How to use a docker image

Imagine that you want to use an interpreter that it is not installed in your system but it is in a docker image.

For example, the C++ interpreter cling is available in the eldipa/cling docker image.

Let’s say that you want to use that.

First, of course, you need to download the image from the docker registry:

$ sudo docker pull eldipa/cling              # byexample: +skip

Now, create a script to run cling in the container:

#!/bin/bash
docker run --rm -it -v "$(pwd):/mnt" -w /mnt eldipa/cling cling "$@"

Call the script docker-cling.sh.

Assuming it is saved in /home/john/, give it execution permissions with chmod u+x /home/john/docker-cling.sh.

The script will start a temporal interactive docker container and run inside cling and it will mount the current directory from your host to /mnt.

When the script is executed by byexample, the current directory will be the same of byexample.

Finally, run byexample with a custom shebang and an extra timeout (running docker is a little slow):

$ byexample -l cpp -x-shebang="cpp:sudo /home/john/docker-cling.sh %a" -x-dfl-timeout 30 <files>    # byexample: +skip

If you prefer you can write this long command line in a file for easy reuse:

# boptions.args file
-l=cpp
-x-shebang=cpp:sudo /home/john/docker-cling.sh %a
-x-dfl-timeout=30

So you can then run byexample as follows:

$ byexample @boptions.args <files>    # byexample: +skip

Shorter, isn’t?

Of course this example for C++ can be applied to any language/interpreter and used in combination with interpreters that don’t require of docker.

The following combines C++ (in a docker) and Python (in the host) without a problem:

$ byexample @boptions.args -l python <files>    # byexample: +skip

Make sudo passwordless to avoid timeouts

In the example above I use sudo to run docker-cling.sh. This is in general required because running docker is a privileged operation.

Unfortunately this makes sudo to ask for a password and byexample currently does not support that.

The solution is to make sudo passwordless for docker-cling.sh editing your sudoers file with visudo.

Run sudo visudo and add the following line:

john ALL=(root) NOPASSWD: /home/john/docker-cling.sh

This tells sudo to not ask for a password (NOPASSWD) when the user john is trying to execute the program /home/john/docker-cling.sh to run it as root.

Of course you will have to put your username and the full path of docker-cling.sh in that line.

You can test that the sudoers rule is properly working running sudo /home/john/docker-cling.sh by hand. It should open the cling shell without asking a password.

Issue: sudo still requires a password / prompt not found

If you see something like

$ byexample @boptions.args <files>    # byexample: +skip
[w] Initialization of Cpp Runner failed.
[!] Something went wrong processing the file <...>:
Prompt not found: the code is taking too long to finish or there is a syntax error.

Last 1000 bytes read:
[sudo] password for john:

Rerun with -vvv to get a full stack trace.

That indicates that sudo is not passwordless for docker-cling.sh yet.

Check that you used the correct full path to the docker-cling.sh script in both byexample and in the sudoers file. (Something like /home/john/docker-cling.sh).

Check also that you correctly typed the username.

Then, run sudo /home/john/docker-cling.sh and it should not require a password.

Ensure also that the script does not call sudo itself (the script should run docker run... and not sudo docker run...).

sudo will not give you a hint of what it is the problem. If something does not match perfectly, sudo will fallback and ask a password.

Issue: docker permission denied

If you get something like

docker: Got permission denied

That means that you need to use sudo as explained above.