process ID and session ID) is recorded only for the parent process in the address space. However, z/OS® UNIX resource consumption is accumulated for the parent process and all local child processes.