How can I execute a command on an existing Docker container?

How to Execute a Command on an Existing Docker Container

To run a command on a Docker container that is already there, we can use the docker exec command. This lets us run commands in a container that is already running. It’s a strong tool for managing our Docker apps. The basic way to write it is docker exec -it <container_name_or_id> <command>. Here <container_name_or_id> is the name or ID of our container. <command> is the command we want to run.

In this article, we will look at different ways to run commands on Docker containers that are already there. We will learn how to use Docker exec to run commands in active containers. We will also see how to run commands in stopped containers. We will talk about running interactive commands and using Docker attach. We will also learn how to run shell commands in Docker containers with Docker Compose. Lastly, we will answer some common questions about these topics. Here is a short list of what we will talk about:

  • How to Execute a Command on an Existing Docker Container
  • Using Docker Exec to Execute Commands on Running Containers
  • Executing Commands Inside Stopped Docker Containers
  • How to Run Interactive Commands on Docker Containers
  • Using Docker Attach to Execute Commands in Running Containers
  • Executing Shell Commands in Docker Containers with Docker Compose
  • Frequently Asked Questions

Using Docker Exec to Execute Commands on Running Containers

We can use the docker exec command to run a command on a running Docker container. This command helps us run commands inside a container that is already running. It gives us access to the container’s environment.

Basic Syntax

docker exec [OPTIONS] CONTAINER COMMAND [ARG...]
  • CONTAINER: This is the name or ID of the running container.
  • COMMAND: This is the command we want to run inside the container.
  • ARG: These are optional arguments for the command.

Example Usage

  1. Run a Simple Command

    To list files in the /tmp directory of a container called my-container, we can use this command:

    docker exec my-container ls /tmp

  2. Run an Interactive Shell

    If we want to open an interactive shell like bash in the container, we use:

    docker exec -it my-container /bin/bash

    We use -it to set up a pseudo-TTY and keep the input open.

  3. Execute a Command with Environment Variables

    If we need to set environment variables when running a command, we can use the -e flag like this:

    docker exec -e MY_VAR=value my-container printenv MY_VAR

Common Options

  • -d: This runs the command in the background.
  • --user: This lets us specify the username or UID for running the command.
  • --privileged: This gives extra privileges to the command.

Example of Running a Command as a Different User

To run a command as a specific user inside the container, we can do:

docker exec --user username my-container whoami

Checking Running Containers

Before we run commands, we should check if our container is running. We can list all running containers with this command:

docker ps

For more details about using Docker commands, we can check the Docker documentation.

Executing Commands Inside Stopped Docker Containers

To run commands in a stopped Docker container, we first need to restart the container. Docker does not let us run commands directly in a stopped container. Here is how we can do it:

  1. Start the Stopped Container: We can start the container using the docker start command. Just replace <container_name_or_id> with the name or ID of your container.

    docker start <container_name_or_id>

  2. Execute the Command: After we start the container, we can run the command inside it using docker exec. The -it flags help us interact with the container.

    docker exec -it <container_name_or_id> <command>

    For example, if we want to list files in the /app directory of the container, we can do:

    docker exec -it <container_name_or_id> ls /app

  3. Stop the Container After Execution: If we want to stop the container again after running the command, we can use:

    docker stop <container_name_or_id>

Remember, if we want to run a command without keeping the container running, we can use a different way with Docker. We can create a new container from the same image and run the command we need:

docker run --rm <image_name> <command>

This way, we can execute commands without needing to restart a stopped container.

How to Run Interactive Commands on Docker Containers

We can run interactive commands on Docker containers by using the docker exec command with the -it flags. This helps us to access the terminal of a running container.

Example of Running Interactive Commands

  1. Find the Container ID or Name: First, we need to find the container where we want to run commands. We can list running containers by using:

    docker ps

  2. Execute a Command: Next, we use the docker exec command with -it to run a command interactively. For example, to start a bash shell in a running container called my_container, we use:

    docker exec -it my_container /bin/bash

    If the container has a different shell, like sh, we can run:

    docker exec -it my_container /bin/sh

Running Other Commands Interactively

We can also run other commands interactively. For example, to start an interactive Python shell, we can use:

docker exec -it my_container python

Detaching from the Container

To exit from the shell without stopping the container, we can use the escape sequence Ctrl + P then Ctrl + Q. This lets us leave the interactive session while the container continues to run.

