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md5sum is a computer program that calculates and verifies 128-bit MD5 hashes, as described in RFC 1321. The MD5 hash (or checksum) 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. The underlying MD5 algorithm is no longer deemed secure, thus while md5sum 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 highly recommended.
Virtually any non-malicious change to a file will cause its MD5 hash to change; therefore md5sum is used to verify the integrity of files. Most commonly, md5sum 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 md5sum program is installed by default in most Unix, Linux, and Unix-like operating systems or compatibility layers. Other operating systems, including Microsoft Windows and BSD variants — such as Mac OS X - have similar utilities (see external links). On FreeBSD this utility is called 'md5' and contains additional features.
All of the following files are assumed to be in the current directory.
$ md5sum filetohashA.txt filetohashB.txt filetohashC.txt > hash.md5
File contains filename and hash pairs.
$ cat hash.md5 595f44fec1e92a71d3e9e77456ba80d1 filetohashA.txt 71f920fa275127a7b60fa4d4d41432a3 filetohashB.txt 43c191bf6d6c3f263a8cd0efd4a058ab filetohashC.txt
Note: There must be two spaces between each md5sum value and filename to be compared. Otherwise, the following error will result: "no properly formatted MD5 checksum lines found".
Note: The file must be also UNIX line ending formatted otherwise you will see "md5sum: WARNING: x listed files could not be read". 'dos2unix' will convert it quickly if it is DOS/Windows formatted.
$ md5sum -c hash.md5 filetohashA.txt: OK filetohashB.txt: OK filetohashC.txt: OK
$ echo "595f44fec1e92a71d3e9e77456ba80d1 filetohashA.txt" | md5sum -c - filetohashA.txt: OK
Notice the 2 spaces between the md5 value and the filename.
Notice the dash at the end.