A sandbox for playing with cron on alpine in docker.

Getting the logs from cron jobs running in a Docker container doesn't seem to be as easy as I expect. I want them to be written to stdout and stderr so docker logs can see them. I also want to uphold docker recommendations about having a single process per container so backgrounding cron and then tailing a log file in the foreground doesn't seem right.

This example runs cron in the foreground and captures the output from the jobs that are run.

Quick start

1. run the container with a TTY attached with:

   docker-compose up --build

2. or, run the container in the background and tail the logs:

   docker-compose up --build -d && docker logs -f ts_act

3. to clean up

   docker-compose down

The cron schedule is all stars (*) so it'll run every minute. Once you've started the container, wait until the change of the minute and you should see some log output.

Example output

The output should look like the following. The entrypoint.sh script will echo the commands it runs, then when the cron job runs, we'll see the stdout and stderr output.

ts_act | + pwd
ts_act | + thisDir=/app
ts_act | + : supplied
ts_act | + CRON_SCHEDULE='* * * * *'
ts_act | + export 'OPTIONAL_ARG=somedefaultvalue'
ts_act | + echo '* * * * * SOME_VAR=something sh /app/run.sh'
ts_act | + crond -f
ts_act | [WARN] this line is going to stderr
ts_act | [INFO] in the run.sh script
ts_act | [INFO] should be able to see the value of SOME_VAR=something
ts_act | [INFO] can we also see MANDATORY_ARG=supplied or OPTIONAL_ARG=somedefaultvalue ?