-su: /dev/tty: No such device or address

Question

could please someone explain to me why this happens?

# su - someone -s /bin/bash -c "ls -la /dev/tty"
crw-rw-rw- 1 nobody nogroup 5, 0 Dec  7 20:53 /dev/tty
# BUT:
# su - someone -s /bin/bash -c "echo hello > /dev/tty"
-su: /dev/tty: No such device or address

I'm trieng to build a docker Container which has two services inside. Those services a startet by a Shell-Script:

CMD ["./starter.sh"]

Withing the Dockerfile I have redirected the Logs to /dev/stderr or /dev/tty

# None of the following works:
RUN ln -sf /dev/tty /var/log/thelog.log
RUN ln -sf /dev/stdout /var/log/thelog.log
RUN ln -sf /dev/stderr /var/log/thelog.log

The problem is that I'm trying to run one of the services as not root (su - someone -c "service"), which give's the following error:

unable to open log file [/var/log/thelog.log]: [6] No such device or address

How could I solve this problem? I want the logs to be linked to /dev/* AND want to run the User as non-root. Also I tried to add the User to the group tty, which did not work out.

Thanks.

Solution

The manpage for su states that the executed command will have no controlling terminal. Any writes to /dev/tty will return the ENXIO error:

$ errno ENXIO
ENXIO 6 No such device or address

sudo does allocate a controlling terminal:

sudo -u someone /bin/bash -c "echo hello > /dev/tty"

There's no need for you to make a symbolic link to /dev/tty (/dev/stdout and /dev/stderr is enough) or use sudo if you use the USER directive in the Dockerfile or supervisor.

Comments

[tripleee]

Replacing devices with symlinks seems like a very brittle and dangerous thing to do. The kernel manipulates the tty, it's not a static file descriptor, that's why you get the error in the first place.

[tripleee]

Wrap the service in something which captures its output to a sensible place. Most sane services don't really print anything to the console anyway.

[manifestor]

Hi, thanks, but doing so is a common practice with docker. Redirecting to /dev/* makes you see the logs with docker logs mycontainer. So the crux of this matter is the su - and starting the process as another (!=root) user.

[manifestor]

Hi thanks. Yes, true I could use supervisor and I know about USER. But how do you start 2 services with a Shell-Script where one must run as non root and one as root without using supervisor? If I use USER non_root I could not start the root-service as I have only one RUN command.

[Ricardo Branco]

Either use supervisor or run each service in a different container.

[manifestor]

Just wanted to complement, that even with supervisord it is not possible to use /dev/tty as a non-root User.