Directory Overlay


The Directory Overlay Update Module installs a user defined file tree structure into a given destination directory in the target.

Before the deploy into the destination folder on the device, the Update Module will take a backup copy of the current contents, allowing restore of it using the rollback mechanism of the Mender client if something goes wrong. The Update Module will also delete the current installed content that was previously installed using the same module, this means that each deployment is self contained and there is no residues left on the system from the previous deployment.

Example use-cases:

  • Deploy root filesystem overlays


Module name dir-overlay
Supports rollback yes
Requires restart no
Artifact generation script yes
Full system updater no
Source code Update Module, Artifact Generator

Please be aware that Mender Update modules are meant to update parts of the system and if not configured properly they could potentially delete parts or the complete system. Always inspect the code carefully and only test modules on a systems that you can recover easily.

It is not recommended to install a Mender Artifact on your workstation

Prepare the device

This section describes how to setup your target device, i.e. the device to be updated. This will also be referred to as the device environment.

All commands outlined in this section should be run in the device environment.


This update module has the following prerequisites for the device environment:

The dir-overlay module will store state information in /var/lib/mender, and that is:

  • Manifest files of current and previous installation
    • list of files
  • Backup of current installed files based on previous manifest, which is used in case of roll-back

Sample directory layout:

$ ls /var/lib/mender/dir-overlay/
backup.tar     manifest       manifest.prev

The dir-overlay directory is created by the module.

This Update Module has the following prerequisites for the development environment:

Install the Update Module

Download the latest version of this Update Module by running:

mkdir -p /usr/share/mender/modules/v3 && wget -P /usr/share/mender/modules/v3 && chmod +x /usr/share/mender/modules/v3/dir-overlay

Prepare the development environment on your workstation

This section describes how to set up your development environment on your workstation.

All commands outlined in this section should be run in the development environment.


This Update Modules has the following prerequisites for the development environment:

Create Mender Artifacts

For convenience, an Artifact generator tool dir-overlay-artifact-gen is provided along the module. This tool will generate Mender Artifacts in the same format that the Update Module expects them.

To download dir-overlay-artifact-gen, run the following:


Make it executable by running,

chmod +x dir-overlay-artifact-gen

Create example root filesystem content to deploy,

# The permissions must match DEST_DIR on the device, otherwise this module will
# overwrite the set permissions.
# You can check this by running the following command on the device (if DEST_DIR="/"):
#    stat -c %a /
sudo install -d -m 755 -g root -o root rootfs_overlay

# Create directory structure
sudo install -d -m 755 -g root -o root rootfs_overlay/etc
sudo install -d -m 755 -g root -o root rootfs_overlay/usr/bin
sudo install -d -m 755 -g root -o root rootfs_overlay/usr/share/app

# Create sample files
sudo touch rootfs_overlay/etc/app.conf
sudo touch rootfs_overlay/usr/bin/app
sudo touch rootfs_overlay/usr/share/app/data.txt

# You would typically chmod above files to have the appropriate mode in a production environment

The result should look like this:

$ tree rootfs_overlay/
├── etc
│   └── app.conf
└── usr
    ├── bin
    │   └── app
    └── share
        └── app
            └── data.txt

5 directories, 3 files

Now generate your Artifact using the following command:

./dir-overlay-artifact-gen -n ${ARTIFACT_NAME} -t ${DEVICE_TYPE} -d ${DEST_DIR} -o ${OUTPUT_PATH} ${OVERLAY_TREE}
  • ARTIFACT_NAME - The name of the Mender Artifact
  • DEVICE_TYPE - The compatible device type of this Mender Artifact
  • OUTPUT_PATH - The path where to place the output Mender Artifact. This should always have a .mender suffix
  • DEST_DIR - The path on target device where content of OVERLAY_TREE will be installed.
  • OVERLAY_TREE - The path a folder containing the contents to be installed on top of the DEST_DIR on the device in the update.

You can either deploy this Artifact in managed mode with the Mender server as described in Deploy to physical devices or by using the Mender client only in Standalone deployments.

Artifact technical details

The Mender Artifact used by this Update Module has three payload files:

  • update.tar - a tarball containing all the files to be deployed
  • dest_dir - a regular file with the DEST_DIR in plain text
  • manifest - a regular file with a list of files installed in plain text

The Mender Artifact contents will look like:

Artifact my-update-1.0.mender generated successfully:
Mender artifact:
  Name: my-update-1.0
  Format: mender
  Version: 3
  Signature: no signature
  Compatible devices: '[my-device-type]'
  Provides group:
  Depends on one of artifact(s): []
  Depends on one of group(s): []
  State scripts:

    Type:   dir-overlay
    Provides: Nothing
    Depends: Nothing
    Metadata: Nothing
      name:     update.tar
      size:     10240
      modified: 2019-04-01 12:01:41 +0000 UTC
      checksum: fabfd7f6739d35130dd05dbab42ff7fe8baa76fea22f134b254671bd52604807
      name:     dest_dir
      size:     2
      modified: 2019-04-01 12:01:41 +0000 UTC
      checksum: f465c3739385890c221dff1a05e578c6cae0d0430e46996d319db7439f884336
      name:     manifest
      size:     48
      modified: 2019-04-01 12:01:41 +0000 UTC
      checksum: fc582174946ec48e2782ed9357e9d4389cc0e2a3bfbc261b418d4fd5a326d472
1 Like

hmm, what is the difference between Directory and Directory Overlay modules?

Directory - Here you can choose one path and all the files inside the selected path will be updated.
Ex: /etc/conf where all files inside the conf directory will update.
Please note that, this is selecting only a single path.

Directory Overlay - Here you can select multiple directories and all files from multiple directories can be updated.
Ex: /etc/conf and /etc/bin where all files of these 2 directories can be updated.
Please note that, here the user can select and update multiple locations at once.

The advantage of each method depends on the usage of the update :slight_smile:


4 posts were split to a new topic: State-script support in dir-overlay Update Module

10 posts were split to a new topic: Issues with dir-overlay Update Module rollback