SYSIN

SYSIN is a conventional data input mechanism used in IBM mainframe batch processing and JCL (Job Control Language). It refers to the DDNAME SYSIN, a data definition that supplies input to a program or utility invoked in a job step. A SYSIN data source can be provided in-stream using a DD * ... /* construct, or it can reference an existing dataset via a DSN= parameter in the JCL. The content and format of SYSIN data are defined by the program that reads it, and may consist of control statements, configuration data, or raw records.

Common uses include utilities such as IDCAMS for data set management, DFSORT or SyncSort for sorting and reporting, and other IBM utilities that accept command or control input through SYSIN. For example, IDCAMS may receive DEFINE or ALTER statements in SYSIN to create or modify datasets; DFSORT may receive sorting criteria and job control statements in SYSIN. In many setups, the SYSIN stream is short, but larger configurations can be supplied through a file.

In a typical JCL job, the SYSIN DD line names the data source, and the step executes the program with SYSIN as the standard input. If in-stream data is used, the content follows the SYSIN DD * line and ends with /*. If a dataset is used, SYSIN DD DSN=... provides a dataset containing the necessary control statements.

Best practices include keeping SYSIN content version-controlled, validating syntax with test runs, and documenting the expected format for maintainers, since the exact syntax is specific to the invoked program.