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Linux Pocket Guide, 2nd Edition by Daniel J. Barrett

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md5sum — stdin  stdout  - file  -- opt  --help  --version

Synopsis

md5sum files | --check file

The md5sum command works with checksums to verify that files are unchanged. The first form produces the 32-byte checksum of the given files, using the MD5 algorithm:

$ md5sum myfile
dd63602df1cceb57966d085524c3980f  myfile

while the second form tests whether a checksum matches its file, using --check:

$ md5sum file1 file2 file3 > mysum
$ cat mysum
90a022707ca5b5fc8f465e7cbb954987  file1
86d19ef79d33c28cf0c9ba882f25cdb8  file2
d0dc53c9941e33a10e7f38ecc0de772f  file3
$ md5sum --check mysum
file1: OK
file2: OK
file3: OK
$ echo "new data" > file2
$ md5sum --check mysum
file1: OK
file2: FAILED
file3: OK
md5sum: WARNING: 1 of 3 computed checksums did NOT match

Two different files are highly unlikely to have the same MD5 checksum, so comparing checksums is a reasonably reliable way to detect if two files differ:

$ md5sum myfile1 | cut -c1-32 > sum1
$ md5sum myfile2 | cut -c1-32 > sum2
$ diff -q sum1 sum2
Files sum1 and sum2 differ

Some other programs similar to md5sum are sum and cksum, which use different algorithms to compute their checksums. sum is compatible with other Unix systems, specifically BSD Unix (the default) or System V Unix (-s option), and cksum produces a CRC checksum:

$ sum myfile
12410     3
$ sum -s myfile
47909 6 myfile
$ cksum myfile
1204834076 2863 myfile

The first integer is a checksum and the second is a block count. But as you can see, these checksums are small numbers and therefore ...

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