When files are being moved are they stored in a memory?

Question

When I move a file using the mv command, is this file first read, fully cached into a memory and then saved as another file? Or is it the same as writing

while read line; do
    echo "$line" >> output_file
done < input_file

Similarly with cp. When copying one file to another, is the file first fully cached into memory and then copied?

Answer 1

Often a mv is closer to a rename than a copy. In a classic unix type file system, the inodes that contain the file data won't be replicated if the source and the destination are on the same mount point. Instead a new filename is created that points to the same inodes, and the old filename is unlinked.

If the mv is to another mount point, then it will be an actual copy. What fraction of it is in RAM at a time is an OS detail whose transparency to the user is not defined.

Just to give an example, if the data was considered actually copied to a new place, the file might be, to the user, conceptually 'written' to the disk. But it could, at the OS level, actually be residing in a buffer that hasn't been flushed to the hardware yet.