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Docker will automatically exit a container when it is executed in the background; however, in some cases, we need to ensure that the instance continues to run despite the process being completed. This article will discuss how to continue running a docker container in the background.

Running Docker containers in the background

If we want to simplify the way we run containers, we should run our container in the background like this:

$ docker run -d image sample

Instead of running:

$ docker run -i -t image sample

It is recommended to use -d because we can run our container with only one command and we don’t need to detach the container’s terminal by pressing Ctrl+P + Q.

However, there is something wrong with the -d parameter. As a result, our container stops immediately unless the command is not running in the foreground.

Let's explain this with a case where we want to run an Apache service on a container. The intuitive way to do this is:

$ docker run -d apache-server apachectl start

However, the started container stops immediately because apachectl exits once the Apache daemon is detached.

Docker doesn’t like this. Docker requires our command to be kept running in the foreground; otherwise, it thinks our application is stopped and shuts down the container.

Running a container in the foreground

We can solve this problem by running the apache executable directly with the foreground option.

$ docker run -d apache-server /usr/sbin/apache2 -D NO_DETACH -D FOREGROUND

Here we manually do what apachectl does for us and runs the apache executable. In this way, apache is always running in the foreground.

Again, keep in mind that the process is different for each Unix-based image.

The problem is that some applications do not run in the foreground. Also, we need to do some extra work, such as exporting the environment variables ourselves.

In this case, we can tail -f /dev/nulladd to your command. Our container will not stop because tail is always running in the foreground.

We can use this technique in the previous example.

$ docker run -d apache-server apachectl start && tail -f /dev/null

Since tail -f /dev/nullit does not harm us, we can use this workaround for any application.

Run the container indefinitely

daemon -dAnother simple way to keep a container alive indefinitely in daemon mode is to run sleep infinity as an argument to the container. This has nothing to do with doing weird things like allocating a TTY in daemon mode .

Although this command is very fancy, like making sleep your default command.

$ docker run -d apache-server sleep infinity

Also, the -t parameter allocates a pseudo-tty, which should do the trick. This will trick bash into continuing to run because it thinks it is connected to an interactive TTY.

$ docker run -t -d apache-server