why executing docker log -f it returns data from scratch? - docker

Question

I have a simple code in Dotnet core console and it simply counted forever :
class Program
{
static void Main(string[] args)
{
int counter = 1;
while (true)
{
counter++;
Console.WriteLine(counter);
}
}
}
I contained it with docker and ran it via VS Docker run.
but every time that I execute > docker logs -f dockerID it starts and shows counting from scratch! (1,2,3,....). I expected that whenever I run this command it shows me logs from the last integer that it counts!
Is "docker logs -f" cause to run a new instance of my application every time?

Answer 1

When running docker help logs you see this:
--tail string Number of lines to show from the end of the logs
(default "all")
So you should do:
docker logs --tail 100 -f dockerID
Additional reference: docker logs documentation > Options

Answer 2

docker logs -f should work similar to tail -f, so, it should show just new logs.
Therefore, if you're watching logs repeated, it looks your program is being executed in loop.
The only thing that comes to my mind is a little trick, although it's not an elegant solution, it can be useful for other similar situations:
Define in your system history time format according to docker logs
export HISTTIMEFORMAT="%Y-%m-%dT%H:%M:%S "
Get your last docker logs command date with a simple script.
LAST_DOCKER_LOG=`history | grep " docker logs" | tac | head -1 | awk '{print $2}'`
Get logs after your last docker logs execution:
docker logs --since $LAST_DOCKER_LOG <your_docker>
In one line:
docker logs -t --since `history | grep " docker logs" | tac | head -1 | awk '{print $2}'` <your_docker>
Note: history time and docker log times have to be in the same timezone.
In spite of everything, I'd have a look to your loop execution. Maybe, docker log -t timestamp show option can help you to solve it.