import implementation

.travis.yml

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+language: generic
+sudo: required
+services:
+  - docker
+
+before_script:
+  - docker login -u "${DOCKERHUB_USERNAME}" -p "${DOCKERHUB_PASSWORD}"
+script:
+  - docker build -t infolinks/crond:${TRAVIS_COMMIT} .
+  - |
+    if [[ ${TRAVIS_TAG} =~ ^v[0-9]+$ ]]; then
+      docker tag infolinks/crond:${TRAVIS_COMMIT} infolinks/crond:${TRAVIS_TAG}
+      docker push infolinks/crond:${TRAVIS_TAG}
+      docker tag infolinks/crond:${TRAVIS_COMMIT} infolinks/crond:latest
+      docker push infolinks/crond:latest
+    fi
+after_script:
+  - |
+    if [[ "${TRAVIS_BRANCH}" == "master" ]]; then
+      docker run infolinks/github-release \
+          --token="${GITHUB_ACCESS_TOKEN}" \
+          --repo="${TRAVIS_REPO_SLUG}" \
+          --commit="${TRAVIS_COMMIT}"
+    fi

Dockerfile

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+FROM google/cloud-sdk:171.0.0-alpine
+MAINTAINER Arik Kfir <arik@infolinks.com>
+COPY ./bin/*.sh /usr/local/bin/
+RUN apk add --no-cache dcron bash && \
+    chmod a+x /usr/local/bin/crond.sh /usr/local/bin/job-executor.sh
+ENTRYPOINT ["/usr/local/bin/crond.sh"]

README.md

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 # crond
-Docker image running cron daemon for executing periodic jobs.
+
+Docker image running cron daemon for executing a periodic job.
+
+This container receives (via environment variables) a job to run and the
+periodic schedule to run it on, and it will start a `crond` server which
+will execute that given job on the specified schedule.
+
+## Running
+
+This image can be either extended by another Docker image, preloading
+its job, or alternatively it can be run as is, with the actual job and
+its schedule to be provided externally (via a mount & environment
+variables).
+
+### Running as a standalone image
+
+Here's how to run an hourly job at the beginning of each hour:
+
+    docker run \
+        -v /path/to/my-job.sh:/usr/local/bin/my-job.sh \
+        -e "SCHEDULE='0 * * * *'" \
+        -e "JOB_CMDLINE='/usr/local/bin/my-job.sh arg1 arg2 arg3'" \
+        infolinks/crond
+
+This will run `/usr/local/bin/my-job.sh arg1 arg2 arg3` at `00:00`, at
+`01:00`, `02:00`, etc.
+
+### Extending this image
+
+Extending image's `Dockerfile`:
+
+    FROM infolinks/crond
+    COPY my-job.sh /usr/local/bin/
+    ENV SCHEDULE="0 * * * *"
+    ENV JOB_CMDLINE="/usr/local/bin/my-job.sh arg1 arg2 arg3"
+
+Build it:
+
+    docker build -t my-images/my-job .
+
+And how to run it:
+
+    docker run my-images/my-job
+
+Violla!
+
+### Kubernetes
+
+You can also use this container as part of a Kubernetes pod - since you'll
+be doing the same things as running this image yourself (mounting the
+job script, and setting environment variables) the exact specifics will
+be left as a reader's exercise :)
+
+## Contributions
+
+Any contribution to the project will be appreciated! Whether it's bug
+reports, feature requests, pull requests - all are welcome, as long as
+you follow our [contribution guidelines for this project](CONTRIBUTING.md)
+and our [code of conduct](CODE_OF_CONDUCT.md).

bin/crond.sh

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+#!/bin/sh
+
+# trap script termination for cleanup
+PIDS=""
+trap '[ ! -z "${PIDS}" ] && kill -TERM ${PIDS}' EXIT
+
+# print header and verify required parameters
+echo
+echo "************************************************************"
+echo "STARTING crond:"
+echo "  SCHEDULE: ${SCHEDULE}"
+echo "       JOB: ${JOB_CMDLINE}"
+echo "     DEBUG: ${DEBUG}"
+echo "************************************************************"
+echo
+[[ ${DEBUG} == "true" ]] && set -x
+[[ -z "${SCHEDULE}" ]] && echo "missing SCHEDULE environment variable (formatted as cron expression)" && exit 1
+[[ -z "${JOB_CMDLINE}" ]] && echo "missing JOB_CMDLINE environment variable (the command to execute)" && exit 1
+
+# exit if any command fails
+set -e
+
+# generate special file nodes that the job-executor redirects job stdout/stderr to; we will tail on these files into
+# this process's stdout/stderr. this enables the container to essentially print the job's stdout/stderr back to the
+# docker orchestrator, so it can be harvested into whatever logging solution is used
+mkdir -p /var/lib/crond
+mknod /var/lib/crond/stdout p
+mknod /var/lib/crond/stderr p
+
+# generate the crontab file to be fed into crontab daemon and feed it to crontab
+echo "${SCHEDULE} /usr/local/bin/job-executor.sh ${JOB_CMDLINE}" > /root/crontab
+crontab /root/crontab
+
+# tail on our special file nodes; redirect '/var/lib/crond/stdout' into this script's stdout, and
+# '/var/lib/crond/stderr' to our stderr; we push the tail commands into the background (notice the trailing "&") so both
+# will run in parallel, and then we use "wait" to wait indefinitely until "crontab" and the "tail" commands exit
+tail -f /var/lib/crond/stdout >&1 &
+PIDS="$!"
+tail -f /var/lib/crond/stderr >&2 &
+PIDS="${PIDS} $!"
+
+# run the crond (the crontab daemon) in the foreground ("-f") but send that thing to the background (trailing "&")
+# it's a little weird, but essentially we don't want crond to simply start, schedule background jobs, and exit; we
+# want it to stay alive as a foreground process, but we want THAT foreground process to be in the background so we
+# can wait for everything (crond and the tail commands). we also want it to stop if this script is interrupted.
+crond -f &
+PIDS="${PIDS} $!"
+
+# now wait until interrupted
+wait

bin/job-executor.sh

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+#!/bin/sh
+
+# expose proper PATHs to job
+PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
+
+# executes given command-line, but redirect its stdout to /var/lib/crond/stdout and its stderr to /var/lib/crond/stderr
+# both are special file nodes created by the container entrypoint and are used to redirect job output to the main
+# container process's stdout and stderr.
+exec $@ 1>>/var/lib/crond/stdout 2>>/var/lib/crond/stderr