Bind mounts

Bind mounts have been around since the early days of Docker. Bind mounts have limited functionality compared to volumes. When you use a bind mount, a file or directory on the host machine is mounted into a container. The file or directory is referenced by its absolute path on the host machine. By contrast, when you use a volume, a new directory is created within Docker's storage directory on the host machine, and Docker manages that directory's contents.

The file or directory does not need to exist on the Docker host already. It is created on demand if it does not yet exist. Bind mounts are very performant, but they rely on the host machine's filesystem having a specific directory structure available. If you are developing new Docker applications, consider using named volumes instead. You can't use Docker CLI commands to directly manage bind mounts.

Tip

Working with large repositories or monorepos, or with virtual file systems that are no longer scaling with your codebase? Check out Synchronized file shares. It provides fast and flexible host-to-VM file sharing by enhancing bind mount performance through the use of synchronized filesystem caches.

Choose the -v or --mount flag

In general, --mount is more explicit and verbose. The biggest difference is that the -v syntax combines all the options together in one field, while the --mount syntax separates them. Here is a comparison of the syntax for each flag.

Tip

New users should use the --mount syntax. Experienced users may be more familiar with the -v or --volume syntax, but are encouraged to use --mount, because research has shown it to be easier to use.

• -v or --volume: Consists of three fields, separated by colon characters (:). The fields must be in the correct order, and the meaning of each field is not immediately obvious.

  • In the case of bind mounts, the first field is the path to the file or directory on the host machine.
  • The second field is the path where the file or directory is mounted in the container.
  • The third field is optional, and is a comma-separated list of options, such as ro, z, and Z. These options are discussed below.

• --mount: Consists of multiple key-value pairs, separated by commas and each consisting of a <key>=<value> tuple. The --mount syntax is more verbose than -v or --volume, but the order of the keys is not significant, and the value of the flag is easier to understand.

  • The type of the mount, which can be bind, volume, or tmpfs. This topic discusses bind mounts, so the type is always bind.
  • The source of the mount. For bind mounts, this is the path to the file or directory on the Docker daemon host. May be specified as source or src.
  • The destination takes as its value the path where the file or directory is mounted in the container. May be specified as destination, dst, or target.
  • The readonly option, if present, causes the bind mount to be mounted into the container as read-only.
  • The bind-propagation option, if present, changes the bind propagation. May be one of rprivate, private, rshared, shared, rslave, slave.
  • The --mount flag does not support z or Z options for modifying selinux labels.

The examples below show both the --mount and -v syntax where possible, and --mount is presented first.

Differences between -v and --mount behavior

Because the -v and --volume flags have been a part of Docker for a long time, their behavior cannot be changed. This means that there is one behavior that is different between -v and --mount.

If you use -v or --volume to bind-mount a file or directory that does not yet exist on the Docker host, -v creates the endpoint for you. It is always created as a directory.

If you use --mount to bind-mount a file or directory that does not yet exist on the Docker host, Docker does not automatically create it for you, but generates an error.

Start a container with a bind mount

Consider a case where you have a directory source and that when you build the source code, the artifacts are saved into another directory, source/target/. You want the artifacts to be available to the container at /app/, and you want the container to get access to a new build each time you build the source on your development host. Use the following command to bind-mount the target/ directory into your container at /app/. Run the command from within the source directory. The $(pwd) sub-command expands to the current working directory on Linux or macOS hosts. If you're on Windows, see also Path conversions on Windows.

The --mount and -v examples below produce the same result. You can't run them both unless you remove the devtest container after running the first one.

$ docker run -d \
  -it \
  --name devtest \
  --mount type=bind,source="$(pwd)"/target,target=/app \
  nginx:latest

$ docker run -d \
  -it \
  --name devtest \
  -v "$(pwd)"/target:/app \
  nginx:latest

Use docker inspect devtest to verify that the bind mount was created correctly. Look for the Mounts section:

"Mounts": [
    {
        "Type": "bind",
        "Source": "/tmp/source/target",
        "Destination": "/app",
        "Mode": "",
        "RW": true,
        "Propagation": "rprivate"
    }
],

This shows that the mount is a bind mount, it shows the correct source and destination, it shows that the mount is read-write, and that the propagation is set to rprivate.

Stop the container:

$ docker container stop devtest

$ docker container rm devtest

Mount into a non-empty directory on the container

If you bind-mount a directory into a non-empty directory on the container, the directory's existing contents are obscured by the bind mount. This can be beneficial, such as when you want to test a new version of your application without building a new image. However, it can also be surprising and this behavior differs from that of docker volumes.

This example is contrived to be extreme, but replaces the contents of the container's /usr/ directory with the /tmp/ directory on the host machine. In most cases, this would result in a non-functioning container.

The --mount and -v examples have the same end result.

$ docker run -d \
  -it \
  --name broken-container \
  --mount type=bind,source=/tmp,target=/usr \
  nginx:latest

docker: Error response from daemon: oci runtime error: container_linux.go:262:
starting container process caused "exec: \"nginx\": executable file not found in $PATH".

