Echo

Enhanced Definition

In the mainframe context, "echo" primarily refers to the immediate display of characters typed by a user on a terminal screen, confirming input. It also describes the process of displaying JCL statements or command input back to the job log or console as they are processed, often for verification or debugging purposes. In the context of IBM z/OS, "echo" primarily refers to the system's action of displaying or printing input data, commands, or parameters back to the user or an output stream. This behavior is crucial for verifying input, debugging, and providing clear traceability of executed instructions, especially within JCL procedures and TSO/ISPF environments.

Key Characteristics

- Terminal Echoing: For interactive sessions (e.g., TSO/ISPF), the terminal emulator or communication software typically echoes characters back to the screen as they are typed, providing visual feedback to the user.
- JCL ECHO Parameter: The ECHO parameter on the JOB statement or EXEC statement in JCL controls whether the JCL statements themselves are printed to the job's output listing (sysout) before execution. ECHO=YES (or ECHO=Y) prints them, ECHO=NO (or ECHO=N) suppresses them.
- Command Echoing: When issuing commands (e.g., MVS operator commands, TSO commands), the system or application may echo the command back to the console or terminal before processing it.
- Debugging and Tracing: Echoing can be a valuable debugging tool, allowing developers to verify the exact input received by a program or the sequence of JCL steps being executed.
- REXX SAY and PULL: In REXX, the SAY instruction displays output, which can be seen as echoing data, while PULL reads input, and the terminal typically echoes the user's typed input.

Use Cases

- Interactive TSO Sessions: When a user types commands or data into a TSO session, the characters are echoed to the screen, allowing the user to see what they are typing and correct errors before pressing Enter.
- JCL Job Verification: Setting ECHO=YES on a JOB statement ensures that all JCL statements for the job are printed in the job log, which is crucial for auditing, debugging, and understanding the job's configuration.
- MVS Operator Commands: An operator typing a command like D A,L (Display Active, Long) on the system console will see the command echoed back before the system processes and responds to it.
- REXX Script Development: During development of REXX execs, using SAY statements to echo variable values or progress messages helps in tracing execution flow and debugging logic.
- Input Data Verification: In some batch programs, input records might be echoed to a print file to verify the data being processed, especially for critical data transformations.

Related Concepts

Echoing is fundamental to TSO/ISPF for interactive user experience, providing immediate feedback on typed input. In JCL, the ECHO parameter on the JOB statement directly influences the content of the job log and SYSOUT datasets, making it a key aspect of job control and output management. It's also related to system consoles for operator command interaction and REXX for displaying program output or user input. The concept underpins the basic human-computer interaction model on the mainframe.

Best Practices:

Control JCL Echoing: For production jobs, consider ECHO=NO to reduce the size of job logs and save spool space, especially for jobs with many JCL statements or when the JCL is stable. For development or debugging, always use ECHO=YES.
Security for Sensitive Input: Be mindful of echoing when handling sensitive data (e.g., passwords). While terminals typically echo, applications should mask or suppress echoing for security-critical input fields.
Use for Debugging: Leverage JCL ECHO=YES and REXX `