running cron in foreground in Docker container produces no output

Question

I want to run some cron jobs in a Docker container and send the output to stdout. I read this post: How to run a cron job inside a docker container?

To try this out with a simple example, I created a demo crontab:

my-crontab:

* * * * * date > /dev/stdout 2> /dev/stderr
# empty line

Then I run an interactive shell inside a Docker container based on the image my scripts will need:

docker run -it --entrypoint bash python:3.10.3-bullseye

/# apt update
/# apt install cron
/# crontab < my-crontab
/# cron -f

If I wait 60 seconds, I expect to see some output to the console attached to the container once every minute. But there is no output.

Finally, I found the output in /var/spool/mail/mail. Here is one message:

From root@5e3c82cb3651 Tue May 10 20:04:02 2022
Return-path: <root@5e3c82cb3651>
Envelope-to: root@5e3c82cb3651
Delivery-date: Tue, 10 May 2022 20:04:02 +0000
Received: from root by 5e3c82cb3651 with local (Exim 4.94.2)
    (envelope-from <root@5e3c82cb3651>)
    id 1noW5S-0000SA-0T
    for root@5e3c82cb3651; Tue, 10 May 2022 20:04:02 +0000
From: root@5e3c82cb3651 (Cron Daemon)
To: root@5e3c82cb3651
Subject: Cron <root@5e3c82cb3651> date > /dev/stdout 2> /dev/stderr
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
X-Cron-Env: <SHELL=/bin/sh>
X-Cron-Env: <HOME=/root>
X-Cron-Env: <PATH=/usr/bin:/bin>
X-Cron-Env: <LOGNAME=root>
Message-Id: <E1noW5S-0000SA-0T@5e3c82cb3651>
Date: Tue, 10 May 2022 20:04:02 +0000

Tue May 10 20:04:01 UTC 2022

Then it looks like /bin/sh is completely ignoring the shell redirection in the crontab.

`/dev/stdout` points at wherever the process's stdout would normally go; for something launched by cron, that will by default be captured and sent out via email. You might try `/proc/1/fd/1` instead to target the main container process's stdout, but you might not be allowed to do that. (`>/dev/stdout 2>/dev/stderr` redirect stdout and stderr to where they're already going; it doesn't really do anything.) — David Maze, May 10 '22 at 20:30

score 0 · Answer 1 · answered May 10 '22 at 21:02

@DavidMaze answered this in his comment (above - can't find a link to it). Redirecting to /proc/1/fd/1 and /proc/1/fd/2 (for stderr) totally works. Thank you, David.

Nevertheless, that's counterintuitive. The filesystem nodes /dev/stdout and /dev/stderr already exist as symlinks that point to /proc/1/fd/1 and /proc/1/fd/2, respectively, independent of cron. Why wouldn't cmd > /dev/stdout and cmd > /proc/1/fd/1 be interchangeable in a crontab?

running cron in foreground in Docker container produces no output

1 Answers1