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I am confused how md5sum --check is supposed to work:

$ man md5sum
-c, --check
    read MD5 sums from the FILEs and check them

I have a file, I can pipe it to md5sum:

$ cat file | md5sum
44693b9ef883e231cd9f90f737acd58f  -

When I want to check the integrity of the file tomorrow, how can I check if the md5sum is still 44693b9ef883e231cd9f90f737acd58f?

Note

cat file might be a stream. So I want to use the pipe as in my example, not md5sum file.

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2 Answers 2

up vote 19 down vote accepted

You do this:

cat file | md5sum > sumfile

And the next day you can do this:

cat file | md5sum --check sumfile

Which prints:

-: OK

if everything is alright.

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The default syntax for md5sum is:

$ md5sum file 
068a9a19124df814e52ff5461598cfe4  file

To create a checksum file, redirect standard output to a file:

$ md5sum file > md5.checksum

To verify the file against the checksum file:

$ cd path/to/file
$ md5sum --check path/to/md5.checksum
file: OK

That said, m13r's implementations is equally valid.

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