diffmk

diffmk old_file new_file marked_file

diffmk is a specialized command designed to highlight differences between two text files in a format suitable for typesetting systems like nroff and troff.

Unlike the standard diff command, which produces a diff format (e.g., unified, context), diffmk generates a third file where changes are visually indicated on a line-by-line basis. Specifically, lines that have been modified between the old_file and new_file are marked with a vertical bar (|) at the beginning, while lines that were present in old_file but deleted in new_file are marked with an asterisk (*). Lines that are common to both files or newly added lines in new_file are typically presented without special marking or might be inserted directly.

Its primary application is in documentation generation, where it helps authors track and display changes in printed or formatted documents, making it easier for readers to identify revisions. The command employs an algorithm to find the longest common subsequence of lines, ensuring a reasonable alignment of changes. This makes diffmk particularly useful for generating revision marks in technical manuals or legal documents processed by nroff or troff.

• diffmk is specifically tailored for nroff/troff output. Its marking conventions (| and *) are intended to be interpreted by these typesetting systems. Using its output directly in other contexts might not be meaningful without further processing.
• It primarily marks changed and deleted lines from the perspective of the old_file compared to the new_file. The treatment of added lines might vary or require understanding how nroff/troff handles them in the marked_file.
• The algorithm used is based on finding the longest common subsequence, which is generally effective but might not always produce the 'human-readable' diff that diff -u or git diff might offer for source code. Its focus is on line-level changes for document revision.
• The command is quite old and might be considered a legacy tool, often found in environments still relying on groff or troff for document preparation.

When diffmk generates the marked_file, the | and * characters are placed at the beginning of lines. These characters are not just visual indicators; they are often interpreted by nroff or troff macro packages to apply specific formatting (e.g., bolding, italicizing, or using different margins) to the changed or deleted text within the final formatted document. This allows for automated visual differentiation of revisions in printed output.

The old_file represents the original version of the document, and new_file is the revised version. The marked_file is the generated output that combines content from both, with the specific difference markers. It's crucial that these files are text-based for diffmk to operate correctly.

diffmk is a venerable Unix utility, originating from Bell Labs, alongside other text processing tools like nroff and troff. It was designed specifically to integrate with these typesetting systems for document revision control.

Its purpose emerged from the need to easily highlight changes in technical documentation and manuals, which were extensively prepared using troff in the early Unix era. As part of the troff ecosystem, its development paralleled the evolution of professional document formatting on Unix systems. While diff evolved to serve a broader range of file comparison needs (especially for source code), diffmk remained focused on the specialized task of generating revision marks for formatted output, reflecting a specific design philosophy for document management within the Unix environment.

diff(1), nroff(1), troff(1), groff(1)