JCL (job control language) is a language for describing jobs (units of work) to the MVS, OS/390, and VSE operating systems, which run on IBM's S/390 large server (mainframe) computers. These operating systems allocate their time and space resources among the total number of jobs that have been started in the computer. Jobs in turn break down into job steps. All the statements required to run a particular program constitute a job step. Jobs are background (sometimes called batch) units of work that run without requiring user interaction (for example, print jobs). In addition, the operating system manages interactive (foreground) user requests that initiate units of work. In general, foreground work is given priority over background work.
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One IBM manual compares a set of JCL statements to a menu order in a restaurant. The whole order is comparable to the job. Back in the kitchen, the chefs divide the order up and work on individual dishes (job steps). As the job steps complete, the meal is served (but it has to be served in the order prescribed just as some job steps depend on other job steps being performed first).
JCL statements mainly specify the input data sets (files) that must be accessed, the output data set to be created or updated, what resources must be allocated for the job, and the programs that are to run, using these input and output data sets. A set of JCL statements for a job is itself stored as a data set and can be started interactively. MVS and OS/390 provide an interactive menu-like interface, ISPF, for initiating and managing jobs.
In MVS and OS/390, the part of the operating system that handles JCL is called the Job Entry Subsystem (JES). There are two versions, JES2 and a later version with additional capabilities, JES3.