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Copybooks are COBOL's #include — text files of code copied into your program at compile time. They're how you share record layouts and constants across many programs.

The basics

A copybook is just a text file (typically .cpy or .cob) with COBOL declarations:

      *> file: customer.cpy
       01 CUSTOMER-RECORD.
           05 CUST-ID         PIC X(10).
           05 CUST-NAME       PIC X(40).
           05 CUST-ADDRESS.
               10 STREET      PIC X(30).
               10 CITY        PIC X(20).
               10 ZIP         PIC 9(5).
           05 CUST-BALANCE    PIC 9(7)V99.

Use it with COPY:

       DATA DIVISION.
       WORKING-STORAGE SECTION.
       COPY "customer.cpy".
       PROCEDURE DIVISION.
           MOVE "123" TO CUST-ID.
           ...

The COPY statement is replaced verbatim by the file contents at compile time.

Replacing

       COPY "customer.cpy" REPLACING ==CUST== BY ==BUYER==.

This would substitute BUYER for every CUST in the copybook — useful when you want to use the same layout twice with different prefixes.

Why copybooks matter

In mainframe shops, a single record layout might be used by 100+ programs. Storing it in one place means:

• Schema changes update everywhere on rebuild
• Programs can't disagree about field offsets
• A central data dictionary is enforced

Most financial mainframe shops have thousands of copybooks defining records, constants, screen formats, etc.

Common copybook contents

• Record layouts — for files, messages, screens
• Constants — error codes, status flags
• Procedure code — common sequences (validation, logging) included into PROCEDURE DIVISION
• CICS / IMS structures — system parameters for transaction processing

Conventions

• File extension: .cpy, .cob, .cobcopy, varies by shop
• Naming: <DOMAIN>-<DESCRIPTION> — e.g., CUST-DTLS.cpy, ORDER-HDR.cpy
• One record/group per copybook (not always followed)
• Comment header with revision history

Modern alternatives

• GnuCOBOL supports COPY (we'd use it if Judge0 had a real filesystem)
• Procedural COBOL on z/OS uses CA Endevor or similar SCM tools to manage copybooks
• Modern COBOL (Cobol-2002+) has better module systems but copybooks remain ubiquitous

For this lesson we can't use real COPY (no filesystem). Below: simulate the pattern by inlining what would be a copybook.