Can't overwrite a file by moving another over it

Question

I have file /some/foo which is a plain text file. I can edit it with vim and I can cat something else to it. But whenever I try to automate the editing process with sed, it says

sed: cannot move '/some/sedGl29Uj': Device or resource busy

I tried redirecting sed's output to a temporary file and cat, it's OK. But if I try to move the temporary file onto it, it will say

root@localhost:/some # mv tmp foo
mv: Device or resource busy

I'm quite concerned why vim and cat can write the file but mv can't.
Ubuntu 16.04.1 LTS. /some is a subfolder of / (root mount point, ext4).

Answer 1

Found the answer when managing drive mounts. A strange line showed up when I was checking mounted drives.

~ # mount | grep /etc/hosts
/tmp/hosts on /etc/hosts type tmpfs (rw,nosuid,nodev,noexec,bind)

That made me surprised. After digging all the scripts that I've used, I found this line in a hosts modifier script

sudo mount -o bind /tmp/hosts /etc/hosts

Then I tried something else:

/tmp # echo aaa > a
/tmp # mount -o bind a b; cat b
aaa
/tmp # echo bbb > b; cat a
bbb
/tmp # echo ccc > c
/tmp # mv c b
mv: Device or resource busy

The solution is, of course, cancel that mount point:

sudo umount /etc/hosts

Answer 2

It is likely you don't have write access to the directory. The file operations: create, delete and rename all modify the directory.

cat and vim just modify the file contents and don't need to modify the file. sed -i writes a temporary file and renames it.