How to Compare Two Text Files in Linux

Linux includes several tools for comparing text files, from the classic diff command to interactive side-by-side viewers. Here’s how to use each and when to reach for them.

The diff command

diff is the foundational tool — installed on every Linux system, available in scripts, and the basis for most other comparison tools.

diff file1.txt file2.txt

Basic output (normal format):

3c3
< old line three
---
> new line three
5d4
< line only in file1

Reading the output:

• 3c3 — line 3 in file1 was changed to line 3 in file2
• 5d4 — line 5 in file1 was deleted (file2 has no corresponding line)
• < — lines from file1
• > — lines from file2

Exit codes:

• 0 — files are identical
• 1 — files differ
• 2 — error

This makes diff useful in scripts:

if ! diff -q file1.txt file2.txt > /dev/null; then
  echo "Files differ"
fi

Unified format (diff -u)

The unified format is much more readable and is what git uses:

diff -u file1.txt file2.txt

Output:

--- file1.txt   2026-04-25 10:00:00
+++ file2.txt   2026-04-25 10:01:00
@@ -1,6 +1,5 @@
 line one
 line two
-old line three
+new line three
 line four
-line only in file1

• --- marks the original file, +++ the modified file
• Lines starting with - were removed; + were added; space is context
• @@ -1,6 +1,5 @@ means: 6 lines from file1 starting at line 1; 5 lines from file2 starting at line 1

Save a patch:

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

Common flags

Flag	Effect
-u	Unified format (most readable)
-c	Context format (older style)
-y	Side-by-side view
-q	Quiet — only report whether files differ
-i	Ignore case differences
-w	Ignore all whitespace
-b	Ignore changes in amount of whitespace
-B	Ignore blank lines
-r	Recursive (compare directories)
--color	Colorized output (GNU diff 3.4+)
-U N	Show N lines of context (default 3)

Examples:

# Ignore all whitespace differences
diff -w file1.txt file2.txt

# Recursive directory comparison
diff -r dir1/ dir2/

# Colorized unified diff
diff --color -u file1.txt file2.txt

# Show more context
diff -U 10 file1.txt file2.txt

Side-by-side comparison (diff -y)

diff -y file1.txt file2.txt

Output:

line one                    line one
line two                    line two
old line three            | new line three
line four                   line four
line only in file1        <

Markers:

• | — line differs
• < — only in file1
• > — only in file2
• (space) — lines are the same

Control the column width with --width=N (default 130).

Colorized diff with colordiff

colordiff wraps diff and adds color:

# Install (Debian/Ubuntu)
sudo apt install colordiff

# Install (RHEL/Fedora)
sudo dnf install colordiff

# Use as a drop-in replacement
colordiff file1.txt file2.txt
colordiff -u file1.txt file2.txt

You can alias it: alias diff=colordiff

Interactive diff with vimdiff

For reviewing large files interactively:

vimdiff file1.txt file2.txt

Opens both files in a split Vim window with differences highlighted. Navigate between changes with ]c (next diff) and [c (previous diff). Use :diffput and :diffget to resolve differences.

comm — comparing sorted files

comm is for sorted files and outputs three columns: lines only in file1, lines only in file2, and lines in both:

# Files must be sorted first
sort file1.txt > sorted1.txt
sort file2.txt > sorted2.txt

comm sorted1.txt sorted2.txt

Suppress columns selectively:

# Show only lines common to both files
comm -12 sorted1.txt sorted2.txt

# Show only lines in file1 but not file2
comm -23 sorted1.txt sorted2.txt

cmp — byte-level comparison

cmp reports the first byte that differs, useful for binary files:

cmp file1.txt file2.txt
# file1.txt file2.txt differ: byte 47, line 3

With -l, prints all differing bytes.

When to use which tool

Situation	Tool
Quick check if files differ	diff -q
Generate a patch file	diff -u
Read a diff easily	colordiff -u
Compare directories	diff -r
Interactive review/merge	vimdiff or meld
Compare sorted text streams	comm
Binary file check	cmp
Compare in a browser	textdiff.pro

Comparing files without a terminal

For quick comparisons without SSH or a terminal, textdiff.pro provides an online diff with unified and side-by-side views — paste your text directly in the browser.