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Installation and configuration manual

In most cases the end user can download and execute udocker without system administrator intervention. udocker itself is written in Python, but also uses external binaries and libraries to provide a chroot like environment where containers are executed in user space. These tools do not require any privileges and constitute the udocker tools and libraries for engines that is downloaded and installed by udocker itself.

Redistribution, commercial use and code changes must regard all licenses shipped with udocker. For more information see section 6 External tools and libraries.

1. Dependencies

The udocker dependencies are minimal and should be supported by most Linux installations. udocker requires:

2. Installation

2.1. Install from a released version

Download a release tarball from

tar zxvf udocker-1.3.16.tar.gz
export PATH=`pwd`/udocker-1.3.16/udocker:$PATH

Alternatively use curl instead of wget as follows:

curl -L \
  > udocker-1.3.16.tar.gz
tar zxvf udocker-1.3.16.tar.gz
export PATH=`pwd`/udocker-1.3.16/udocker:$PATH

udocker executes containers using external tools and libraries that are enhanced and packaged for use with udocker. For more information see section 6 External tools and libraries. Therefore to complete the installation invoke udocker install to download and install the required tools and libraries.

udocker install

2.2. Install from the GitHub repositories

To install the latest stable code from the github master branch:

git clone --depth=1
(cd udocker/udocker; ln -s udocker)
export PATH=`pwd`/udocker/udocker:$PATH

Alternatively, install the latest development code from the github dev-v1.4 branch:

git clone -b dev-v1.4 --depth=1
(cd udocker/udocker; ln -s udocker)
export PATH=`pwd`/udocker/udocker:$PATH

Alternatively, install the latest development code from the github devel3 branch:

git clone -b devel3 --depth=1
(cd udocker/udocker; ln -s udocker)
export PATH=`pwd`/udocker/udocker:$PATH

udocker executes containers using external tools and libraries that are enhanced and packaged for use with udocker. For more information see section 6 External tools and libraries. Therefore to complete the installation invoke udocker install to download and install the required tools and libraries.

udocker install

2.3. Install from PyPI using pip

For installation with pip it is advisable to setup a Python3 virtual environment before proceeding, see Creating a virtual environment.

python3 -m venv udockervenv
source udockervenv/bin/activate
pip install udocker

The udocker command will be udockervenv/bin/udocker.

udocker executes containers using external tools and libraries that are enhanced and packaged for use with udocker. For more information see section 6 External tools and libraries. Therefore to complete the installation invoke udocker install to download and install the required tools and libraries.

udocker install

2.4. Install without outbound network access

When the installation is attempted without having outbound network connectivity, the installation of the binary tools and libraries with udocker install will fail the download step. The solution is to fetch the the tarball in advance and then install the tools and libraries directly from the tarball file.

The tarballs containing the tools and libraries are available at

First, download a tarball file using:

curl -L > udocker-englib-1.2.8.tar.gz

Second, transfer the udocker code plus the tarball containing the tools and libraries to the target destination host.

Finally perform the udocker install step using the transferred tarball file.

export UDOCKER_TARBALL=udocker-englib-1.2.8.tar.gz
udocker install

The environment variable UDOCKER_TARBALL can also point to an URL to fetch the tarball from a specific or alternate location. To fetch from multiple possible locations for redundancy the environment variable UDOCKER_TARBALL can contain multiple URLs separated by a blank.

export UDOCKER_TARBALL="https://... https://... http://..."
udocker install

2.5. Force the re-installation of the tools and libraries

To force download and re-installation of the udocker tools and libraries. Invoke udocker install with the flag --force:

udocker install --force

3. Configuration

The configuration of udocker has the following hierarchy:

  1. The default configuration options are set in the conf dictionary in the Config class.
  2. If a configuration file is present (section 4. Configuration files), it will override 1.
  3. If environment variables are set (section 5. Environment variables), they will override 2.
  4. The presence of general udocker command line options, will override 3.

3.1. Directories

With the default configuration, udocker creates files and subdirectories under $HOME/.udocker these are:

Both installed files, as well as the containers to be downloaded or created with udocker, will be installed by default under $HOME/.udocker.

A default configuration file is available at udocker.conf it can be copied to your udocker directory as $HOME/.udocker/udocker.conf and customized.

4. Configuration files

The configuration files allow overriding of the udocker Config class conf dictionary. Example of the udocker.conf syntax:

dockerio_registry_url = "https://myregistry.mydomain:5000"
http_insecure = True
verbose_level = 5

udocker loads the following configuration files if they are present:

  1. /etc/udocker.conf
  2. $HOME/.udocker/udocker.conf: overrides the options set in 1.
  3. $UDOCKER_DIR/udocker.conf (if different from the above): overrides the options set in 2.
  4. Configuration file set with the general CLI option --config=: overrides the options set in 3.