$ docker run -d \
  -it \
  --name broken-container \
  -v /tmp:/usr \
  nginx:latest

docker: Error response from daemon: oci runtime error: container_linux.go:262:
starting container process caused "exec: \"nginx\": executable file not found in $PATH".

The container is created but does not start. Remove it:

$ docker container rm broken-container

Use a read-only bind mount

For some development applications, the container needs to write into the bind mount, so changes are propagated back to the Docker host. At other times, the container only needs read access.

This example modifies the one above but mounts the directory as a read-only bind mount, by adding ro to the (empty by default) list of options, after the mount point within the container. Where multiple options are present, separate them by commas.

The --mount and -v examples have the same result.

$ docker run -d \
  -it \
  --name devtest \
  --mount type=bind,source="$(pwd)"/target,target=/app,readonly \
  nginx:latest

$ docker run -d \
  -it \
  --name devtest \
  -v "$(pwd)"/target:/app:ro \
  nginx:latest

Use docker inspect devtest to verify that the bind mount was created correctly. Look for the Mounts section:

"Mounts": [
    {
        "Type": "bind",
        "Source": "/tmp/source/target",
        "Destination": "/app",
        "Mode": "ro",
        "RW": false,
        "Propagation": "rprivate"
    }
],

Stop the container:

$ docker container stop devtest

$ docker container rm devtest

Recursive mounts

When you bind mount a path that itself contains mounts, those submounts are also included in the bind mount by default. This behavior is configurable, using the bind-recursive option for --mount. This option is only supported with the --mount flag, not with -v or --volume.

If the bind mount is read-only, the Docker Engine makes a best-effort attempt at making the submounts read-only as well. This is referred to as recursive read-only mounts. Recursive read-only mounts require Linux kernel version 5.12 or later. If you're running an older kernel version, submounts are automatically mounted as read-write by default. Attempting to set submounts to be read-only on a kernel version earlier than 5.12, using the bind-recursive=readonly option, results in an error.

Supported values for the bind-recursive option are:

Configure bind propagation

Bind propagation defaults to rprivate for both bind mounts and volumes. It is only configurable for bind mounts, and only on Linux host machines. Bind propagation is an advanced topic and many users never need to configure it.

Bind propagation refers to whether or not mounts created within a given bind-mount can be propagated to replicas of that mount. Consider a mount point /mnt, which is also mounted on /tmp. The propagation settings control whether a mount on /tmp/a would also be available on /mnt/a. Each propagation setting has a recursive counterpoint. In the case of recursion, consider that /tmp/a is also mounted as /foo. The propagation settings control whether /mnt/a and/or /tmp/a would exist.

Warning

Mount propagation doesn't work with Docker Desktop.

Before you can set bind propagation on a mount point, the host filesystem needs to already support bind propagation.

For more information about bind propagation, see the Linux kernel documentation for shared subtree.

The following example mounts the target/ directory into the container twice, and the second mount sets both the ro option and the rslave bind propagation option.

The --mount and -v examples have the same result.

$ docker run -d \
  -it \
  --name devtest \
  --mount type=bind,source="$(pwd)"/target,target=/app \
  --mount type=bind,source="$(pwd)"/target,target=/app2,readonly,bind-propagation=rslave \
  nginx:latest

$ docker run -d \
  -it \
  --name devtest \
  -v "$(pwd)"/target:/app \
  -v "$(pwd)"/target:/app2:ro,rslave \
  nginx:latest

Now if you create /app/foo/, /app2/foo/ also exists.

Configure the selinux label

If you use selinux you can add the z or Z options to modify the selinux label of the host file or directory being mounted into the container. This affects the file or directory on the host machine itself and can have consequences outside of the scope of Docker.

• The z option indicates that the bind mount content is shared among multiple containers.
• The Z option indicates that the bind mount content is private and unshared.

Use extreme caution with these options. Bind-mounting a system directory such as /home or /usr with the Z option renders your host machine inoperable and you may need to relabel the host machine files by hand.

Important

When using bind mounts with services, selinux labels (:Z and :z), as well as :ro are ignored. See moby/moby #32579 for details.

This example sets the z option to specify that multiple containers can share the bind mount's contents:

It is not possible to modify the selinux label using the --mount flag.

$ docker run -d \
  -it \
  --name devtest \
  -v "$(pwd)"/target:/app:z \
  nginx:latest

Use a bind mount with compose

A single Docker Compose service with a bind mount looks like this:

services:
  frontend:
    image: node:lts
    volumes:
      - type: bind
        source: ./static
        target: /opt/app/static
volumes:
  myapp:

For more information about using volumes of the bind type with Compose, see Compose reference on volumes. and Compose reference on volume configuration.

Next steps

• Learn about volumes.
• Learn about tmpfs mounts.
• Learn about storage drivers.