diff — Comparing Files

Which line changed between two versions of a config? diff tells you exactly.

The files for this lesson are in ~/compare:

ls ~/compare
cat ~/compare/app.conf
cat ~/compare/app.conf.new

Normal output

diff ~/compare/app.conf ~/compare/app.conf.new

3,4c3,4
< workers = 2
< log_level = info
---
> workers = 4
> log_level = debug
5a6
> timeout = 30

• 3,4c3,4 — lines 3–4 of the first file change into lines 3–4 of the second (a is added, d deleted)
• < lines are from the first file, > from the second

Unified output: -u

diff -u ~/compare/app.conf ~/compare/app.conf.new

--- /home/student/compare/app.conf	2026-10-30 09:12:44.000000000 +0100
+++ /home/student/compare/app.conf.new	2026-10-30 10:05:13.000000000 +0100
@@ -1,5 +1,6 @@
 # app settings
 port = 8080
-workers = 2
-log_level = info
+workers = 4
+log_level = debug
 cache = on
+timeout = 30

- lines go, + lines come, the rest is context around them. It's the format of git diff and of patches, and the one people send each other:

diff -u ~/compare/app.conf ~/compare/app.conf.new > ~/compare/changes.diff

Just "same or not": -q

diff -q ~/compare/app.conf ~/compare/app.conf.new   # Files ... differ

diff also answers with its exit status — 0 the same, 1 different, 2 trouble — so a script can ask it:

if diff -q ~/compare/app.conf ~/compare/app.conf.new > /dev/null; then echo same; else echo changed; fi

Whole folders: -r

diff -r ~/compare/site-v1 ~/compare/site-v2

Only in names files one folder has and the other doesn't; files in both are compared line by line. diff -qr gives only the names of what's different.

Ignoring what doesn't matter

diff -i a.txt b.txt        # upper/lower case
diff -w a.txt b.txt        # spaces and tabs
diff -b a.txt b.txt        # a different amount of spaces

💡 Before you replace a config with a new one, diff -u old new shows what you're really changing.