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Tips & Tricks

Remove unwanted files#

Especially during development, it can happen that files get generated when running your docker container that can only be removed by the root user again. If you do not have admin permissions (e.g., via sudo) on the machine then you can revert to using docker to remove them again:

  • change into the directory with unwanted files and/or directories

  • the following docker command will map the current directory to /workspace:

    docker run --rm -v `pwd`:/workspace -it bash:5.1.8

  • change into /workspace and remove all files/dirs (or just the files that need removing):

    cd /workspace rm -Rf *

Python#

Alternative Python version#

In case your base Debian/Ubuntu distribution does not have the right Python version available, you can get additional versions by adding the deadsnakes ppa:

add-apt-repository -y ppa:deadsnakes/ppa && \
apt-get update

Notes:

  • You may need to install package software-properties-common before you have add-apt-repository available.
  • Consider installed python3.x-full and libpython3.x to also get the venv and distutils packages installed.

  • For installing pip, use this:

    wget https://bootstrap.pypa.io/get-pip.py -O /tmp/get-pip.py && \ python3.7 /tmp/get-pip.py && \ rm /tmp/get-pip.py

Switch default Python version#

If your base distro has an older version of Python and you do not want to sprinkle the specific newer version throughout your Dockerfile, then you can use update-alternatives on Debian systems to change the default.

The following commands switch from Python 3.5 to Python 3.6 for the python3 executable and also use Python 3.6 as the default for the python executable:

update-alternatives --install /usr/bin/python python /usr/bin/python3.5 1 && \
update-alternatives --install /usr/bin/python python /usr/bin/python3.6 2 && \
update-alternatives --set python /usr/bin/python3.6 && \
update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.5 1 && \
update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.6 2 && \
update-alternatives --set python3 /usr/bin/python3.6 && \

Install packages with pip ignoring errors#

An installation with pip will fail if a dependency is not available, even if that dependency should not be required. The following command-line install each dependency separately, therefore continuing, even if errors are encountered (source):

cat requirements.txt | sed -e '/^\s*#.*$/d' -e '/^\s*$/d' | xargs -n 1 pip install

Screen#

The screen command-line utility allows you to pick up remote sessions after detaching from them. This is especially helpful when running docker in interactive mode via ssh sessions. If such an ssh session should accidentally close (e.g., internet connection lost, laptop closed), then the docker command would terminate as well.

On the remote host, start the screen command before you launch your docker command (you can run multiple screen instances as well) and skip the screen with the licenses etc using Enter or Space. Now you can start up the actual command. If you want to run multiple screen sessions on the same host, then you should name them using the -S sessionname option to easily distinguish them.

When running multiple sessions on a single host that use the auto-generated names, it can get difficult to distinguish them. You can use screen -S <PID>.<sessionName> -X sessionname <newSessionName> to rename an existing session to use a more suitable name (source).

For detaching a session, use the CTRL+A+D key combination. This will leave your process running in the background and you can close the remote connection.

In order to reconnect, simply ssh into the remote host and run screen -r. If there is only one screen session running, then it will automatically reconnect. Otherwise, it will output a list of available sessions. Supply the name of the session that you want to reattach to the -r option. In case a session got interrupted and not properly detached, use the -d flag to first detach it.

You can exit a screen session by either typing exit or using CTRL+D (just like with a bash shell).

If you need to increase the scrollback buffer, let us say to 100,000 lines, then you can do it

  • in the current session as follows:

    CTRL A : <Enter> scrollback 100000<Enter>

  • for all new sessions by adding the following line in your $HOME/.screenrc config file:

    defscrollback 100000

Default runtime#

Instead of always having to type --runtime=nvidia or --gpus=all, you can simply define the default runtime to use with your docker commands (source):

  • edit the /etc/docker/daemon.json file as root user
  • insert the key default-runtime with value nvidia so that it looks something like this:
{
    "default-runtime": "nvidia",
    "runtimes": {
        "nvidia": {
            "path": "nvidia-container-runtime",
            "runtimeArgs": []
        }
    }
}
  • save the file
  • restart the docker daemon (e.g., sudo systemctl restart docker)

Rebuild#

Rebuilding some layers#

Since docker hashes the layer instructions rather than the content, it will not detect changes to a file (e.g., modifications in a script) since the last build and will keep using the cached version of that layer.

Assuming the following Dockerfile snippet, where file1 changed:

RUN ...<layer X>...
COPY file1 ...<layer X+1>...
COPY file2 ...<layer X+2>...

You can inject a dummy ARG variable as follows to rebuild the layers from layer X+1 onwards:

RUN ...<layer X>...
ARG blah=1
COPY file1 ...<layer X+1>...
COPY file2 ...<layer X+2>...

Every time you need rebuild from there on, simply change the value that you are assigning to the variable, to change the hash value.

Of course, once your Dockerfile has been finalized, you can remove all these unnecessary operations and build your image properly.

Complete rebuild#

Especially during development of a docker image, it can happen that library versions have not been added to the Dockerfile just yet. Since docker hashes the command-lines, it will not rebuild a layer (and subsequent ones) if the hash does not change. If you do not want to clear your complete system (due to time constraints or capped internet usage), then you can force docker to build the image without using the hashed layers by adding the following flag to your build command:

--no-cache

Thorough clean up#

You can stop/remove all containers and images with this one-liner:

docker stop $(docker ps -a -q) && docker rm $(docker ps -a -q) && docker system prune -a

If you need to remove dangling volumes, use this:

docker volume prune

Excluding files/dirs from docker context#

By default, docker will use all files and directories within the directory that contains the Dockerfile to the daemon to use as context. From this context, the image gets built. Should you have test data/models in the same directory then this could mean sending quite a lot of unnecessary data to the daemon. To avoid this, you can take advantage of the .dockerignore file, which lists files and directories to ignore. The following example ignores the test directory and all vim backup files:

test/
*~

For more information, see the official documentation on the dockerignore file.