5. Environment variables

The following environment variables can be used to customize the installation. The location of the udocker directories can be changed via the environment variables:

The Docker index and registry can be overridden via the environment variables.

The verbosity level of udocker can be enforced. Removing banners and most messages can be achieved by executing with UDOCKER_LOGLEVEL=2.

Forcing the use of a given curl executable instead of pycurl can be specified with:

The fakechroot execution modes (Fn modes), the translation of symbolic links to the actual links can be controlled by the environment variable UDOCKER_FAKECHROOT_EXPAND_SYMLINKS. The default value is none which will select automatically the mode to be used, false if mounts are not performed or if the mount points pathname for the host and container are equal (e.g -v /home:/home), true otherwise (e.g -v /data:/home).

The location of some executables used by the execution modes can be enforced with the environment variables described below together with the default behavior. A value of UDOCKER will force the usage of the executables provided by the udocker installation.

A full pathname can be used to force selection of a specific executable (or library) from the host or from the udocker installation.

Several executables and libraries are shipped with udocker. For instance the executable for the Rn modes can be selected to be either runc or crun. This can be accomplished by setting UDOCKER_USE_RUNC_EXECUTABLE to the path of the desired executable. If runsc is available in the host it can also be selected in this manner.

# Forcing the use of crun instead of runc
export UDOCKER_USE_RUNC_EXECUTABLE=$HOME/.udocker/bin/crun-x86_64
udocker run <mycontainerid>

6. External tools and libraries

6.1. Source code repositories

udocker uses external tools and libraries to execute the created containers. The source code for the udocker tools and libraries is taken from several repositories. The F modes need heavily modified Fakechroot libraries and also a modified Patchelf both specifically improved to work with udocker. The Fakechroot for musl is a port of the Fakechroot library for the musl libc performed by the udocker development team. The P modes need a modified PRoot that includes fixes and enhancements to work with udocker. The R modes use the original runc and crun software with small changes for static compilation. The following table highlights the repositories used by udocker containing the modified source code and the original repositories.

Mode Engine Repository used by udocker Original repository
P PRoot
F Fakechroot glibc
F Fakechroot musl
F Patchelf

6.2. Software Licenses

Redistribution, commercial use and code changes must regard all licenses shipped with udocker. These include the udocker license and the individual licenses of the external tools and libraries packaged for use with udocker.

Mode Engine License
P PRoot GPL v2
F Fakechroot glibc LGPL v2.1
F Fakechroot musl LGPL v2.1
F Patchelf GPL v3
R runc Apache v2.0
R crun GPL v2

6.3. Binaries

As mentioned in the previous sections the compiled binaries can be installed with udocker install. Optionally they can be downloaded from the repository containing the binary builds at:

The executables are provided statically compiled for use across systems. The shared libraries that support the F modes need to match the libc within the container and are provided for several Linux distributions. See $HOME/.udocker/lib for the supported distributions and corresponding versions. The tools are also delivered for several architectures.

Mode Supported architecture
P x86_64, i386, aarch64 and arm
F x86_64, aarch64, ppc64le
R x86_64 aarch64
S uses the binaries present in the host system

6.3. Compiling

udocker already provides executables and libraries for the engines. These are statically compiled to be used across different Linux distributions. In some cases these executables may not work and may require recompilation. Use the repositories in section 6.2 if you which to compile the executables or libraries. Notice that the git repositories that are specific of udocker have branches or tags like UDOCKER-x where x is a number. Use the branch or tag with the highest number.

A notable case are the fakechroot libraries used in the Fn modes that need to match the libc in the container. This means that a must be produced for each different distribution release and intended architecture. Two implementations of the libc are supported glibc and musl, choose the one that matches the distribution inside the container. Once compiled the selection of the library can be forced by setting the environment variable UDOCKER_FAKECHROOT_SO.

udocker setup --execmode=F3 <mycontainerid>
UDOCKER_FAKECHROOT_SO=$HOME/  udocker run <mycontainerid>

The latest binary tarball can be produced from the source code using:

git clone -b devel3
cd udocker/utils

7. Central installation

udocker can be installed and made available system wide from a central location such as a shared read-only directory tree or file-system. The following guidelines should be followed when installing udocker in a central shared location or in a read only file system.

