The `docker container cp` utility copies the contents of `SRC_PATH` to the `DEST_PATH`.
You can copy from the container's file system to the local machine or the
reverse, from the local filesystem to the container. If `-` is specified for
either the `SRC_PATH` or `DEST_PATH`, you can also stream a tar archive from
`STDIN` or to `STDOUT`. The `CONTAINER` can be a running or stopped container.
The `SRC_PATH` or `DEST_PATH` can be a file or directory.

The `docker container cp` command assumes container paths are relative to the container's 
`/` (root) directory. This means supplying the initial forward slash is optional; 
The command sees `compassionate_darwin:/tmp/foo/myfile.txt` and
`compassionate_darwin:tmp/foo/myfile.txt` as identical. Local machine paths can
be an absolute or relative value. The command interprets a local machine's
relative paths as relative to the current working directory where `docker container cp` is
run.

The `cp` command behaves like the Unix `cp -a` command in that directories are
copied recursively with permissions preserved if possible. Ownership is set to
the user and primary group at the destination. For example, files copied to a
container are created with `UID:GID` of the root user. Files copied to the local
machine are created with the `UID:GID` of the user which invoked the `docker container cp`
command.  If you specify the `-L` option, `docker container cp` follows any symbolic link
in the `SRC_PATH`. `docker container cp` does *not* create parent directories for
`DEST_PATH` if they do not exist.

Assuming a path separator of `/`, a first argument of `SRC_PATH` and second
argument of `DEST_PATH`, the behavior is as follows:

- `SRC_PATH` specifies a file
    - `DEST_PATH` does not exist
        - the file is saved to a file created at `DEST_PATH`
    - `DEST_PATH` does not exist and ends with `/`
        - Error condition: the destination directory must exist.
    - `DEST_PATH` exists and is a file
        - the destination is overwritten with the source file's contents
    - `DEST_PATH` exists and is a directory
        - the file is copied into this directory using the basename from
          `SRC_PATH`
- `SRC_PATH` specifies a directory
    - `DEST_PATH` does not exist
        - `DEST_PATH` is created as a directory and the *contents* of the source
           directory are copied into this directory
    - `DEST_PATH` exists and is a file
        - Error condition: cannot copy a directory to a file
    - `DEST_PATH` exists and is a directory
        - `SRC_PATH` does not end with `/.` (that is: _slash_ followed by _dot_)
            - the source directory is copied into this directory
        - `SRC_PATH` does end with `/.` (that is: _slash_ followed by _dot_)
            - the *content* of the source directory is copied into this
              directory

The command requires `SRC_PATH` and `DEST_PATH` to exist according to the above
rules. If `SRC_PATH` is local and is a symbolic link, the symbolic link, not
the target, is copied by default. To copy the link target and not the link, 
specify the `-L` option.

A colon (`:`) is used as a delimiter between `CONTAINER` and its path. You can
also use `:` when specifying paths to a `SRC_PATH` or `DEST_PATH` on a local
machine, for example  `file:name.txt`. If you use a `:` in a local machine path,
you must be explicit with a relative or absolute path, for example:

    `/path/to/file:name.txt` or `./file:name.txt`

It is not possible to copy certain system files such as resources under
`/proc`, `/sys`, `/dev`, tmpfs, and mounts created by the user in the container.
However, you can still copy such files by manually running `tar` in `docker exec`.
For example (consider `SRC_PATH` and `DEST_PATH` are directories):

    $ docker exec foo tar Ccf $(dirname SRC_PATH) - $(basename SRC_PATH) | tar Cxf DEST_PATH -

or

    $ tar Ccf $(dirname SRC_PATH) - $(basename SRC_PATH) | docker exec -i foo tar Cxf DEST_PATH -


Using `-` as the `SRC_PATH` streams the contents of `STDIN` as a tar archive.
The command extracts the content of the tar to the `DEST_PATH` in container's
filesystem. In this case, `DEST_PATH` must specify a directory. Using `-` as
the `DEST_PATH` streams the contents of the resource as a tar archive to `STDOUT`.

# EXAMPLES

Suppose a container has finished producing some output as a file it saves
to somewhere in its filesystem. This could be the output of a build job or
some other computation. You can copy these outputs from the container to a
location on your local host.

If you want to copy the `/tmp/foo` directory from a container to the
existing `/tmp` directory on your host. If you run `docker container cp` in your `~`
(home) directory on the local host:

    $ docker container cp compassionate_darwin:tmp/foo /tmp

Docker creates a `/tmp/foo` directory on your host. Alternatively, you can omit
the leading slash in the command. If you execute this command from your home
directory:

    $ docker container cp compassionate_darwin:tmp/foo tmp

If `~/tmp` does not exist, Docker will create it and copy the contents of
`/tmp/foo` from the container into this new directory. If `~/tmp` already
exists as a directory, then Docker will copy the contents of `/tmp/foo` from
the container into a directory at `~/tmp/foo`.

When copying a single file to an existing `LOCALPATH`, the `docker container cp` command
will either overwrite the contents of `LOCALPATH` if it is a file or place it
into `LOCALPATH` if it is a directory, overwriting an existing file of the same
name if one exists. For example, this command:

    $ docker container cp sharp_ptolemy:/tmp/foo/myfile.txt /test

If `/test` does not exist on the local machine, it will be created as a file
with the contents of `/tmp/foo/myfile.txt` from the container. If `/test`
exists as a file, it will be overwritten. Lastly, if `/test` exists as a
directory, the file will be copied to `/test/myfile.txt`.

Next, suppose you want to copy a file or folder into a container. For example,
this could be a configuration file or some other input to a long running
computation that you would like to place into a created container before it
starts. This is useful because it does not require the configuration file or
other input to exist in the container image.

If you have a file, `config.yml`, in the current directory on your local host
and wish to copy it to an existing directory at `/etc/my-app.d` in a container,
this command can be used:

    $ docker container cp config.yml myappcontainer:/etc/my-app.d

If you have several files in a local directory `/config` which you need to copy
to a directory `/etc/my-app.d` in a container:

    $ docker container cp /config/. myappcontainer:/etc/my-app.d

The above command will copy the contents of the local `/config` directory into
the directory `/etc/my-app.d` in the container.

Finally, if you want to copy a symbolic link into a container, you typically
want to  copy the linked target and not the link itself. To copy the target, use
the `-L` option, for example:

    $ ln -s /tmp/somefile /tmp/somefile.ln
    $ docker container cp -L /tmp/somefile.ln myappcontainer:/tmp/

This command copies content of the local `/tmp/somefile` into the file
`/tmp/somefile.ln` in the container. Without `-L` option, the `/tmp/somefile.ln`
preserves its symbolic link but not its content.
