md5sum
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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 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 recommended.
md5sum is used to verify the integrity of files, as virtually any change to a file will cause its MD5 hash to change. 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 included in most Unix-like operating systems, or compatibility layers such as Cygwin.
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.md5
595f44fec1e92a71d3e9e77456ba80d1 filetohashA.txt
71f920fa275127a7b60fa4d4d41432a3 filetohashB.txt
43c191bf6d6c3f263a8cd0efd4a058ab filetohashC.txt
Note: There must be two spaces or a space and an asterisk between each md5sum 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.
Note: The file must be also UNIX line ending formatted otherwise "md5sum: WARNING: x listed files could not be read" will be seen. 'dos2unix' will convert it quickly if it is DOS/Windows formatted.
Check MD5
$ md5sum -c hash.md5
filetohashA.txt: OK
filetohashB.txt: OK
filetohashC.txt: OK
Check single MD5
$ echo D43F2404CA13E22594E5C8B04D3BBB81 filetohashA.txt | md5sum -c
filetohashA.txt: OK