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Docker

Useful Docker Commands to troubleshoot a container

David Lam

David Lam

— 2 min read
Useful Docker Commands to troubleshoot a container

Inspect container details:

Provides detailed configuration and state information about the container, including network settings, mounts, and environment variables.

docker inspect <container_name>

Check logs:

Displays the log of the container.

docker logs <container_name>

View real-time logs:

docker logs -f <container_name>

View environment variables:

docker exec <container_name> printenv

Access into a container directly:

Use the following command to access a container's shell interactively:

docker exec -it <container name> sh

Replace <container_name> with the name or ID of the container

Ping to outside from container:

(Check if the container network can access to outside)

docker exec -it <container name> ping <ip address>

Test if a container have open port:

To test if a specific port is open in a container, use this command:

docker exec -it <container name> netstat -tuln | grep <port number>

Replace <container_name> with the name or ID of the container, port number with the port number

Command Breakdown:

  • netstat -tuln: list all listening TCP and UDP ports.
  • grep <port number>: filters out the output to check if the specific <port number> is actively listening

Test if container can access to other ip:port

docker exec -it <container name> nc -zv <ip> <port>

Replace <container_name> with the name or ID of the container, ip port with the ip address and port number

Command Breakdown:

  • nc: (Netcat) A networking utility for reading from and writing to network connections
  • -z: Checks for open ports without sending data.
  • -v: Provides verbose output.

Add current user to the docker group:

This is useful when you need permission to manage Docker resources, such as editing files within Docker containers or executing Docker commands without requiring sudo.

sudo usermod -aG docker $USER

Command Breakdown:

  1. sudo: Runs the command with superuser (root) privileges, necessary for modifying user groups.
  2. usermod: A Linux utility to modify user account settings.
  3. -aG:
    1. -a (append): Ensures the user is added to the specified group without being removed from other groups.
    2. -G (groups): Specifies the group(s) the user should be added to.
  4. docker: The group to which the user is being added. The docker group allows members to execute Docker commands without needing sudo.
  5. $USER: A shell variable representing the username of the currently logged-in user.

Log out and log back in for the group membership to take effect:

newgrp docker

These commands are useful for debugging container network configurations and verifying connectivity.

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