Running Docker containers in the background is essential when deploying applications that should operate continuously without binding directly to the terminal session. This tutorial explores how to start and manage Docker containers in daemon mode, which allows them to run as background processes.
In this tutorial you will learn:
- How to start a Docker container in the background using daemon mode
- Methods to keep a Docker container running indefinitely
| Category | Requirements, Conventions or Software Version Used |
|---|---|
| System | Any system that supports Docker |
| Software | Docker Engine |
| Other | Access to a terminal or command prompt |
| Conventions | # – requires given linux commands to be executed with root privileges either directly as a root user or by use of sudo command$ – requires given linux commands to be executed as a regular non-privileged user |
Running Docker Containers in Daemon Mode
Starting a Docker container in daemon mode is a straightforward and essential practice for deploying services that require uninterrupted operation, such as web servers or databases. This mode allows the container to run in the background, independent of the initiating terminal, which is vital for ensuring that these services are always available and not interrupted when the user session ends. This capability is particularly crucial in production environments where continuous service availability and reliability are paramount.
--restart flag in conjunction with the -d option. This flag enables automatic container restarts in case of failure or upon system reboot, depending on the specified policy, such as always, unless-stopped, or on-failure. For example, using docker run -d --restart=always [image] ensures that the container will continuously attempt to restart, improving the resilience and uptime of critical background services.- Starting a Basic Container in Daemon Mode: Here is how you start a basic Docker container in daemon mode using the NGINX image as an example.
docker run -d -p 80:80 nginx
This command starts an NGINX container and runs it in the background. The
-dflag is used to run the container in detached mode, which means it operates independently of the terminal session that started it. The-p 80:80maps port 80 of the container to port 80 on the host, allowing HTTP traffic to reach the NGINX server. However, it’s important to note that the-doption only effectively keeps the container running if the main process inside the container remains active. In cases where the main process exits immediately (for example, in base OS images like CentOS without a specified command), the container will also stop, despite being started with-d.
Starting a Basic Container in Daemon Mode - Keeping a Container Running Indefinitely: To prevent a container from exiting immediately when the `-d` option does not maintain it due to the lack of a foreground process, use a looping command. For example, with the CentOS image:
docker run -d --name centos-linux centos /bin/bash -c 'while true; do sleep 30; done'
This command initializes a Bash script that continuously executes a
sleepcommand in an infinite loop every 30 seconds. This method is particularly useful for ensuring that containers, which would otherwise exit immediately after starting, remain operational indefinitely. It’s an effective workaround when containers do not have an inherent long-running process or command to execute.
Keeping a Container Running Indefinitely
Conclusion
In this tutorial, we covered how to run Docker containers as background processes using the daemon mode. This allows applications and services to remain active without tying up a terminal session. By understanding these techniques, you can better manage your Docker environments and ensure your services are reliably available.
