On Sat, Oct 11, 2008 at 07:29:05PM -0600, Bob Proulx wrote: > Larry Clapp wrote: > > He asserts "The wait command waits until a program has *completely* > > finished", or words to that effect. > > That is specious reasoning at best.
Agree. > > Basically he says he's seen where a program does some redirection, > > exits, and the file isn't done yet: > > > > some_program > some_file > > # some_file isn't finished yet! > > wait > > # Now it's done! > > I rarely state things in absolutes and it makes me uncomfortable to > do so but let me say that this is just plain wrong. That isn't what > is happening. There is no basis for it. This might as well be fear > of stepping on a sidewalk crack or skipping the number 13 or > knocking on wood. This reads more of superstition and not of any > actual causality. Agree. > > I have not cornered him on the exact circumstances in which he's > > seen this behavior, but I certainly never have -- > > I think I can shed some light on this behavior. I am sure that he > is misinterpreting NFS filesystem buffering and its lack of cache > coherency. This type of problem is common in NFS environments. But > it has nothing to do with the above example. It occurs when > accessing files from different hosts using different filesystem > buffer caches. People accidentally trip into this problem very > often when using ssh/rsh or job queue systems or anything that > coordinates processes on different machines that access the same > files. This is a very common problem. I feel confident that this > is the root of the superstition and that workarounds for it are > being applied inappropriately in other contexts. > > Search the web for nfs cache coherency and specifically > close-to-open cache consistency and you should find much discussion > of the problems. See specifically the Linux NFS FAQ. I'll look into that, but ... > > Here are my thoughts on this behavior, most likely first: > > > > - I tend to think he had some code at one point that he didn't > > completely understand (or had forgotten some details of) that > > was running stuff in background and he didn't realize it, and he > > experienced this problem, and the "wait" fixed it, so he's > > scattered "waits" around his scripts ever since. > > I am sure you are correct here. I am sure that this person used to > work in an NFS environment across multiple machines and ran into NFS > cache coherency issues. I am confident this guess is very likely. ... even assuming cache coherency problems, I don't see how the extra "wait" would have fixed the problem. On the other hand, as I said, I've not nailed down the exact circumstances; I'd suspect he saw this problem at the command line, not in a script. Just the extra time of typing "wait" might solve the problem, leading to misguided belief in waiting unnecessarily. In any case, the exact *cause* of the superstition doesn't matter (though I definitely appreciate the pointer to NFS problems), I just wanted to poll the bash community whether anyone had seen a bug like this anywhere. > Good luck! > > Bob Thanks, Bob! -- Larry
