To compare today’s version of a file to last year’s version you might do something like this:

$ sha256sum newcopy.jpg oldcopy.jpg
ce5d948cf9bfe9d3709cef57016b4e1c6db5391b9efca4c1ef958fe557d81e1b  newcopy.jpg
56bb613661ebb20ab212f748caff2023f80f9854ada17c7001af4a1f9d30b6aa  oldcopy.jpg

The different checksums indicate differences in the file that a comparison of file size might not show up. But checking sums of JPEGs is complicated by software which modifies the EXIF header. Digikam, for example, will change the image header whenever you correct the timestamp, do a rotation, add a comment or tag, or basically do any metadata operation. The image itself might never be altered, but the JPEG file can go through many iterations. If your incremental backups are perfect, you should still be able to find a backed-up copy of what today’s file should be, but unless you’re using some kind of wicked insane ZFS system, your last backup will have a non-zero age.

In my particular case, I suspected a photo had become corrupted, but couldn’t prove it, as past versions from my rdiff-backup snapshots had a different checksum at different points in time, reflecting changes that had been made to the metadata by successive versions of Digikam and occasional tagging operations. Because the file had been changed, I couldn’t use checksums to prove the difference was due to corruption.

jhead to the rescue!

jhead is a JPEG metadata utility (and an Ubuntu package of the same name). Use it like this to strip all metadata out of a JPEG file, leaving only the pure image:

$ jhead -purejpg newcopy.jpg oldcopy.jpg
Modified: newcopy.jpg
Modified: oldcopy.jpg
$ sha256sum newcopy.jpg oldcopy.jpg
42f61a8e8d78153a41cc9c7fcb20055ed3e403c88366395a577bea64166218a9 newcopy.jpg
42f61a8e8d78153a41cc9c7fcb20055ed3e403c88366395a577bea64166218a9 oldcopy.jpg

As it turned out, the image was not corrupted at all! Yay!

Meanwhile, One Laptop Per Child appears to be running into difficulties. The project’s “extreme dependence on scale to bring down cost” is worryingly familiar; like collective farming, their economies of scale have not yet been great enough to overcome the massive disadvantages of central planning.

And now Brazil is rolling out KDE to 52,000,000 children. The project is called “Um Computador por Aluno” - One Computer Per Student. It may not match the headline features of OLPC, like mesh networking and a cool low-power display, but it looks more likely to work in the real world. In this respect, OLPC may be more like the Soviet space programme than the Soviet state farms: the standout achievements of the Russians, like Sputnik and Yuri Gagarin, were impressive and noteworthy, but the system ultimately couldn’t compete with the more open and dynamic American space programme.