diff / patch

diff [options] file1 file2
patch [options] [original-file] [patch-file]

Command / Option	Description
diff file1 file2	Shows differences between two files. Lines marked with < are from file1; lines marked with > are from file2.
diff -u file1 file2	Shows differences in unified format — the same style used by git diff.
diff -r dir1 dir2	Recursively compares two directories.
diff -i file1 file2	Ignores case differences when comparing.
diff -b file1 file2	Ignores differences in the amount of whitespace.
diff -q file1 file2	Reports only whether the files differ, without showing the actual differences.
diff -y file1 file2	Shows the comparison in two side-by-side columns.
patch file < patch	Applies the differences in a patch file to the specified file.
patch -R file < patch	Reverses a patch (undoes the applied changes).
patch -p1 < patch	Strips the first directory component from file paths in the patch before applying it.

The following examples use these two files.

original.txt

line1
line2
line3

modified.txt

line1
line2 modified
line3
line4 added

Use diff -u to display the differences in unified format (the same style as git diff).

diff -u original.txt modified.txt
--- original.txt	2026-03-05 10:00:00.000000000 +0900
+++ modified.txt	2026-03-05 10:01:00.000000000 +0900
@@ -1,3 +1,4 @@
 line1
-line2
+line2 modified
 line3
+line4 added

Save the differences as a patch file.

diff -u original.txt modified.txt > changes.patch

Apply the patch file to the original.

patch original.txt < changes.patch
patching file original.txt

Use -R to reverse the patch and restore the file to its previous state.

patch -R original.txt < changes.patch
patching file original.txt

Use -q to check only whether files differ — useful for conditionals in scripts.

diff -q original.txt modified.txt
Files original.txt and modified.txt differ

Use -r to recursively compare two directories.

diff -r dir1/ dir2/
diff -r dir1/config.txt dir2/config.txt
1c1
< debug=false
---
> debug=true
Only in dir2/: newfile.txt

Use -y to display the comparison in two side-by-side columns.

diff -y original.txt modified.txt
line1                 line1
line2               | line2 modified
line3                 line3
                    > line4 added

In unified format ('-u'), lines beginning with - were removed and lines beginning with + were added. This is the same diff format used internally by Git. In modern software development, you rarely call diff and patch directly — Git handles all of that for you. However, being able to read their output is essential for applying patches on a server or understanding the output of code review tools.

When comparing files and copying them, see also cp.