Running multiple shells in the same docker container

The exec way

First run a container

docker run -it test

Next, you can use the exec

Get the docker container id

docker ps

Run exec on the container id

docker exec -it 0ec2333ec307 /bin/bash/

In this way, you can execute multiple commands in the docker container.

Alternatively, you can also directly execute the commands you want instead of /bin/bash

I have encountered a use case where I have to run two Java programs in the same container. So, I started an interactive shell, which could also be done by directly executing the java command instead of /bin/bash

The sshd way,

You can have the sshd service up and running so that you can directly start an ssh session to your container. This is a bit more complex method compared to the aforementioned, but you can find it useful if you want access the container from outside of your host machine where there is no docker client.

For CentOS, RHEL

yum install openssh-server

systemctl enable sshd
systemctl start sshd

If you want to have root access, you need to add the following line to sshd-config

vi /etc/ssh/sshd_config

PermitRootLogin yes

If the line is already there uncomment it (just remove the # before the line)

Note: If you cannot access ssh from outside of the host, it may be because of the iptables rules. Make sure that the rules allow incoming connections to your ssh port.

You may also need to map your container’s ssh port to your host port using the -p option.
The above command then would be:

docker run -it p 22 test

This will map the port 22 of the test container to a randomly chosen port on the host. You can view the port that is assigned using the docker ps command.

Now, allow incoming connections to this randomly chosen port by editing the iptables rules of the host machine to allow incoming ssh connections to this port.

Tip: Though in this example, I have used -p 22, it is not recommended you do the same in production. You need to manually assign a port of your choice using -p <my_port>:22

iptables -A INPUT -i eth0 -p tcp --dport 22 -j ACCEPT

Instead of eth0 you can use your interface name or you can omit -i eth0 to apply the rule for all interfaces.

• -A is for appending a rule to a chain (here INPUT).
• -i is for interface
• -p is the type of packet (tcp, here)
• --dport means destination port (22 for ssh)
• -j is jump which tells what to do when a tcp packet from port 22 comes

Note: Make sure that you don’t append this rule after the drop-all packets rule. The above is just an example, but the command might change for you depending on your iptables rules. So, change them accordingly.

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