DIFF(1plan9)

📦 version 6, in debian bookworm, debian trixie, ubuntu jammy, ubuntu noble, ubuntu questing, ubuntu resolute, ubuntu stonking

✨ NAME ✨

diff — the differential file comparator, aka the little gremlin that tells you exactly which lines gotta change so two files finally agree on something 💅

🌸 SYNOPSIS 🌸

diff [ -acefmnbwr ] file1 ... file2

💖 DESCRIPTION 💖

Okay bestie, listen. diff tells you what lines must be changed in two files to bring them into agreement. That's it. That's the whole job. Matchmaker energy but for text files 🫶

Now the plot twists:

  • If one file is a directory, then a file in that directory with basename the same as that of the other file is used. Sneaky, right?
  • If both files are directories, similarly named files in the two directories are compared by the method of diff for text files and cmp(1) otherwise. (Shoutout cmp(1), our binary bestie.)
  • If more than two file names are given, then each argument is compared to the last argument as above. The last one is literally the main character and everyone else is compared to it 👑

🌈 The flags, one by one 🌈

  • -r — causes diff to process similarly named subdirectories recursively. It goes deep. 🕳️
  • -m — when you're processing more than one file, diff prefixes file differences with a single line listing the two differing files, in the form of a diff command line. The -m flag causes this behavior even when processing single files. No cap.
  • -b — causes trailing blanks (spaces and tabs) to be ignored and other strings of blanks to compare equal. Basically it's not gonna fight you over invisible characters. So chill of it.
  • -w — causes all white-space to be removed from input lines before applying the difference algorithm. Not "compared loosely" — removed. Gone. Deleted. 💀
  • -n — prefixes each range with file: and inserts a space around the a, c, and d verbs. Tiny little formatting glow-up ✨
  • -e — produces a script of a, c and d commands for the editor ed, which will recreate file2 from file1. Time machine behavior, lowkey.
  • -f — produces a similar script, not useful with ed, in the opposite order. It may, however, be useful as input to a stream-oriented post-processor. Different vibe, still valid 🦋
  • -c — includes three lines of context around each change, merging changes whose contexts overlap. Three. Lines. Of. Context. Cozy.
  • -a — displays the entire file as context. All of it. Every single line. It said "no secrets" 🙈

🎀 The normal output 🎀

The normal output contains lines of these forms:

n1 a n3,n4
n1,n2 d n3
n1,n2 c n3,n4

These lines resemble ed commands to convert file1 into file2. The numbers after the letters pertain to file2. In fact, by exchanging a for d and reading backward one may ascertain equally how to convert file2 into file1. Reversible!!! The way that this is a two-way street 😭

As in ed, identical pairs where n1 = n2 or n3 = n4 are abbreviated as a single number. Efficiency icon.

Following each of these lines come all the lines that are affected in the first file flagged by <, then all the lines that are affected in the second file flagged by >. < = the old one, > = the new one, iykyk 💜

🌟 One more thing: except in rare circumstances, diff finds a smallest sufficient set of file differences. It's not just doing stuff — it's doing the minimum. Ate. Periodt.

🗂️ FILES 🗂️

  • /tmp/diff[12] — yes, both of them, it's a whole duo 🧚

🧵 SOURCE 🧵

/src/cmd/diff — the origin story, right there.

🔍 SEE ALSO 🔍

Go read them, they're the friend group 🌸

🩺 DIAGNOSTICS 🩺

Exit status is the empty string for no differences, for some, and for trouble.

Yeah bestie that's literally what it says. All three cases. Empty string. We don't make the rules, we just report them 💅

🐛 BUGS 🐛

  • Editing scripts produced under the -e or -f option are naive about creating lines consisting of a single . — like, they just do not handle that. Watch out 🙈
  • When running diff on directories, the notion of what is a text file is open to debate. Philosophical king behavior honestly 🌈

and that's the whole page, bestie 💖 this was diff(1plan9) — section 1 of the Plan 9 manual, brought to you by the good people who wrote it way back when, in all its /src/cmd/diff glory. mwah 🫶

📦 where this page comes from