Md5sum Explained

md5sum
Author:	Ulrich Drepper, Scott Miller, David Madore^[1]
Operating System:	Unix and Unix-like
Platform:	Cross-platform
Genre:	Command
License:	coreutils GNU GPL v3

is a computer program that calculates and verifies 128-bit MD5 hashes, as described in RFC 1321. The MD5 hash functions as a compact digital fingerprint of a file. As with all such hashing algorithms, there is theoretically an unlimited number of files that will have any given MD5 hash. However, it is very unlikely that any two non-identical files in the real world will have the same MD5 hash, unless they have been specifically created to have the same hash.^[2]

The underlying MD5 algorithm is no longer deemed secure. Thus, while is well-suited for identifying known files in situations that are not security related, it should not be relied on if there is a chance that files have been purposefully and maliciously tampered. In the latter case, the use of a newer hashing tool such as sha256sum is recommended.

is used to verify the integrity of files, as virtually any change to a file will cause its MD5 hash to change. Most commonly, is used to verify that a file has not changed as a result of a faulty file transfer, a disk error or non-malicious meddling. The program is included in most Unix-like operating systems or compatibility layers such as Cygwin.

The original C code was written by Ulrich Drepper and extracted from a 2001 release of .^[3]

Examples

All of the following files are assumed to be in the current directory.

Create MD5 hash file hash.md5

$ md5sum filetohashA.txt filetohashB.txt filetohashC.txt > hash.md5

File produced

File contains hash and filename pairs:$ cat hash.md5595f44fec1e92a71d3e9e77456ba80d1 filetohashA.txt71f920fa275127a7b60fa4d4d41432a3 filetohashB.txt43c191bf6d6c3f263a8cd0efd4a058ab filetohashC.txt

Please note:

• There must be two spaces or a space and an asterisk between each value and filename to be compared (the second space indicates text mode, the asterisk binary mode). Otherwise, the following error will result: no properly formatted MD5 checksum lines found. Many programs don't distinguish between the two modes, but some utils do.
• The file must be also be UNIX line ending formatted, otherwise this will be seen: md5sum: WARNING: x listed files could not be read. will convert it quickly if it is DOS/Windows formatted.

Check MD5

$ md5sum -c hash.md5filetohashA.txt: OKfiletohashB.txt: OKfiletohashC.txt: OK

Check single MD5

$ echo 'D43F2404CA13E22594E5C8B04D3BBB81 filetohashA.txt' | md5sum -cfiletohashA.txt: OK

On non-GNU systems

is specific to systems that use GNU coreutils or a clone such as BusyBox. On FreeBSD and OpenBSD the utilities are called,,, and . These versions offer slightly different options and features. Additionally, FreeBSD offers the "SKEIN" family of message digests.

Notes and References

1. https://linux.die.net/man/1/md5sum
2. Web site: HowToMD5SUM - Community Help Wiki . help.ubuntu.com . 2018-11-02.
3. Web site: md5sum: Compute MD5 Checksums . rdrr.io . 2018-11-02.