The jjjordan/docker-gen-cron image runs a cron daemon that can be used by
other containers running on the host. It is useful for smaller
installations (e.g. set up with docker-compose) that don't have
high-level job scheduling capabilities. Jobs are specified in environment
variables on the containers where they will run. docker-gen-cron will
pick up changes (via docker-gen)
and the jobs will be executed via docker exec.
A typical installation may look like this:
# docker-compose.yml
version: '3'
services:
cron:
image: jjjordan/docker-gen-cron
environment:
TZ: America/Los_Angeles
volumes:
- /var/run/docker.sock:/var/run/docker.sock
some_other_container:
# ...
environment:
CRON_0: '0 */1 * * * hourly-backup.sh'
CRON_1: '*/1 * * * * touch /tmp/minutely'
another_container:
# ...
environment:
CRON_0: '@daily a-daily-job.sh'docker-gen-cron uses fcron internally, so all of
the extra options available
in fcron can be used here.
Jobs are read from all environment variables of the form CRON_### where
### is an integer. Lines will be processed in numeric (not lexical)
order. Additionally, CRON_START_### can be used to set a schedule for the
container to be started if they have exited and similarly CRON_RESTART_###
will restart containers. In these cases, no command needs to be specified.
The prefix can be changed by supplying a PREFIX environment variable to
the cron container (an underscore will be added to this value).
fcron is flexible and many options can be set on jobs. Most of these are
untouched, but a couple are handled specially:
| Option | Behavior |
|---|---|
| runas | Translated to the -u option in docker exec, no effect in the cron container. (This should be the intuitive behavior) |
| n, nice | nice value. Ignored. |
| SHELL=value | If this environment variable is set in the job specification, then it will be used to execute the command in the target container. |
| Other environment variables | Passed to the job via -e options to docker exec |
Job output can be found in the logs of the cron container. This is a
departure from the standard behavior of mailing output, but the normal
behavior can be restored. docker-gen-cron has
msmtp installed as the system mailer, which can
be configured to deliver mail to a remote SMTP server by volume mounting a
configuration file() to
/etc/msmtprc and explicitly telling fcron to mail output. To
illustrate:
# docker-compose.yml
version: '3'
services:
cron:
image: jjjordan/docker-gen-cron
environment:
TZ: America/Los_Angeles
volumes:
- /var/run/docker.sock:/var/run/docker.sock
- ./msmtprc:/etc/msmtprc
some_other_container:
# ...
environment:
CRON_0: '!mailto([email protected]),mailfrom([email protected]),erroronlymail(true)'
CRON_1: '@stdout(false),mail(true) 1d daily-job.sh'/etc/msmtprc might look something like this. See
another example as well as
the documentation.
defaults
auth plain
tls on
tls_trust_file /etc/ssl/certs/ca-certificates.crt
syslog off
account cron
host mail.domain.tld
port 587
from "[email protected]"
user "[email protected]"
password "password123"
account default : cron
# docker-compose.yml
version: '3'
services:
cron:
image: jjjordan/docker-gen-cron
environment:
# Defaults to Etc/UTC
TZ: America/Los_Angeles
# Read environment variables with this prefix. Default: CRON
PREFIX: CRON
# Much more output for troubleshooting
DEBUG: 1
volumes:
- /var/run/docker.sock:/var/run/docker.sock
# Keep fcron spool on local host
- /var/spool/docker-gen-cron:/var/spool/fcron- fcron is the cron daemon used in this container.
- crontab format manual page
- User manual in English
- msmtp is installed in this container as the default mailer.
- docker-gen is used to monitor containers, though you will likely not need to interface with it directly.