Script to check if SSH service in Docker is up - bash

Question

My docker container has two services: a web service and a SSH server.
The SSH server is openssh-server and I need to run the command docker exec -it my-container sudo service ssh restart from outside the container to start the SSH server.
However, the command doesn't succeed all the time. Every time I need to manually check if the SSH server is up in the container using the command: ssh root#localhost:
1) If the SSH server fails to start, the result is ssh_exchange_identification: Connection closed by remote host
2) Otherwise, it asks for the password. (Which indicates that the SSH server is up)
Since I have to deploy multiple containers at the same time, it is unrealistic to check every container manually. Therefore, I want to retry the docker exec -it my-container sudo service ssh restart command automatically if the SSH serve fails to start. But I am not sure how to write the bash script to achieve this. It basically should work like this:
while (ssh_server_fails_to_start):
docker exec -it my-container sudo service ssh restart
Any comments or ideas are appreciated. Thanks in advance!

Answer 1

If the sshd is up an running, it will accept connections on its certain port. Otherwise, the connection attempt will fail.
If you run the following command:
ssh -o PasswordAuthentication=No root#localhost true
This will fail in either way, but the output will be different. In the case that the server is running and accepting connections, the explicit switch-off of password authentication will make it fail with this message:
Permission denied (publickey,password).
Otherwise it will print out a message like this:
ssh: connect to host localhost port 22: Connection refused
So I propose to scan the error message for a hint like this:
if ssh -o PasswordAuthentication=No root#localhost true \
|& grep -q "Connection refused"
then
echo "No server reachable!"
else
echo "Server reachable."
fi
So you could write your script like this:
while ssh -o PasswordAuthentication=No root#localhost true \
|& grep -q "Connection refused"
do
docker exec -it my-container sudo service ssh restart
done
You might want to add some sleep delays to avoid hurried restarts. Maybe the ssh server just needs some time to accept connections after being restarted.

Answer 2

To test the ssh connection, we can use sshpass package to provide a password in the command line.
while : ; do
docker exec -it my-container sudo service ssh restart
sleep 5s
sshpass -p 'root' ssh -q root#localhost -p 2222 exit
if [ $? == 0 ]; then
echo "SSH server is running."
break
fi
echo "SSH server is not running. Restarting..."
done