gdiffmk(1)

💅 gdiffmk(1) — General Commands Manual 🎀

okay so bestie, welcome to gdiffmk, straight out of the General Commands Manual section (section 1, the "just a normal command, nothing spooky" section 💖). this whole page is literally gonna teach you how to mark up the diff between groff/nroff/troff files, and it's giving main character energy fr fr ✨

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✨ Name ✨

gdiffmk — mark differences between groff/nroff/troff files 🦋

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🌸 Synopsis 🌸

gdiffmk [-a add-mark] [-c change-mark] [-d delete-mark] [-x diff-
        command] [-D [-B] [-M mark1 mark2]] [--] file1 file2 [output-
        file]

gdiffmk --help

gdiffmk --version

so yeah, that's the shape of it 💅 two files in, optionally an output file, and a whole pile of flags to make it yours 🫶 (the diff-command thing is wrapped across lines up there, don't panic, it's still just -x diff-command).

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🦋 Description 🦋

okay so here's the whole vibe: gdiffmk compares two roff(7) documents, file1 and file2, and writes another document derived from both of them — either to the standard output stream or to output-file 📝 — and it sprinkles margin character (mc) requests into the output at exactly the spots where the input documents differ ✨

and here's the important bit, bestie, so listen up: gdiffmk does not interpret the source documents 🙈 it treats roff comments and nilpotent changes to formatting as meaningful. like, it genuinely does not know that \h'3m', \h'(1 * 3m)', and \h'3m+0' all mean the same thing. no cap, it's just not that kind of girl 💀

other deets:

• if file1 or file2 is -, gdiffmk reads the standard input stream for that input 🍓
• if the output operand is present, it writes output to a file of that name 📁
• if output is - or absent, it writes to the standard output stream 🌈
• - cannot be both an input and an output operand ⚠️ like, pick a lane, babe

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💅 OPTIONS 💅

first up, the two ride-or-die info flags:

• --help — displays a usage message, then exits 🫶
• --version — shows version information, then exits ⭐

(both of them dip out right afterward, very "my work here is done" 💅)

now the actual diff-flavouring ones:

• -a add-mark — annotate material that's absent from file1 but present in file2 with add-mark (default: +) 🌸
• -B — suppress br requests that are normally inserted by the -D option. use with caution 😭 because those requests are literally the only way to guarantee that deletions and small changes get marked ⚠️
• -c change-mark — annotate material that differs between file1 and file2 with change-mark (default: |) 🦋
• -d delete-mark — annotate material that's present in file1 but absent from file2 with delete-mark (default: *) 🍓
• -D — mark changed and deleted material with surrounding delimiters 👑
• -M mark1 mark2 — use mark1 (default: [[) and mark2 (default: ]]) as the delimiters when you're using the -D option 🎀
• -x diff-command — use the diff-command program to do the comparison of file1 and file2. diff-command (default: diff) must accept GNU diff(1)'s -D extension option 🔥
• -- — treat all the arguments after it as file names, even if they start with - 🫶 (yes, even the chaotic ones)

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⭐ Exit status ⭐

gdiffmk exits with these, iykyk:

• 0 — the input files are the same 😌
• 1 — the input files differ 👀
• 2 — usage error 🙈
• 3 — your system's diff(1) or sh(1) commands don't support features gdiffmk requires 💀
• 4 — the output argument is a duplicate of file1 or file2 😭

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⚠️ Caveats ⚠️

bestie, the output is not necessarily compatible with all macro packages or preprocessors 💀 a reliable workaround? run gdiffmk on the output of the final preprocessor instead of on the input source ✨ (yes, that's the move, trust)

also: gdiffmk relies on the -D option of GNU diff to make a merged #ifdef output format 🧚 Busybox diff is known to not support it 🙈 also see the -x diff-command option above 👑

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👑 Authors 👑

gdiffmk was written by Mike Bianchi, now retired 🌸 it is maintained by the groff developers 🫶

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🌈 See also 🌈

groff(1), nroff(1), troff(1), roff(7), diff(1)

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and that's the whole page, bestie 💖 this iconic little number is groff 1.24.2, dated 2026-10-01, and it's literally the gdiffmk(1) page — go mark some differences, slay 🎀🦋✨