A minimal container used to run Cron jobs.
This is an image based on Alpine Linux that provides a minimal construct to run cron-jobs within a Docker container. msmtp has been installed to replace BusyBox sendmail as the default mail transfer agent. cronic has also been installed to interpret the execution and format output of jobs run within cron to help control its emailed output.
The simplest way to use this image is to use a docker-compose.yml file. Here is a fully configured example.
version: '2.1'
services:
mykron:
image: rjlasko/minikron
containername: kron
restart: unless-stopped
environment:
- [email protected]
- CRONTAB_USEDEFAULT=true
volumes:
- "${MSMTPRC}:/etc/msmtprc:ro"
- "${CRONTAB}:/etc/periodic/cron.tab:ro"
- "${HOURLY_01}:/etc/periodic/hourly/01.sh:ro"
- "${DAILY_01}:/etc/periodic/daily/01.sh:ro"
- "${DAILY_02}:/etc/periodic/daily/02.sh:ro"
- "${TZ}:/etc/localtime:ro"
Slightly more work, but just as functional is to use the CLI interface to configure and build your own container from just the Dockerfile.
docker run -d \
--name kron \
--restart=always \
--env [email protected] \
--env CRONTAB_USEDEFAULT=true \
-v "${MSMTPRC}":/etc/msmtprc:ro \
-v "${CRONTAB}":/etc/periodic/cron.tab:ro \
-v "${HOURLY_01}":/etc/periodic/hourly/01.sh:ro \
-v "${DAILY_01}":/etc/periodic/daily/01.sh:ro \
-v "${DAILY_02}":/etc/periodic/daily/02.sh:ro \
-v "${TZ}":/etc/localtime \
rjlasko/minikron
Cron jobs may be called by two different methods
If the following crontab standard format file is found during container start time, it will be appended to the root user's crontab.
/etc/periodic/cron.tab
On container start, the environment variable CRONTAB_USEDEFAULT is evaluated. If it is equal (case-insensitive) to TRUE, then the default crontab is not cleared. In this case, placing executable files in a directory structure corresponding to pre-defined cron-jobs calling the run-parts executable on the given directories occurring with the stated periodicity.
/etc/periodic/15min
/etc/periodic/hourly
/etc/periodic/daily
/etc/periodic/weekly
/etc/periodic/monthly
A primary goal of this project is to support crond's ability to send emails to the email address defined by the MAILTO variable in the crontab definition. The CRONTAB_MAILTO environment variable is provided to specify the destination of any cron generated emails.
The Alpine Linux default email program sendmail has been replaced by msmtp due to its ease of use, and active development. This will allow us to leverage all of its functionality, mostly through the specification of its standard configuration file, to be located at:
/etc/msmtprc
Typically, we want Cron to email us when a job failed. To ensure that we get as much output from a failed script as possible, we utilize the great utility cronic. This allows cron to generate a message (that gets logged and/or emailed) that contains the exit code of the job, as well as its STDOUT and STDERR. To use this, simply pass your commands as arguments to cronic, within the crontab.
* * * * * cronic boom
The default OS-defined time zone is set to UTC. Changing of the timezone is supported by overriding the TZ environment variable, as defined by the 'TZ database name' enumeration. This enumeration set can be found on Wikipedia.
TZ=US/Mountain