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docker-alpine-cron

github-actions github-release docker-image-size

Dockerized alpine with busybox crond and useful tools.

Packages included for all images: curl, wget

Tags

Tag Dockerfile Build Context
:3.17, :latest View
:3.17-mysqlclient View
:3.17-openssl View
:3.17-mysqlclient-openssl View
:3.15, :latest View
:3.15-mysqlclient View
:3.15-openssl View
:3.15-mysqlclient-openssl View
:3.12, :latest View
:3.12-mysqlclient View
:3.12-openssl View
:3.12-mysqlclient-openssl View

Usage

Example 1: Create one cron that runs as root

docker run -it \
    -e CRON='* * * * * /bin/echo "hello"' \
    theohbrothers/docker-alpine-cron:3.17

Example 2: Create two crons that run as root

docker run -it \
    -e CRON='* * * * * /bin/echo "hello"\n* * * * * /bin/echo "world"' \
    theohbrothers/docker-alpine-cron:3.17

Example 3: Create two crons that run as UID 3000 and GID 4000

docker run -it \
    -e CRON='* * * * * /bin/echo "hello"\n* * * * * /bin/echo "world"' \
    -e CRON_UID=3000 \
    -e CRON_GID=4000 \
    theohbrothers/docker-alpine-cron:3.17

Environment variables

Name Default value Description
CRON '' Required: The cron expression. For multiple cron expressions, use \n. Use crontab.guru to customize crons. This will be set as the content of the crontab at /etc/crontabs/$CRON_USER
CRON_UID 0 Optional: The UID of the user that the cron should run under. Default is 0 which is root
CRON_GID 0 Optional: The GID of the user that the cron should run under. Default is 0 which is root

Entrypoint: docker-entrypoint.sh

  1. A /etc/environment file is created at the beginning of the entrypoint script, which makes environment variables available to everyone, including crond.
  2. A user of CRON_UID is created if it does not exist.
  3. A group of CRON_GID is created if it does not exist.
  4. The user of CRON_UID is added to the group of CRON_GID if membership does not exist.
  5. Crontab is created in /etc/crontabs/<CRON_USER>

Secrets

Since a /etc/environment file is created automatically to make environment variables available to every cron, any sensitive environment variables will get written to the disk. To avoid that:

  1. Add shell functions like this at the beginning of your cron-called script
  2. Optional: Specify the secrets folder by using environment variable ENV_SECRETS_DIR. By default, its value is /run/secrets
  3. Declare environment variables using syntax MY_ENV_VAR=DOCKER-SECRET:my_docker_secret_name, where my_docker_secret_name is the secret mounted on $ENV_SECRETS_DIR/my_docker_secret_name
  4. When the cron script is run, the env var MY_ENV_VAR gets populated with the contents of the secret file $ENV_SECRETS_DIR/my_docker_secret_name

FAQ

Q: Why use a cron container?

This image is only needed if there is no container orchestrator (E.g. Docker in standalone mode), or if there is a container orchestrator but without a cron scheduler (E.g. Docker in Swarm mode).

If your orchestrator already provides an external scheduler (E.g. Kubernetes via CronJob), there's no need to use this image.

Q: Why use CRON environment variable? Just use crontab -

Right. If the cron command is complex, and there is no need for customizing CRON_UID and CRON_GID, it is better to skip this image altogether and simply write everything in the entrypoint of the container manifest. For example, in Compose:

version: '2.2'
services:
  some-cron:
    image: alpine
    tty: true
    entrypoint:
      - /bin/sh
    command:
      - -c
      - |
          set -eu

          crontab - <<'EOF'
          * * * * * /bin/echo "My super 'complex' cron command with quotes is better written in shell"
          EOF

          exec crond -f

Q: My cron is not running!

  • Use docker logs to check whether crond has spit out any messages about the syntax of your cron
  • If bind-mouting your own crontab in /etc/crontabs, ensure:
    • the crontab is owned by root:root with 0600 permissions. See here
    • the crontab has a newline at the end of the file

Development

Requires Windows powershell or pwsh.

# Install Generate-DockerImageVariants module: https://github.com/theohbrothers/Generate-DockerImageVariants
Install-Module -Name Generate-DockerImageVariants -Repository PSGallery -Scope CurrentUser -Force -Verbose

# Edit ./generate templates

# Generate the variants
Generate-DockerImageVariants .