CLI

Command Line Interface / Call Level Interface

Enhanced Definition

CLI is an acronym with two distinct yet important meanings within the mainframe and z/OS ecosystem. As **Command Line Interface**, it refers to a text-based user interface for interacting with the operating system or applications by typing commands. As **Call Level Interface**, it denotes a standardized application programming interface (API) for database access, allowing applications to connect to and manipulate data in a database management system.

Key Characteristics

- Command Line Interface (CLI):
- Text-based Interaction: Users type commands and parameters at a prompt to execute functions, utilities, or system operations.
- Ubiquitous on z/OS: Found in environments like TSO/ISPF (e.g., READY prompt, ISPF command line), OMVS (UNIX System Services shell), and for issuing z/OS operator commands.
- Scripting and Automation: Often used in conjunction with scripting languages like REXX or CLISTs to automate repetitive tasks or build custom tools.
- Direct System Control: Provides direct access to system functions, dataset management, job submission, and program execution.
- Call Level Interface (CLI):
- Standardized API: Defines a set of functions for connecting to a database, executing SQL statements, and retrieving results. A prominent example is ODBC (Open Database Connectivity), which has z/OS implementations for DB2.
- Programmatic Database Access: Allows application programs (e.g., written in C/C++, COBOL, Java via JDBC-ODBC bridge) to access relational (DB2) or hierarchical (IMS) databases without embedding SQL directly into the source code (though dynamic SQL is used).
- Driver-Based Architecture: Relies on specific database drivers that translate generic CLI calls into the native API calls of the target database system.
- Cross-Platform Connectivity: Facilitates connectivity between distributed applications (e.g., Windows, Linux) and mainframe databases like DB2 for z/OS.

Use Cases

- Command Line Interface (CLI):
- System Administration: Issuing z/OS operator commands (e.g., DISPLAY M, VARY CPU) to monitor and control system resources.
- Job Submission and Management: Submitting JCL jobs using SUBMIT command in TSO, or checking job status with STATUS.
- Dataset Manipulation: Allocating, deleting, copying, or listing datasets using TSO commands like ALLOCATE, DELETE, COPY, LISTCAT.
- Program Execution: Running TSO-authorized programs or utilities directly from the READY prompt.
- Call Level Interface (CLI):
- Distributed Application Development: Enabling client-server applications running on distributed platforms to access and update DB2 for z/OS data.
- Reporting and Business Intelligence: Connecting BI tools (e.g., Cognos, Tableau) to mainframe data sources via ODBC drivers.
- Data Integration: Facilitating data exchange between mainframe databases and other enterprise systems.
- Custom Database Utilities: Developing specialized C/C++ or COBOL programs that interact with DB2 using the CLI API for specific data processing tasks.

Related Concepts

The Command Line Interface is fundamental to interacting with TSO/ISPF, JCL, and z/OS operator commands, serving as the primary user interface for system administrators and developers. It is often augmented by REXX or CLIST for automation. The Call Level Interface, particularly in its ODBC form, is crucial for DB2 for z/OS and sometimes IMS, providing a standardized, vendor-neutral way for application programs to access these database systems. It complements embedded SQL (e.g., in COBOL programs) by offering dynamic SQL capabilities and facilitating distributed data access and data integration across heterogeneous environments.

Best Practices:

For Command Line Interface (CLI):
Automate with Scripts: For repetitive tasks, leverage REXX or CLISTs to create reusable scripts, improving efficiency and reducing errors.
Understand Command Scope: Be aware of the security context and potential impact of commands, especially for z/OS operator commands.
Utilize ISPF Panels: For complex operations, use ISPF panels which often provide a more guided and error-resistant interface than raw commands.
Use TSO PROFILE NOPREFIX: To avoid automatic prefixing of dataset names, which can simplify command entry.
For Call Level Interface (CLI):
Connection Pooling: Implement connection pooling to minimize the overhead of establishing new database connections, especially for high-volume applications.
Parameter Markers: Always use parameter markers (?) in SQL statements to prevent SQL injection vulnerabilities and allow DB2 to reuse access paths, improving performance.
Proper Resource Management: Ensure that all database connections, statements, and