diff and patch

Before you change a configuration file, copy it: cp app.conf app.conf.orig. After the change, diff tells you precisely what you did, line by line. That output is not just for reading. Saved to a file it becomes a patch, a recipe that patch can replay on another copy of the original, on another machine, or undo later. This is how changes to source code travelled by email for decades, and it is still how git shows you changes.

~% cd lab/l95
l95% cat old/app.conf
host=localhost
port=8080
debug=yes
l95% diff -u old/app.conf new/app.conf
--- old/app.conf
+++ new/app.conf
@@ -1,3 +1,3 @@
 host=localhost
-port=8080
-debug=yes
+port=9090
+debug=no
l95% echo $?
1

Reading a unified diff

The --- line names the old file and +++ the new one. Each block starting with @@ is a hunk: -1,3 +1,3 means "lines 1 to 3 of the old file become lines 1 to 3 of the new one". Inside a hunk, lines starting with a space are unchanged context, - lines were removed and + lines were added. A changed line therefore shows up as a - followed by a +. The context lines are what lets patch find the right spot even if the file has moved a little.

Other systems print an older "normal" format by default and need -u for this one. BusyBox diff only knows the unified format, so here diff and diff -u print the same thing; write -u anyway, it is the habit that works everywhere. The exit status is useful in scripts: 0 means identical, 1 means different and 2 means trouble, such as a missing file.

l95% diff -rq site site.bak
Files site/index.html and site.bak/index.html differ
Only in site: new.html

Making and applying a patch

l95% diff -u old/app.conf new/app.conf > app.patch
l95% cp old/app.conf live.conf
l95% patch live.conf app.patch
patching file live.conf
l95% cat live.conf
host=localhost
port=9090
debug=no
l95% patch -R live.conf app.patch
patching file live.conf

patch FILE PATCHFILE (or patch FILE < PATCHFILE) applies the recipe to FILE. -R reverses it, turning + into -, which is your undo button. Without a FILE argument patch reads the file names from the ---/+++ headers, and -p1 strips the first directory from them, the usual way to apply a patch for a whole project from its top directory: patch -p1 < fix.patch. Without an explicit FILE, BusyBox patches the file named on the +++ line, so name the target when in doubt.