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how do I use this utility?
Do you mean the md5sum command? For that it's just md5sum , or use the switch -b before the filename to use it in binary mode.
What it does is generate a string of bizarre characters, technically known as an md5 checksum. These bizarre characters have the following useful properties:
- It is virtually impossible to work out the original file from the characters
- A small change in the file will result in a large change in the checksum
- The same file will always produce the same bizarre string
- The checksum is always the same (fairly short) length, regardless of the length of the file
This means that if someone puts a file up on a website, they can fairly easily put up the checksum as well.
And if you download the file, and then run md5sum , and your checksum is the same as the checksum on the website, then you can be very sure that there were no transmission errors and you've got the whole file. (Technically speaking, there is a tiny tiny (1 in 2^128, roughly) chance that you've got an erroneous or truncated file that produces the same checksum; but you've got several million times more chance of winning the lottery than that).