7.1. Install executables and libraries centrally

The executables and libraries can be installed with any of the methods described in section 2 of this manual. The directory tree should contain the following subdirectories: bin, containers, layers, lib, repos. For the binaries and libraries the only directories required are bin and lib.

The udocker tool should be installed as shown in section 2.1:

cd /sw
tar zxvf udocker-1.3.16.tar.gz

Directing users to the central udocker installation can be done using the environment variables described in section 5, or through the configuration files described in section 6. The recommended approach is to set environment variables at the user level as in the example where the assumed central location will be under /sw/udocker:

export UDOCKER_BIN=/sw/udocker/bin
export UDOCKER_LIB=/sw/udocker/lib
export PATH=$PATH:$UDOCKER_BIN:/sw/udocker

Note that the command udocker will be in /sw/udocker with all the python directory structure, while /sw/udocker/bin has all execution engines.

Make sure that the file protections are adequate namely that the files are not modifiable by others.

7.2. Images and layers in central installations

The repository of pulled images can also be placed in a different location than the user home directory $HOME/.udocker. Notice that if the target location is not writable then the users will be unable to pull new images, which may be fine if these images are managed centrally by someone else. Make sure that the file protections are adequate to the intended purpose.

From the images in the common location the users can then create containers whose content will be placed in the user home directory under $HOME/.udocker. This can be accomplished by redirecting the directories layers and repos to a common location. The users will need to set the following environment variables. Therefore assuming that the common location will be /sw/udocker:

export UDOCKER_REPOS=/sw/udocker/repos
export UDOCKER_LAYERS=/sw/udocker/layers

7.3. Containers in central installations

If a container is extracted to the common location, it is possible to point udocker to execute the container from that location. Making udocker pointing at different containers directory such as for example /sw/udocker/containers can be accomplished with:

export UDOCKER_CONTAINERS=/sw/udocker/containers

Assuming that the container is to be created under /sw/udocker/containers it can be extracted with:

export UDOCKER_CONTAINERS=/sw/udocker/containers
udocker --allow-root pull  centos:centos7
udocker --allow-root create  --name=myContainerId  centos:centos7
udocker --allow-root run  -v /tmp myContainerId

Notice the --allow-root should only be used when running from the root user. However depending on the execution mode and several other factors the limitations described in the next sections apply.

7.3.1. Selection of execution mode for central installations

The selection of the execution mode requires writing in the containers directory, therefore if the container is in a read-only location the execution mode cannot be changed. If a container is to be executed in a mode other than the default then this must be set in advance. This must be done by someone with write access. A table summarizing the execution modes and their implications:

Mode Engine Execution from readonly location
P1 PRoot OK
P2 PRoot OK
F1 Fakechroot OK
F2 Fakechroot OK
F3 Fakechroot OK see restrictions in section
R1 runc / crun OK requires udocker version above v1.1.7
R2 runc / crun OK see restrictions in section
R3 runc / crun OK see restrictions in section
S1 Singularity OK

Changing the execution mode can be accomplished with the following udocker command where <MODE> is one of the supported modes in column one.

udocker --allow-root setup --execmode=<MODE> myContainerId

Notice the --allow-root should only be used when running from the root user.

If the same container is to be provided for execution using more than one execution mode (e.g. to be executed with P1 and F3), then make copies of the initial container and setup each one of them with the intended mode. The command udocker clone can be used to create copies of existing containers. Mode F4 is not supported

The mode F4 is not suitable for readonly containers as it is meant to support the dynamic creation of new executables and libraries inside of the container, which cannot happen if the container is readonly. Use the mode F3 instead of F4. Mode F3 restrictions

The F3 mode (and also F4) perform changes to the container executables and libraries, in particular they change the pathnames in ELF headers making them pointing at the container location. This means that the pathname to the container must be always the same across all the hosts that may share the common location. Therefore if the original location pathname is /sw/udocker/containers then all hosts must also mount it at the same exact path /sw/udocker/containers. Modes R1, R2 and R3 general restrictions

These modes make use of runc or crun and require that user namespaces are enabled in the kernel. Older distributions may either not have support for namespaces (e.g. CentOS 6) or may have the support for user namespaces disabled at the system level (e.g. CentOS 7). More recent releases of Linux distributions do have support for user namespaces (e.g. CentOS 8 and CentOS 9).

