Docker Start

Introduction

Docker containers provide lightweight, isolated environments for running applications. One of the fundamental operations in Docker management is starting containers that have been created or previously stopped. The docker start command is essential for bringing containers to life and is a core part of the container lifecycle management process.

In this tutorial, you'll learn how to use the docker start command effectively, understand its options, and see practical examples of how to integrate it into your Docker workflows.

Container Lifecycle Basics

Before diving into the docker start command, let's understand where it fits in the Docker container lifecycle:

As shown in the diagram, containers can exist in different states:

• Created: The container is set up but not running
• Running: The container is active and executing its processes
• Stopped: The container has been stopped but still exists

The docker start command transitions containers from the Created or Stopped state to the Running state.

Basic Usage

The basic syntax of the docker start command is:

bash

docker start [OPTIONS] CONTAINER [CONTAINER...]

You can specify one or more containers by their names or IDs.

Example 1: Starting a Single Container

bash

# Create a container without starting it
docker create --name my-nginx nginx

# Start the container
docker start my-nginx

Output:

my-nginx

The output is the name of the container that was started. A successful start command returns the container name or ID.

Example 2: Checking Container Status

You can verify the container is running using the docker ps command:

bash

docker ps

Output:

CONTAINER ID   IMAGE     COMMAND                  CREATED         STATUS         PORTS     NAMES
3a7f8e9d2b1c   nginx     "/docker-entrypoint.…"   2 minutes ago   Up 5 seconds   80/tcp    my-nginx

Start Command Options

The docker start command comes with several useful options:

Option	Description
-a, --attach	Attach to STDOUT/STDERR and forward signals
-i, --interactive	Attach container's STDIN
--detach-keys	Override the key sequence for detaching from a container

Example 3: Interactive Start

To start a container and connect to its input stream:

bash

docker start -i my-ubuntu-container

This is particularly useful for containers that run interactive shells or command-line applications.

Practical Use Cases

Starting Multiple Containers

You can start multiple containers in a single command:

bash

docker start container1 container2 container3

Output:

container1
container2
container3

Restarting a Stopped Application

When your application container stops due to an error or after system maintenance:

bash

# Check which containers are stopped
docker ps -a

# Start the application container
docker start my-app

Attaching to Container Output

When you want to see the output of a container as it starts:

bash

docker start -a my-logging-container

This will display the container's standard output and error streams in your terminal.

Automating Container Starts

Using Restart Policies

Instead of manually starting containers, you can use Docker's restart policies when creating containers:

bash

docker create --restart unless-stopped --name always-up nginx
docker start always-up

With this configuration, Docker will automatically restart the container if it stops unexpectedly or if the Docker daemon restarts, unless the container was explicitly stopped by the user.

Common Restart Policies:

• no - Never automatically restart (default)
• on-failure[:max-retries] - Restart only on non-zero exit status
• always - Always restart regardless of exit status
• unless-stopped - Like "always" but won't start on daemon startup if container was explicitly stopped

Integration with Docker Compose

In real-world applications, you'll often use Docker Compose to manage multiple containers. Starting containers with Compose is as simple as:

bash

docker-compose up -d

To restart services that are already defined in your docker-compose.yml:

bash

docker-compose start [SERVICE...]

Troubleshooting

Container Won't Start

If a container refuses to start, you can investigate using:

bash

# Check the exit status
docker ps -a

# Look at the container logs
docker logs my-container

Common issues include:

• Port conflicts
• Missing volume mounts
• Configuration errors
• Resource constraints

Example: Fixing a Port Conflict

bash

# Stop the container
docker stop my-nginx

# Remove it
docker rm my-nginx

# Recreate with a different port mapping
docker create --name my-nginx -p 8080:80 nginx

# Start the container
docker start my-nginx

Advanced Start Techniques

Starting with Resource Limits

While you can't change resource limits on docker start, you can specify them at container creation:

bash

docker create --name limited-app --memory="512m" --cpus="0.5" my-app
docker start limited-app

Start with Healthchecks

For critical applications, create containers with healthchecks:

bash

docker create --name healthchecked-app --health-cmd="curl -f http://localhost/ || exit 1" --health-interval=5s nginx
docker start healthchecked-app

You can then monitor the health with:

bash

docker inspect --format='{{.State.Health.Status}}' healthchecked-app

Summary

The docker start command is an essential tool for Docker container management that:

• Transitions containers from a stopped or created state to a running state
• Can be used with options like -a for attaching output or -i for interactive mode
• Works with single or multiple containers
• Integrates with other Docker features like restart policies

Understanding how to effectively use docker start gives you greater control over your containerized applications and is fundamental to Docker management.

Further Learning

To deepen your Docker management skills, consider exploring:

1. The docker run command, which combines docker create and docker start
2. Container orchestration tools like Docker Compose and Kubernetes
3. Docker's logging and monitoring capabilities
4. Service discovery and networking between containers

Practice Exercises

1. Create a container without starting it, then start it and verify it's running.
2. Start a container in interactive mode and execute a command inside it.
3. Set up a container with a restart policy and test that it automatically restarts after stopping.
4. Create multiple containers and start them all with a single command.
5. Use a healthcheck to monitor a container's status after starting it.