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docker-ejecutar-comandos-con-exec

Executing Commands in Containers with Exec

  • 3 min
  • Intermediate

The command docker exec is a tool that allows us to run a new process inside a container that is already in a running state.

Imagine this situation: You have your Nginx server running perfectly in the background (-d). Everything seems fine, but suddenly, the website gives an Error 500.

You need to check the configuration file, verify if the disk is full, or ping the database from inside.

In the world of Virtual Machines, you would connect via SSH. But in Docker, SSH is not used. Instead, we have a much more powerful and direct command: docker exec.

Why not use SSH?

It’s a classic question. Why not install openssh-server on my Docker image to connect with PuTTY? Because it’s an anti-pattern.

  1. ❌ Unnecessary weight: You are adding a heavy service to a container that should be lightweight.
  2. ❌ Security: You have to manage keys, users, and open port 22. More attack vectors.
  3. ❌ Complexity: You have to configure systemd or supervisord to run two processes at the same time (your app + ssh), which goes against the “one container, one process” philosophy.

Docker already has its own secure backdoor: it’s the Docker Socket via docker exec. Use it.

What is docker exec?

This command allows us to run a new process inside a container that is already running.

docker exec [OPTIONS] <CONTAINER_NAME> <COMMAND>
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  • <CONTAINER_NAME>: The name or ID of the container in which we want to execute the command.
  • <COMMAND>: The command we wish to run inside the container.
  • [OPTIONS]: Additional options to customize the command execution.

Basic example Suppose we have a running container named my_container that is running a web server. If we want to see the files inside the container, we can use docker exec to list the contents of the current directory:

docker exec my_container ls -l
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This command will run ls -l inside the my_container container and display the file listing in the terminal.

Entering as another user

Sometimes you enter a container and realize you are a limited user (for example, the node or postgres user) and do not have permissions to install tools (apt-get install vim).

You can force entry as root using the -u flag:

docker exec -u 0 -it my-container /bin/bash

The user ID for root in Linux is always 0

Or conversely, if you want to test if your application has the correct write permissions as a normal user:

docker exec -u node -it my-node-app /bin/sh

Running commands in containers with multiple processes

In containers that run multiple processes (for example, a container running a web server and a database server), it is important to ensure that the command we run does not interfere with the main processes.

For this, we can use the --detach or -d option to run the command in the background.

For example, if we want to run a maintenance script in the background inside a container, we can do it like this:

docker exec -d my_container /path/to/maintenance_script.sh
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This command will run the script in the background, allowing the container’s main processes to continue functioning without interruption.

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