Why unix mv program doesn't need -R (recursive) option for directories but cp does need it?

A directory is (conceptually) a special "file" which contains a list of names, and the inode numbers those names point to. Some of names can be subdirectories. There is a special entry .. which points to the parent directory.

So, its clear, changing the name of a file is easy: you just change the name in the directory entry, nothing else. This holds whether the file is actually a file, or is a "file" used to store another directory's contents. Indeed, the same rename syscall does both.

Copying, however, is a much less trivial operation. You could just copy the directory "file", but then you'd have two directories where are the files are the same (they'd be hardlinks). If you had a system that allows hardlinks to directories, those would be to, but since no modern system allows that, at least to non-root, you have to do that copy for each subdirectory. You can actually ask cp for this behavior with cp -lR: -l for hard link, -R for that recursion.

But leaving everything linked is likely not what you want. Instead, you want cp to copy each file. That's a fairly expensive operation: each file must be read into memory, and written back out to disk in a second location. It actually takes several syscalls, to open, read, write, and close the files, and that has to be repeated for each file.

Traditional filesystems work this way on disk, too. There isn't any way to copy a bunch of files, other than to go through each one individually and copy it, and those are the types of filesystems that were in use when the basic command line utilities were designed.

Let me start by asking another question:

What is the difference between cp and cp -R?

Well, without the -R flag, it's only possible to copy files, because it's rather unusual that someone wants to non-recursively copy a directory: A non-recursive copy would just result in a second name for the directory, pointing to directly the same directory structure. Because that's rarely what people want, and there is actually a separate program that does this (ln), a non-recursive copy of directories is not allowed.

What then could be the difference between mv and mv -R?

mv a b just renames a single entry in the directory, so if a directory is mved, its contents are automatically also moved. In that sense, mv already provides the recursive property, i.e. the "renaming" of all entries in the renamed directory, e.g. from a/1 to b/1. A mv that does not do that, i.e. which renames a directory a to b, but keeps a/1 as a/1, is not what people understand when they refer to moving something: When you move a cupboard, the contents of the cupboard are moved as well. That operation is also already available, it's called mkdir.

Usually when I'm messed up with Unix logic I look at Plan9 to see how Unix inventors' implemented the same tasks years later without stucking on backward compatibility.

So Plan9 offers cp and mv tools to operate with files only.

`cp f1 f2` creates f2 and copies f1's contents into it.
`mv f1 f2` renames f1 to f2 if f1 and f2 are in the same dir
           does `cp f1 f2 && rm f1` else
           can rename dirs (`mv d1 d2`) but will not move dir to another dir.

For copying a dir there's dircp that is really @{cd fromdir && tar c .} | @{cd todir && tar xT} (rc shell syntax)

For moving a dir I think there's only dircp d1 d2 && rm -r d1

I think this decision to limit cp and mv for file operations only (not dirs) brings more clearness to disk operations and using tar for copying file trees is very comfortable for understanding and scripting.