On Tue, May 20, 2025 at 11:35:14AM -0400, Lee wrote: > On Tue, May 20, 2025 at 10:08 AM tomas wrote: > > > > On Tue, May 20, 2025 at 09:45:11AM -0400, Lee wrote: > > > On Tue, May 20, 2025 at 9:33 AM wrote: > > > > [...] > > > > > > What do you do if you get two USBs containing file systems with > > > > the same UUID? > > > > > > Is that possible? I suppose it is.. so I'd go looking for how to > > > change the UUID for one of the usb drives. > > > > If you copy a whole file system (e.g. with dd), the UUID will > > travel with it. It's just data, after all. Sometimes you want it, > > sometimes not. > > If I'm copying from one usb to another it seems like I really would > not want the same UUID on both usb sticks. > And it sounds like it would be fairly easy to fix the duplicate UUID problem..
Yes, but copy programs (be it dd or cp or whatever you use) just see a "stream of bytes" and try, as hard as they can, to copy it faithfully. Of course, if you *make* a second file system and copy the content, the UUID will be most probably different (you can set it, for ext* file systems with tune2fs). Sometimes you *want* the same UUID. Imagine an application which starts automatically when it "sees" a medium with a given UUID inserted (I once made such a thing for a customer: an udev rule kicked off a backup when the right USB disk was inserted into a headless box). If you make a backup medium for the case your primary breaks, you *want* the same UUID. > > If you then modify one file system's content, you'll get two > > file systems with the same UUID and... with different data =:-o > > > > The horrors :) > > > > Remember: the second "U" in "UUID" is a lie ;-) > > uhmm... to remember something one first has to know/learn it. > I'm asking all these ignorant questions because I'm not there yet. It wasn't meant personally, I'd be very sorry if I came across like that. I just wanted to counter the all too uncritical perception that UUIDs Are Good. They are useful, but to make use of them one should know their properties :-) Cheers -- tomás
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