mmv can overwrite a file over already existing with the same name if it wasn't included in rename.

[MahouShoujoMivutilde]

How to reproduce

1. Create some files that you could track by content

/tmp/test ❯ for i in $(seq 1 4); do echo $i > "test_$i.txt"; done

/tmp/test ❯ ls
test_1.txt  test_2.txt  test_3.txt  test_4.txt

/tmp/test ❯ cat test_1.txt
1

2. Move test_1.txt -> test_3.txt and test_2.txt -> test_4.txt

/tmp/test ❯ mmv test_1.txt test_2.txt

3. Notice, original 3 and 4 .txt are gone without any warning, replaced by 1 and 2

/tmp/test took 9s ❯ ls
test_3.txt  test_4.txt

/tmp/test ❯ cat test_3.txt
1

/tmp/test ❯ cat test_4.txt
2

/tmp/test ❯

I think expected behavior here is for mmv to abort rename to prevent unintended data loss, or e.g. -f flag if overwrite is truly expected and desired.

[itchyny]

This is an expected behavior of mmv for now.

[MahouShoujoMivutilde]

Got it.

---

Ironically, after finishing implementing my own and 2 days after creating this issue, I found already existing project that has all the features I wanted, including such protection.

I'll leave the link here for anyone who comes from the search.

Docs:

...
  Use the --force flag to overwrite existing files that aren't part
  of a renaming cycle. (Existing directories are never overwritten.
  If you attempt to overwrite a directory the program will exit with
  an error message and a non-zero status code.)
...

I hope you won't find this too offensive...