moby: Echoing to /dev/stdout does not appear in 'docker logs'

The reason this doesn’t work is because /dev/stdout is a link to STDOUT of the process accessing it. So by doing foo > /dev/stdout, you’re saying “redirect my STDOUT to my STDOUT”. Kinda doesn’t do anything 😃. And since /var/log/test.log is a symlink to it, the same thing applies.

What you want is to redirect output to STDOUT of PID 1. PID 1 is the process launched by docker, and its STDOUT will be what docker picks up. Thus if you instead do

RUN ln -sf /proc/1/fd/1 /var/log/test.log

you’ll see it works just fine.

Note however this will only work if you don’t launch your container with --pid=host. If you do, you’ll need to set up the symlink in your daemon.sh script with something such as:

ln -sf /proc/$$/fd/1 /var/log/test.log

/dev/stdout is a symlink to /proc/self/fd/1, not /proc/1/fd/1. Subtle, but very significant difference.

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Hi, When I symlink my log file to /proc/1/fd/1, it gives an error saying permission denied, as only root user and the process that has PID 1 are supposed to have permissions to write to it. How can one solve that issue? Or how did you go ahead to ensure a problem with permissions didn’t occur in your case?

I think the main difference with the nginx image is that it only has one main process in the foreground, while in your case you have daemon.sh but also cronjobs running in the background.

For communication between them and daemon.sh, it might be more suited to use a named pipe. Eg. something like this in the beginning of daemon.sh :

mkfifo /var/log/test.log
tail -f /var/log/test.log &

I guess there’s no need to call mkfifo, since this seems to work fine:

echo "Hello" > /dev/console

Edit: Only if TTY is available (-t); See also: #6880

Ok… Agree I’m seeing it now. However, I’m now getting permission errors when writing to /proc/1/fd/1 .

$ docker exec -it test1 bash
root@55a27c41b408:/# echo "log me" > /var/log/test.log 
bash: /var/log/test.log: Permission denied
root@55a27c41b408:/#

The modified Dockerfile to reflect the changes (aka what I have been taught…! Appreciate it btw!)

Dockerfile

FROM ubuntu:latest

# Run the wrapper script (to keep the container alive)
ADD daemon.sh /usr/bin/daemon.sh
RUN chmod +x /usr/bin/daemon.sh

# Create the pseudo log file to point to stdout
RUN ln -sf /proc/1/fd/1 /var/log/test.log

# Create a cronjob to echo into the logfile just created
RUN echo '* * * * * root date >> /var/log/test.log' > /etc/crontab

CMD "/usr/bin/daemon.sh"

I moved the symlink creation to daemon.sh and it still fails to produce anything in ‘docker logs’. However if I remove the symlink so it just writes to the /var/log/test.log file it writes just fine.

Why does this work in the nginx dockerfile?

https://github.com/nginxinc/docker-nginx/blob/a8b6da8425c4a41a5dedb1fb52e429232a55ad41/Dockerfile