How to run a command inside Docker Container?

After you build a Docker image and create a running instance of it (a Docker container), you might want to execute commands inside the container to install packages, browse directories, or perform various tasks. Docker provides several methods to access the container environment and execute commands. In this article, we will explore different ways to run commands inside Docker containers.

Using Docker exec Command

The docker exec command allows you to run commands in a running container from your local machine's command line. First, you need the container ID of the target container.

Get Container Information

sudo docker ps -a

This command displays all containers with their names, IDs, and status information. Copy the container ID where you want to execute commands.

Execute Commands

sudo docker exec -it <container-id> echo "Welcome to tutorialspoint"

Important: The container must be in a running state to use docker exec. If the container is stopped, you'll receive an error.

Interactive Shell Access

sudo docker exec -it <container-id> bash

This opens an interactive bash shell inside the running container, allowing you to execute multiple commands.

Using Container Bash at Startup

You can launch a new container and immediately access its bash shell using the docker run command:

sudo docker run -it <image-name> bash

The -it flags create an interactive terminal session. Inside the bash shell, you can execute commands such as:

apt-get update
apt-get install firefox

Using Dockerfile Instructions

The most common approach for running commands is through a Dockerfile during the image build process. The RUN instruction executes commands and creates new image layers.

Example Dockerfile

FROM ubuntu
RUN apt-get -y update
RUN apt-get install -y firefox
RUN mkdir /app
WORKDIR /app

When you build the image with docker build, these commands execute step-by-step, creating a customized image with your required software and configuration.

Comparison of Methods

Common Use Cases

• Development and Debugging: Use docker exec to inspect running containers and troubleshoot issues.

• Package Installation: Use Dockerfile RUN instructions for permanent software installation.

• Interactive Exploration: Use docker run -it to explore container environments interactively.

Conclusion

Docker containers function similarly to Linux terminals, allowing you to execute commands through multiple methods. Use Dockerfile instructions for permanent changes during image building, docker exec for running containers, and docker run -it for interactive sessions with new containers.