Fastest way to tell if two files have the same contents in Unix/Linux

Let's say that we have two files inside a directory called dir1, and at first both these files are different. Different in the sense that the text they contain isn't the same.

The files in the folder −

immukul@192 dir1 % ls -ltr
total 16
-rw-r--r-- 1 immukul staff 7 Jul 7 10:37 2.txt
-rw-r--r-- 1 immukul staff 8 Jul 8 19:05 3.txt

The contents inside the first file (2.txt) looks something like this −

immukul@192 dir1 % cat 2.txt
orange

The contents inside the second file (3.txt) looks something like this −

immukul@192 dir1 % cat 3.txt
uorange

Methods to Compare Files

Using diff Command

We can easily make use of the diff command to check if they have something different. Consider the command shown below −

diff 2.txt 3.txt

Output

1c1
< orange
---
> uorange

But in case where the contents of the file are exactly the same, then the diff command won't return any output.

Using cmp Command (Fastest Method)

In that case it is recommended to make use of the cmp command. The cmp command is a Linux utility command that is used to compare two files byte by byte. It is the fastest way to determine if two files have identical contents because it stops at the first difference.

cmp --silent 2.txt 3.txt || echo "Difference in Files"

Output

immukul@192 dir1 % cmp --silent 2.txt 3.txt || echo "Difference in Files"
Difference in Files

Alternative Methods

Using Checksums

For large files, you can use checksums to quickly compare files without reading the entire content −

md5sum file1.txt file2.txt
sha256sum file1.txt file2.txt

Using test Command

You can also use the test command with cmp for conditional checking −

if cmp --silent file1.txt file2.txt; then
    echo "Files are identical"
else
    echo "Files are different"
fi

Comparison of Methods

Conclusion

The cmp command with the --silent flag is the fastest way to determine if two files have identical contents in Unix/Linux. It performs a byte-by-byte comparison and stops immediately when it finds the first difference, making it more efficient than diff for simple identical content checking.