UNIX for Dummies Questions & Answers

if I rename a dir
mv dir dir2

Is this operation atomic? Suppose there 100 files in dir, does linux rename them one by one or at once?

In other words, is there a time at which both dir and dir2 exist, with dir has, say 30 files and dir2 has the rest 70 files?

I'm really only familiar with the HP-UX version, but I'm guessing that Linux version must be similar. HP's mvdir is confined to operating in a filesystem...never between filesystems. To understand it, consider the first unix implemention of it... If you have /parent/A which has a lot of files in it and you move it to /parent/B, the original version would invoke the link system call and link A to B. At this point, both A and B exist and they are just two names to the same directory. Then, as soon as possible, it would unlink A. This would leave just B. The operation took two very quick system calls. This was NOT atomic. Between the system calls, the filesystem was in an illegal state. A and B should not be links to the same directory even for a millisecond. Today, instead of two system calls, we use just one...and rename(). This is absolutely atomic. For all intents and purposes A just morphs into B. A and B must be in the same filesystem, but not necessarily have the same exact parent as I have indicated here.

Within fileysystems mv may or may not be atomic. Across filesystems mv is not atomic.

Thanks all.
Can I get an official link or resource for whether mv is atomic? I am writing a tech document of them. Thanks.

i would start searching here: IEEE Std 1003.1, 2004 Edition

bakunin