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Mv

The meaning of «mv»

mv (short for move) is a Unix command that moves one or more files or directories from one place to another. If both filenames are on the same filesystem, this results in a simple file rename; otherwise the file content is copied to the new location and the old file is removed. Using mv requires the user to have write permission for the directories the file will move between. This is because mv changes the content of both directories (i.e., the source and the target) involved in the move. When using the mv command on files located on the same filesystem, the file's timestamp is not updated.

On UNIX implementations derived from AT&T UNIX, cp, ln and mv are implemented as a single program with hard-linked binaries. The behavior is selected from the path name argv[0]. This is a common technique by which closely related commands that have been packaged as a unit allow the user to specify the particular course of the intended action. GNU coreutils does the same.

The version of mv bundled in GNU coreutils was written by Mike Parker, David MacKenzie, and Jim Meyering.[1] mv itself dates back to X/Open Portability Guide issue 2 of 1987.[2]

When a filename is moved to an existing filename, the existing file is deleted. If the existing file is not writable but is in a directory that is writable, the mv command asks for confirmation (if run from a terminal) before proceeding, unless the -f (force) option is used.

A related ambiguosity arises when a filename is moved to an existing directory. By default, mv would handle this as one trying to move a name inside this directory. GNU mv has a .mw-parser-output .monospaced{font-family:monospace,monospace}-T switch for disabling this assumption and try to overwrite the directory instead. An inverse -t makes the move-to-directory operation explicit.[1]

Moving files within the same file system is generally implemented differently than copying the file and then removing the original. On platforms that do not support the rename syscall, a new link is added to the new directory and the original one is deleted. The data of the file is not accessed. All POSIX-conformant systems implement the rename call.

An actual move (effectively a rename) is dramatically faster than the circuitous copy-and-move procedure. The file's i-number (short for "i-node number") does not change. No permission is required to read the file being moved insofar as—conceptually speaking—it is only cataloguing information that is being changed as a result of the "move." Since the source and target directories are being modified, to wit, entries are being created within the target directory and erased from within the source directory, "write" permission in both directories is required to complete the move. Moving files from one file system to another may fail entirely or may be automatically performed as an atomic copy-and-delete action; the actual details are dependent upon the implementation.

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