What is a Copybook in COBOL? πŸ€”

What is a Copybook in COBOL? A COBOL copybook is a reusable and separate file that contains data structure definitions, such as record layouts, data fields, and data types, used in COBOL programs.

What is a COBOL Copybook TOC

Copybooks are typically included or “copied” into COBOL source code to provide a consistent and modular way of defining data structures, improving code readability, and facilitating code reuse.

COBOL Copybooks help to ensure uniformity in data definitions across multiple program files, making it easier to maintain and update COBOL applications.

A longer explanation regardingΒ what purpose Copybooks serve in COBOL applications follows.

Purpose of COBOL Copybooks

COBOL copybooks serve as a central repository for data structure definitions and declarations used in COBOL programs.

COBOL copybook data structures can include:

Record Layouts

Copybooks often define the record structure or data blocks used in files or datasets including fields for various pieces of information within each record.

Data Fields

Copybooks specify the characteristics of individual data fields, such as data types, lengths, and formats.

Data Types and Picture Clauses

COBOL Copybooks provide information about the data types used in COBOL applications, including alphanumeric, numeric, or date formats, as well as any associated picture clauses that define the field’s format.


Copybooks may include comments to describe the purpose of each field or record, which can be helpful for programmers and future maintainers.

COBOL Copybook Example

Below I’ve included a COBOL Copybook example file that is used for testing in the R COBOL Data Integration packageΒ (RCOBOLDI).

A COBOL Copybook example.
A COBOL Copybook example

This COBOL Copybook example and an associated COBOL data file are converted into a data frame in R using the RCOBOLDI package and results in the image below when we inspect the data.

After the COBOL Copybook transformation into an R data frame has completed we inspect the contents of the result using the head command which displays the first six rows.
A COBOL Copybook and COBOL data file converted into a data frame in R

Other COBOL Copybook examples can be found in the test directory for the RCOBOLDI project on GitHub.

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