Docker：容器内的 cron 没有错误但仍然无法正常工作

Question

I am new to docker. Managed to create a script which gets info from online source and populates an SQL DB. All works well within docker container.

However, I need to make this to run for example every minute.

So I amended my working Dockerfile and added the following:

Dockerfile:

RUN apt-get install -y cron
COPY cronms /etc/cron.d/cronms
RUN chmod 0644 /etc/cron.d/cronms
RUN crontab /etc/cron.d/cronms
RUN touch /var/log/cron.log
    
CMD cron && tail -f /var/log/cron.log

#CMD [ "python3", "./my_script.py" ]  -- command before cron

my cronms file:

*/1 * * * * /usr/bin/python3 /my_script.py

Image builds without errors however when running it data is not being downloaded.

What I am missing please?

Thanks

Answer 1

Your crontab file has incorrect syntax. From thecron man page :

The system crontab ( /etc/crontab ) and the packages crontabs ( /etc/cron.d/* ) use the same format, except that the username for the command is specified after the time and date fields and before the command.

So your cronms file should look like:

*/1 * * * * root /usr/bin/python3 /my_script.py

I assume the Dockerfile in your question is truncated, but it's worth noting that for this to work you may need to explicitly install python3 as well as cron , depending on which base image you're using.

---

If I build an image using this Dockerfile :

FROM ubuntu:20.04

RUN apt-get update
RUN apt-get install -y cron python3
COPY cronms /etc/cron.d/cronms
RUN chmod 0644 /etc/cron.d/cronms
RUN crontab /etc/cron.d/cronms
RUN touch /var/log/cron.log

COPY my_script.py /my_script.py

CMD cron && tail -f /var/log/cron.log

And this cronms :

*/1 * * * * root /usr/bin/python3 /my_script.py

And this my_script.py :

#!/usr/bin/python3

import time

with open("/tmp/datafile", "a") as fd:
    fd.write(time.ctime())
    fd.write("\n")

Then I can confirm that everything works as expected: the script is executed once a minute.

Note that nothing is written to /var/log/cron.log , however, because cron logs to syslog, not to a file. To see output from cron , you would need to arrange to run a syslog daemon, or use a different cron daemon (for example, the busybox crond command can log to a file or stderr).