Command

Enhanced Definition

A command in the z/OS environment is an instruction issued to the operating system, a subsystem (like CICS or DB2), or an application to perform a specific action, retrieve information, or alter system behavior. It serves as the primary means for operators, administrators, and programs to interact directly with and control the mainframe system.

Key Characteristics

- Syntax-driven: Commands typically follow a strict syntax, often consisting of a verb (the action) followed by operands and parameters that specify the target or details of the action.
- Execution Contexts: Can be issued from various interfaces, including the system console (via MVS or MODIFY commands), TSO/E READY prompt, within JCL (e.g., START commands), or programmatically (e.g., using SVC calls or WTO for operator commands).
- Privilege Levels: Command execution is subject to security authorization (e.g., through RACF), requiring specific permissions based on the command's sensitivity and the issuer's authority level (e.g., operator, system programmer, application user).
- Immediate or Deferred: Many commands execute immediately, while others might initiate a background process or schedule an action for later execution.
- Scope of Impact: Commands can have a broad impact, affecting the entire z/OS system, or a narrow scope, targeting a specific address space, task, or resource.
- Response and Feedback: The system typically provides a response to a command, indicating success, failure, or requested information, often displayed on the console or returned to the issuing program.

Use Cases

- System Monitoring and Control: Operators use MVS commands like D A,L (Display Active, Long) to monitor system health, VARY to activate/deactivate devices, or P (Purge) to stop tasks.
- Subsystem Management: System programmers issue commands to manage subsystems, such as CICS CEMT SET PROG(MYPROG) NEW to refresh a program, or DB2 -DISPLAY DDF to check Distributed Data Facility status.
- TSO/E User Interaction: Users interact with TSO/E by issuing commands like ALLOCATE to define datasets, ISPSTART to launch ISPF, or CALL to execute programs.
- JCL-driven Actions: JCL can include START or MODIFY commands to initiate started tasks or modify their parameters during job execution.
- Programmatic Automation: REXX execs or CLISTs can issue commands to automate operational tasks, such as dynamically allocating resources or querying system status.

Related Concepts

Commands are fundamental to interacting with the z/OS operating system and its subsystems (CICS, DB2, IMS, MQ). They are the primary interface for system operators and system programmers to manage resources, monitor performance, and troubleshoot issues. Commands are often issued from TSO/E sessions or system consoles, and their execution is governed by RACF or equivalent security