Given a folder of files that are being copied from one place to another, sometimes it is difficult to see whether or not the files were copied in their entirety, i.e. a file transfer between computers is interrupted or whatnot. Yes, the file sizes might be different, but there's always the possibility that the contents of the files might be identical as far as the actual *amount* of data, but not for the *contents* of the data. Hence the use of MD5. I've also found this to be one of the few times when the 'transparency' option of Terminal.app is completely indispensible.
In Terminal, cd into the source directory. If it's just a series of files being copied, then typing MD5 * will result in a series of checksums being performed on every file in the directory. Now cd into the desintation directory, and repeat the command. A single byte change in any of the files will result in a different checksum being generated. If the checksums are identical, then the files are identical.
Now comes the fun part with Terminal and transparency. Instead of having to manually check the numbers produced by MD5, try this. Open two Terminal windows, one in the source directory, and one in the target. After you run the MD5 commands, make one window partially transparent, and then slide it over the other until the file names line up. The checksums should also line up. Any discrepancy between the two will be very obvious to see.
Yes, you could always do this:
source_dir: MD5 * > ~/MD5checksum1.txt
target_dir: MD5 * > ~/MD5checksum2.txt
home_dir: diff MD5checksum2.txt MD5checksum1.txt
However, this visual method is far faster for the occasional time when this situation comes up. It's particularly useful when you need to compare files across great distances, instead of transferring them again 'just to be sure.'
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