Hi list! This is more a general POSIX question but I guess here I have the best chance to get a definite answer.
If I want to replace a file with another file without removing the first one and without having a moment in time at which the file name does not exist, I can use the following sequence: # swap $file1 with $file2 on the same file system dir=$(dirname "$file1") tmp=$(mktemp -u -d "$dir") # [1] ln "$file1" "$tmp" mv "$file2" "$file1" mv "$tmp" "$file2" This works because mv does not affect the hardlink to $file1 and a rename operation on a single file system is atomic. This is a handy procedure when you have a background process which occasionally opens $file1 and you don't want it to fail just because of bad timing. Now my question is: How can I do a similar thing for a directory? I cannot usually create hardlinks on directories (are there file systems where this actually works?) and I cannot use mv to overwrite one directory with another. The only idea I currently have is to create a level of indirection via symlinks and then atomically overwrite the symlink. Is there any other way? [1] Yes, I know I need to iterate this and the next line in case a different process creates $tmp in between syscalls. Putting in a few -T and "--" switches might also help but let's keep it simple for the moment. Thanks in advance! Florian Philipp
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