HowToMD5SUM

Revision 1 as of 2006-03-18 20:18:45

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Introduction

MD5SUMs are a type of checksum. Checksums are a way of verifying data integrity, in other words, they can detect errors. When you download a large file like a CD .iso, it is possible that there are errors, simply because the probability of an error in a file is proportional to the size of the file. When you have a file that must be correct, e.g. an install CD for an operating system, it is a very good idea to run a check on it. In addition, checksums provide a measure of authenication, so you know that your copy is the same as the official copy, and has not been altered in some way. There are actually better ways of authentication than simple checksums, but they are better than nothing.

MD5SUM on Unix

Most Linux distributions come with the md5sum utility, so there is usually no need to install it. When you download a CD .iso file, on the same page there should be an md5sum file. If your .iso file is named my_cd.iso, then the md5sum file is generally named my_cd.iso.md5sum, my_cd.md5sum, my_cd.txt, or some similar variant. The Ubuntu CDs all have a common md5sum file named MD5SUM. Download this file to the same place as the .iso file. When you have both files, run

cd download_directory
md5sum --check md5sum_file

This will cause md5sum to look at all the files listed in the md5sum file (one of which should be your .iso), and compare the checksum of the .iso with the checksum recorded in the md5sum file. If they match, you're good to go. If they don't, redownload the .iso file.

You can check files without an md5sum file as well (but you still need to get the checksum from somewhere!). Just run

md5sum file_to_check

and manually compare the output of that command with the checksum from the website.

MD5SUM on Windows

Windows does not come with an md5sum utility. You must download one from another location, and preferably a location that you trust. There is a commandline md5sum.exe that works similarly to the Unix utility, and a graphical version that you may be more comfortable with. Some of the links in the External Links section have additional howtos and offer checksum utility downloads.