For Centos 7 there are steps that system administrators may perform to enable user namespaces, such as:

sudo echo "user.max_user_namespaces=10000" >> /etc/sysctl.conf Modes R2 and R3 specific restrictions

Central installation from readonly location using any of the R modes requires udocker above v1.1.7. These modes require the creation of a mount point inside the container that is transparently created when the container is first executed, therefore (as recommended for all other modes) the container must be executed once by someone with write access prior to making it available to the users. Furthermore these execution modes are nested they use P1 or P2 inside the R engine, the Pn modes require a tmp directory that is writable. Therefore it is recommended to mount the host /tmp in the container /tmp like this:

udocker --allow-root run  -v /tmp myContainerId

Or alternatively:

export PROOT_TMP_DIR=/<path-to-host-writable-directory>
udocker --allow-root run  -v /<path-to-host-writable-directory>  myContainerId

Notice the --allow-root should only be used when running from the root user.

7.3.2. Mount directories and files in central installations

Making host files and directories visible inside the container requires creating the corresponding mount points. The creation of mount-points requires write access to the container. Therefore if a container is in a read-only location these files and directories must be created in advance.

Notice that some default mount points are required and automatically created by udocker itself, therefore the container should be executed by the administrator to ensure that the required files and directories are created. Furthermore if additional mount points are required to access data or other user files from the host, such mount points must also be created by the administrator by executing the container with the adequate volume pathnames. The example shows how to setup the default mount points and in addition create a new mount point named /data.

udocker --allow-root run -v /home:/data  myContainerId

Notice the --allow-root should only be used when running from the root user.

Notice that once /data is setup the end users can mount other directories in /data at runtime, meaning that users are not restricted to mount only the /home directory as the mapping is defined at run time.

7.3.3. Protection of container files and directories

For the container to be executed by other users the files and directories within the container must be readable. When udocker is installed in the user home directory all files belong to the user and are therefore readable by him. If a common location is shared by several users the file protections will likely need to be adjusted. Consider carefully your security policies and requirements when changing the file protections.

The following example assumes making all files readable to anyone and making all files (and directories) that have the executable bit to be also executable by anyone.

export mycdir=$(udocker --allow-root inspect -p myContainerId)
chmod -R uog+r $mycdir
find $mycdir -executable -exec chmod oug+x {} \;

Notice the --allow-root should only be used when running from the root user.

7.4. Using a common directory for executables and containers

If the common directory is used both for executables and containers then the following environment variables can be used:

export UDOCKER_DIR=/sw/udocker

8. Uninstall

udocker does not provide an uninstall command. udocker can be uninstalled by simply removing the created files and directories. The recommended approach is as follows:

  1. Fix permissions for all created containers for id in $(udocker ps | cut -f1 -d" " | grep -v CONTAINER); do udocker setup --fixperm $id; done
  2. Remove all created containers for id in $(udocker ps | cut -f1 -d" " | grep -v CONTAINER); do udocker rm -f $id; done
  3. Remove the udocker directory tree usually under $HOME/.udocker cd $HOME ; rm -Rf .udocker
  4. Remove the udocker Python code

The udocker directory tree contains the external executables, libraries, documentation, container images and container file system trees. By removing it all created containers will be also removed. Changing the file permissions might be required prior to deletion especially for the container file system trees in the containers subdirectory.

9. Quality assurance

The udocker software quality assurance follows the Common Software Quality Assurance Baseline Criteria for Research Projects DOI: available at

udocker uses the Jenkins Pipeline Library to implement Jenkins CI/CD pipelines for quality assurance.

9.1. Functional and integration tests

High level functional and integration tests used for quality assurance are available in These tests are also suitable to be executed by end-users to verify the installation. After cloning the udocker repository with git the bash scripts can be executed using:

cd utils

If the .udocker directory already exists these tests will not execute as they require a clean environment. In this case proceed as follows:

  1. rename the directory $HOME/.udocker, as in mv $HOME/.udocker $HOME/.udocker.ORIG
  2. execute the tests
  3. remove the $HOME/.udocker created by the tests
  4. restore the original .udocker directory as in mv $HOME/.udocker.ORIG $HOME/.udocker

9.2. Unit and security tests

The unit tests used in the software quality assurance pipelines are available at The tests can be executed after creating a virtualenv and installing the development requirements in requirements-dev.txt These tests are meant to be executed by the automated quality assurance pipelines.

virtualenv -p python3 ud3
source ud3/bin/activate
git clone
cd udocker
pip install -r requirements-dev.txt

The unit tests coverage can be executed using:

nosetests -v --with-coverage --cover-package=udocker tests/unit

Other tests configured in tox.ini, can be executed as well, such as linting (code style checking) and static security tests:

pylint --rcfile=pylintrc --disable=R,C udocker
bandit -r udocker -f html -o bandit.html