https://coboltextbook.com/ COBOL tutorials, one complete program per lesson, desk-checked against GnuCOBOL and the ISO standard, from your first program to files, tables, and JSON. 2026-10-30T00:00:00Z https://coboltextbook.com/posts/gnucobol-std-dialects-ibm.html https://coboltextbook.com/posts/gnucobol-std-dialects-ibm.html What cobc -std=ibm actually switches in GnuCOBOL 3.2, read from its dialect files, and one program whose output changes when the dialect does. 2026-10-28T00:00:00Z https://coboltextbook.com/posts/cobol-inspect-replacing-converting.html https://coboltextbook.com/posts/cobol-inspect-replacing-converting.html A GnuCOBOL program that cleans an order export in place with INSPECT REPLACING, CONVERTING and BEFORE/AFTER, and the size and order rules behind them. 2026-10-15T00:00:00Z https://coboltextbook.com/posts/cobol-string-unstring.html https://coboltextbook.com/posts/cobol-string-unstring.html A GnuCOBOL program that splits a comma-separated export with UNSTRING, checks each field with COUNT IN and TALLYING IN, and builds a label with STRING. 2026-10-12T00:00:00Z https://coboltextbook.com/posts/cobol-input-validation-class-inspect-test-numval.html https://coboltextbook.com/posts/cobol-input-validation-class-inspect-test-numval.html A GnuCOBOL edit program that checks every field of a fixed-width feed with CLASS, INSPECT, TEST-NUMVAL and TEST-DATE-YYYYMMDD, and logs every error with a code. 2026-10-08T00:00:00Z https://coboltextbook.com/posts/cobol-json-generate-parse.html https://coboltextbook.com/posts/cobol-json-generate-parse.html A GnuCOBOL program that turns fixed-width records into JSON Lines with JSON GENERATE, the exact output it should write, and why JSON PARSE does nothing in 3.2. 2026-10-05T00:00:00Z https://coboltextbook.com/posts/cobol-reconcile-two-files.html https://coboltextbook.com/posts/cobol-reconcile-two-files.html A complete GnuCOBOL match of two sorted files that reports every reference as matched, amount differs, bank only or book only, with control totals that must balance. 2026-09-24T00:00:00Z https://coboltextbook.com/posts/cobol-call-c-function.html https://coboltextbook.com/posts/cobol-call-c-function.html CALL BY VALUE, BY REFERENCE and RETURNING across the C ABI, then the other direction with cob_init and cob_resolve, with both listings and the build lines. 2026-09-21T00:00:00Z https://coboltextbook.com/posts/cobol-free-format-columns-8-and-12.html https://coboltextbook.com/posts/cobol-free-format-columns-8-and-12.html The same COBOL program written twice, in fixed format and in free format, with the column rules that shaped it and where GnuCOBOL and IBM disagree. 2026-09-17T00:00:00Z https://coboltextbook.com/posts/cobol-copybook-copy-replacing.html https://coboltextbook.com/posts/cobol-copybook-copy-replacing.html One 39-byte copybook in three GnuCOBOL programs: plain COPY, REPLACING LEADING, and REPLACE, with where GnuCOBOL and IBM differ on search paths and nesting. 2026-09-14T00:00:00Z https://coboltextbook.com/posts/cobol-indexed-file-start-read-key-rewrite.html https://coboltextbook.com/posts/cobol-indexed-file-start-read-key-rewrite.html A GnuCOBOL program loads a parts master into an indexed file, reads by key, rewrites a record, browses a key range with START and READ NEXT, and deletes; data ships. 2026-09-10T00:00:00Z https://coboltextbook.com/posts/cobol-write-report-file-page-breaks.html https://coboltextbook.com/posts/cobol-write-report-file-page-breaks.html A GnuCOBOL program writes a paginated report: page header with the run date, column headings, edited detail lines, a page total, and a break every 23 lines. 