Day 40 · Docker

Docker EXEC Operations

By Published Docker / Linux~1h to complete

Technical Overview

When working with containerized systems, debugging, administration, and runtime operations require executing commands directly inside the namespace of a running container.

While docker run starts a new container from an image to run a task, the docker exec command allows you to run new processes inside an already running container.

How docker exec Works

docker exec works by invoking the Docker Daemon to spawn a child process inside the specific namespaces (namespaces for PID, network, mount, IPC, etc.) of a target running container. This means the executed command runs within the container's environment, inheriting its filesystem, network interfaces, and environment variables.

sequenceDiagram actor Admin as Sysadmin (Host) participant Engine as Docker Daemon participant Container as Container Namespace (kkloud) Admin->>Engine: docker exec -it kkloud bash Engine->>Container: Spawn bash shell inside container namespaces (net, pid, mnt) Engine-->>Admin: Attach STDIN/STDOUT (Pseudo-TTY) Admin->>Container: Run "apt-get install -y apache2" Container-->>Admin: Install packages Admin->>Container: Run port configuration commands (sed) Admin->>Container: Run "service apache2 restart" Container-->>Admin: Apache starts on port 8085 Admin->>Container: Run "exit" to detach

Options Breakdown: -i vs -t

To run commands interactively (like entering a shell), you combine two options:

  • -i (or --interactive): Keeps standard input (STDIN) open, allowing you to type commands.
  • -t (or --tty): Allocates a pseudo-TTY (terminal), enabling interactive terminal elements like text coloring, command prompt styling, and tab auto-completion.

Infrastructure & Configuration Requirements

  • Target Host: Nautilus App Server 1 (stapp01) (can vary in labs, e.g., stapp01, stapp02, stapp03)
  • SSH User: tony (associated with stapp01; steve for stapp02, banner for stapp03)
  • Target Running Container: kkloud
  • Package to Install: apache2
  • Target Listen Port: 8085 (instead of default port 80)

Step-by-Step Implementation

Step 1: Connect to the Application Server

Establish an SSH connection from the Jump Host to App Server 1:

ssh tony@stapp01

Provide the server password when prompted.

Step 2: Verify the Target Container is Running

List the running Docker containers to verify that the container kkloud is active:

docker ps

Expected Output:

CONTAINER ID   IMAGE     COMMAND                  CREATED          STATUS          PORTS     NAMES
3a5b6c7d8e9f   ubuntu    "tail -f /dev/null"      12 minutes ago   Up 12 minutes             kkloud

Step 3: Install Apache inside the Container

Run the package updater and installer inside the container using docker exec. You can do this by executing the commands non-interactively from the host terminal:

# Update repository lists inside kkloud
docker exec -it kkloud apt-get update

# Install apache2 without interactive prompts
docker exec -it kkloud env DEBIAN_FRONTEND=noninteractive apt-get install -y apache2

Note: If your host user requires root privileges, prepend the docker command with sudo.

Step 4: Configure Apache Listen Port

Access the container shell interactively to adjust Apache configuration files:

docker exec -it kkloud bash

You are now inside the container filesystem. Run the following configurations:

# 1. Update the ports.conf file to change Listen 80 to Listen 8085
sed -i 's/Listen 80/Listen 8085/g' /etc/apache2/ports.conf

# 2. Update the default VirtualHost site configuration file to use port 8085
sed -i 's/:80/:8085/g' /etc/apache2/sites-available/000-default.conf
sed -i 's/:80/:8085/g' /etc/apache2/sites-enabled/000-default.conf

Step 5: Start Apache Web Server

Restart the Apache service inside the container to apply the port changes:

service apache2 restart

Exit the container shell:

exit

Post-Deployment Verification

1. Verify Listen Ports inside Container

Check if the Apache process is listening on the modified port 8085 inside the container:

docker exec kkloud ss -tuln
# or
docker exec kkloud netstat -tuln

Expected Output:

Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State      
tcp6       0      0 :::8085                 :::*                    LISTEN

2. Verify Web Server Response

Perform a local HTTP request using curl inside the container's network context to confirm the default page is serving correctly on port 8085:

docker exec kkloud curl -I http://localhost:8085

Expected Output:

HTTP/1.1 200 OK
Date: Sat, 11 Jul 2026 22:52:00 GMT
Server: Apache/2.4.41 (Ubuntu)
...

Log out of the Application Server:

exit

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