On 3/20/24 12:43, Bernhard Voelker wrote:
This stems from the fact that although mv(1) is a userland frontend for renameat(2), the user interface is different: while renameat(2) deals exactly with 2 operands, mv(1) has always been able to work on more arguments.
Yes, that's mv's original sin, which we cannot realistically change now.
Now, extending "exchange" to more arguments is confusing and the use is not intuitive: mv -v --exchange a b c d An "exchange" can literally only be applied to 2 files,
Sure, but that's true for "rename" too: a "rename" can be applied only to 2 files.
When d is a directory, "mv a b c d" does three renames so it is like "mv a d/a; mv b d/b; mv c d/c". This remains true if you uniformly replace "mv" with "mv --exchange", which does three exchanges.
I have the gut feeling that we didn't think through all cases, and that some might be surprising, e.g.: $ mkdir d; echo 1 > a; echo 2 > d/a $ src/mv --exchange a a a a d/a versus $ src/mv --exchange a a a a d/a
I don't understand the word "versus" here, as the two examples look the same to me.
If d/a is not a directory, the example is an error, just as it would be without --exchange.
If d/a is a directory and you have permissions etc., "mv a a a a d/a" is like attempting "mv -T a d/a/a; mv -T a d/a/a; mv -T a d/a/a; mv -T a d/a/a". If you use plain "mv" only the first "mv -T a d/a/a" succeeds because "a" goes away, so you get three diagnostics for the remaining three "a"s. If you use "mv --exchange" all four "mv --exchange -T a d/a/a" attempts succeed, and since there are an even number of exchanges the end result is a no-op except for updated directory timestamps. So I don't see any ambiguity about what mv should do with this contrived example.
