How do I do a binary diff on two identically sized files under Linux?

I have two identically sized files, and I need to do a binary diff to check whether they're the same.

I'm currently runnnig diff file1.img file2.img but it's taking quite a while to process my 4 GB files. Is this the most efficient way to do this?

cmp is designed to find differences in binary files. You might also try checksumming (sum) and compare the hashes.

One of the most common ways of determining if two files are identical (assuming their sizes match) is using a program to create a "hash" (essentially a fingerprint) of a file. The most common ones are md5sum and sha1sum.

For example:

$ md5sum file1 file2
e0e7485b678a538c2815132de7f9e878  file1
4a14aace18d472709ccae3910af55955  file2

If you have many files that you need to check, for example if you are transferring a directory full of files from one system to another, you can redirect the output from the original system to a file, then md5sum/sha1sum can automatically use that file to tell you which files are different:

$ md5sum file1 file2 > MD5SUMS
... copy file1, file2, MD5SUMS across
$ md5sum --check MD5SUMS
file1: OK
file2: OK