Docker ubuntu cron tail logs not visible

Question

Trying to run a docker container that has a cron scheduling. However I cannot make it output logs.

Im using docker-compose.

docker-compose.yml

---
version: '3'
services:
  cron:
    build:
      context: cron/
    container_name: ubuntu-cron

cron/Dockerfile

FROM ubuntu:18.10

RUN apt-get update
RUN apt-get update && apt-get install -y cron 

ADD hello-cron /etc/cron.d/hello-cron

# Give execution rights on the cron job
RUN chmod 0644 /etc/cron.d/hello-cron

# Create the log file to be able to run tail
RUN touch /var/log/cron.log

# Run the command on container startup
CMD cron && tail -F /var/log/cron.log

cron/hello-cron

* * * * * root echo "Hello world" >> /var/log/cron.log 2>&1

The above runs fine its outputting logs inside the container however they are not streamed to the docker.

e.g. docker logs -f ubuntu-cron returns empty results

but

if you login to the container docker exec -it -i ubuntu-cron /bin/bash you have logs.

cat /var/log/cron.log 
Hello world
Hello world
Hello world

Now Im thinking that maybe I dont need to log to a file? could attach this to sttoud but not sure how to do this.

This looks similar... How to redirect cron job output to stdout

score 7 · Answer 1 · edited Sep 13 '18 at 09:15

7

I tried your setup and the following Dockerfile works:

FROM ubuntu:18.10

RUN apt-get update
RUN apt-get update && apt-get install -y cron

ADD hello-cron /etc/cron.d/hello-cron

# Give execution rights on the cron job
RUN chmod 0755 /etc/cron.d/hello-cron

# Create the log file to be able to run tail
RUN touch /var/log/cron.log

# Symlink the cron to stdout
RUN ln -sf /dev/stdout /var/log/cron.log

# Run the command on container startup
CMD cron && tail -F /var/log/cron.log 2>&1

Also note that I'm bringing the container up with "docker-compose up" rather than docker. It wouldn't matter in this particular example, but if your actual solution is bigger it might matter.

EDIT: Here's the output when I run docker-compose up:

neekoy@synchronoss:~$ sudo docker-compose up
Starting ubuntu-cron ... done
Attaching to ubuntu-cron
ubuntu-cron | Hello world
ubuntu-cron | Hello world
ubuntu-cron | Hello world

Same in the logs obviously:

neekoy@synchronoss:~$ sudo docker logs daf0ff73a640
Hello world
Hello world
Hello world
Hello world
Hello world

My understanding is that the above is the goal.

edited Sep 13 '18 at 09:15

alexkb

3,216
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answered Jul 20 '18 at 10:33

Neekoy

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Doesnt work for me. "RUN ln -sf" resutls in the file always being empty. take that out and the logs are populated but they are not streamed to the docker compose output – Robbo_UK Jul 20 '18 at 10:58
@Robbo_UK In the OP it was mentioned that we don't need the output in a file. Let me know if you want it both in a file and in stdout. Also, I made an edit to my post with output of stdout when I run the container. – Neekoy Jul 20 '18 at 11:30
output to sttdout is fine. what does your hello-cron look like. – Robbo_UK Jul 20 '18 at 12:14
The same as yours. Basically just add that row I referenced and run "docker-compose up". – Neekoy Jul 20 '18 at 12:39
hmm its not working for me... I do think that this is the correct answer. Ive put it on github. Must be something different that is missed? https://github.com/davidsonr/docker-ubuntu-cron – Robbo_UK Jul 20 '18 at 13:22
For anyone using a debian derived docker image, you also need to make sure rsyslog is installed for the above answer to log things to /dev/stdout properly. So just append it to the install cron command like so `RUN apt-get update && apt-get install -y cron rsyslog`. – alexkb Sep 13 '18 at 06:37

Robert · Accepted Answer · 2018-07-20T13:57:05.600

Due to some weirdness in the docker layers and inodes, you have to create the file during the CMD:

CMD cron && touch /var/log/cron.log && tail -F /var/log/cron.log

This works both for file and stdout:

FROM ubuntu:18.10

RUN apt-get update
RUN apt-get update && apt-get install -y cron

ADD hello-cron /etc/cron.d/hello-cron

# Give execution rights on the cron job
RUN chmod 0644 /etc/cron.d/hello-cron

# Create the log file to be able to run tail
# Run the command on container startup
CMD cron && touch /var/log/cron.log && tail -F /var/log/cron.log

The explanation seems to be this one:

In the original post tail command starts "listening" to a file which is in a layer of the image, then when cron writes the first line to that file, docker copies the file to a new layer, the container layer (because of the nature of copy-and-write filesystem, the way that docker works). So when the file gets created in a new layer it gets a different inode and tail keeps listening in the previous state, so looses every update to the "new file". Credits BMitch

As this will not logrotate the cron.log file it may grow very large depending on your cron output.. — Micronax, Dec 11 '18 at 21:28

score 0 · Answer 3 · answered Jul 18 '18 at 10:26

0

Try to redirect on this > /dev/stdout, after this you should see your logs with a docker logs.

answered Jul 18 '18 at 10:26

Kilian

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think this is on the right path.. However the ubuntu container exits. May need something from here to keep it running https://stackoverflow.com/questions/25775266/how-to-keep-docker-container-running-after-starting-services – Robbo_UK Jul 18 '18 at 10:36

Docker ubuntu cron tail logs not visible

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