2026-09-07T00:00:00Z https://coboltextbook.com/posts/python-fixed-width-test-data-cobol.html https://coboltextbook.com/posts/python-fixed-width-test-data-cobol.html A Python script writes a 64-byte fixed-width transaction file, a copybook pins the layout, and a GnuCOBOL program reads it back and prints control totals. 2026-09-05T00:00:00Z https://coboltextbook.com/posts/cobol-redefines-example.html https://coboltextbook.com/posts/cobol-redefines-example.html REDEFINES overlays a second description on the same bytes: a date as text and as three numbers, record variants switched by type code, and a NUMERIC test first. 2026-09-05T00:00:00Z https://coboltextbook.com/posts/cobol-read-sequential-file.html https://coboltextbook.com/posts/cobol-read-sequential-file.html SELECT, FD, OPEN, the priming READ and AT END: a complete program reads a 60-byte customer file to end of file and prints control totals; data and copybook ship. 2026-09-05T00:00:00Z https://coboltextbook.com/posts/cobol-picture-clause-9-x-a-v-s.html https://coboltextbook.com/posts/cobol-picture-clause-9-x-a-v-s.html What each PICTURE symbol means, how PIC 9(3)V99 occupies five bytes with no decimal point, and what S and P really do, shown by one GnuCOBOL program. 2026-09-05T00:00:00Z https://coboltextbook.com/posts/cobol-perform-varying-until-inline.html https://coboltextbook.com/posts/cobol-perform-varying-until-inline.html Every common COBOL loop in one GnuCOBOL program: PERFORM TIMES, UNTIL with TEST BEFORE and TEST AFTER, inline VARYING over a table, nested AFTER, and EXIT PERFORM. 2026-09-05T00:00:00Z https://coboltextbook.com/posts/cobol-occurs-subscripts-table.html https://coboltextbook.com/posts/cobol-occurs-subscripts-table.html OCCURS builds a table; subscripts index it from 1. A worked program sums twelve monthly totals, finds the best month, fills a 3-by-4 grid and uses DEPENDING ON. 2026-09-05T00:00:00Z https://coboltextbook.com/posts/cobol-move-truncation-rules.html https://coboltextbook.com/posts/cobol-move-truncation-rules.html Seven MOVEs in one GnuCOBOL program show where COBOL truncates on the right, on the left, drops a sign, or copies bytes blindly in a group move, with a rules table. 2026-09-05T00:00:00Z https://coboltextbook.com/posts/cobol-hello-world-four-divisions.html https://coboltextbook.com/posts/cobol-hello-world-four-divisions.html A complete first COBOL program for GnuCOBOL, explained division by division, with DISPLAY, WITH NO ADVANCING, signed and unsigned numbers, and STOP RUN. 2026-09-05T00:00:00Z https://coboltextbook.com/posts/cobol-group-items-elementary-items.html https://coboltextbook.com/posts/cobol-group-items-elementary-items.html How level numbers 01 to 49, 77 and 88 build a record; what a group move does to the bytes; a worked 80-byte customer record with its byte offsets. 2026-09-05T00:00:00Z https://coboltextbook.com/posts/cobol-edited-pictures-money.html https://coboltextbook.com/posts/cobol-edited-pictures-money.html Numeric-edited PICTURE strings format amounts for people. Eight edited items receive the same five values; a table shows every result and the rules that produce it. 2026-09-05T00:00:00Z https://coboltextbook.com/posts/cobol-compute-rounded-on-size-error.html https://coboltextbook.com/posts/cobol-compute-rounded-on-size-error.html One GnuCOBOL program: COMPUTE with and without ROUNDED, ROUNDED MODE IS NEAREST-EVEN on a tie, an overflow caught by ON SIZE ERROR, and a trapped divide by zero. 2026-09-05T00:00:00Z https://coboltextbook.com/posts/cobol-bank-statement-batch-job.html https://coboltextbook.com/posts/cobol-bank-statement-batch-job.html A complete GnuCOBOL batch program that reads sorted transactions, rejects bad records to a file, breaks on account change, and writes a statement with totals. 2026-09-05T00:00:00Z