Using Docker Compose

If we are using Docker Compose, we can run interactive commands in a service from our docker-compose.yml file. For example:

docker-compose exec my_service /bin/bash

We change my_service with the name of our service.

For more information on managing Docker containers interactively, we can check this article on Docker commands.

Using Docker Attach to Execute Commands in Running Containers

The docker attach command helps us connect our terminal to a running Docker container. It lets us access the standard input, output, and error streams. This is great for working with the main process of the container. Let’s see how we can use it right.

Basic Syntax

docker attach [OPTIONS] CONTAINER

Example Usage

  1. Attach to a Running Container: To attach to a container, we need the container ID or name. For example, if our container is named my_container:

    docker attach my_container

  2. Detaching from the Container: If we want to detach without stopping the container, we can use the CTRL + P and then CTRL + Q keys. This helps us exit the attach session while keeping the container running.

Options

  • --no-stdin: This option does not attach STDIN.
  • --detach-keys: This lets us change the key combination for detaching.

Limitations

  • We can only attach to the main process of the container. If the main process is not interactive, like a web server, we can’t interact well.
  • If we start a container with -d (detached mode), we can still attach to it. But we won’t see any output until we do so.

Use Case Scenario

If we have a container running a shell, we can attach to it to run commands directly:

docker run -it --name my_container ubuntu /bin/bash
# After running the above, we can detach using CTRL + P, CTRL + Q
docker attach my_container

This way, we can run commands interactively in the running container. The docker attach command is a good tool for debugging or monitoring in real time.

For more information on Docker commands and features, we can check this article on how to list running Docker containers.

Executing Shell Commands in Docker Containers with Docker Compose

We can execute shell commands in Docker containers using Docker Compose. We use the docker-compose exec command for this. This command lets us run commands in a specific service container that we define in our docker-compose.yml file.

Basic Syntax

docker-compose exec <service_name> <command>

Example

Let’s say we have a service called web in our docker-compose.yml. We can run a shell command like this:

docker-compose exec web ls -la

Running an Interactive Shell

If we want to open an interactive shell session like bash or sh inside a running container, we can do this:

docker-compose exec -it <service_name> /bin/bash

or

docker-compose exec -it <service_name> /bin/sh

Handling Multiple Commands

We can also run multiple commands at once. We can chain them with &&:

docker-compose exec <service_name> sh -c "command1 && command2"

Environment Variables

If we need to send environment variables while running commands, we can use the -e flag:

docker-compose exec -e VAR_NAME=value <service_name> <command>

Example with Environment Variables

docker-compose exec -e MY_VAR=value web printenv MY_VAR

For more help on Docker Compose and what it can do, we can read the article on what is Docker Compose and how does it simplify multi-container applications.

Frequently Asked Questions

How do we execute a command on a running Docker container?

To execute a command on a running Docker container, we use the docker exec command. We write the container name and the command we want to run after that. For example, to run a bash shell inside a container called my_container, we can use:

docker exec -it my_container bash

This lets us interact with the container’s environment directly.

Can we execute commands in a stopped Docker container?

No, we cannot execute commands in a stopped Docker container because it is not running. But we can start the container again using the docker start command. Then, we can use docker exec to run commands when it is running. To restart a stopped container called my_container, we can use:

docker start my_container

After that, we can execute the command we want.

How do we run an interactive command on a Docker container?

To run an interactive command on a Docker container, we use the docker exec command with the -it flags. This allows us to interact with the container’s terminal. For example:

docker exec -it my_container /bin/bash

This command opens a bash shell inside the running container. We can then execute commands interactively.

What is the difference between docker exec and docker attach?

docker exec is for running a new command in a running container. It allows us to open a new shell session or execute a command without changing the main process. On the other hand, docker attach connects our terminal to the main process of a running container. This lets us interact with it directly. We should use docker exec for running commands and docker attach for interacting with the main process.

How can we execute commands in a Docker container using Docker Compose?

To execute commands in a Docker container managed by Docker Compose, we can use the docker-compose exec command. For example, to run a bash shell in a service named web, we use:

docker-compose exec web bash

This command lets us interact with the specified service container directly. It works like docker exec.