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Inspecting and Debugging Containers

1. Viewing Container Logs (docker logs)

Option Values Description
docker logs CONTAINER View logs for a running or stopped container
docker logs -f CONTAINER Follow log output in real-time
docker logs --since 2024-01-01T00:00:00 Only show logs since a specific time
docker logs --until 2024-01-02T00:00:00 Only show logs until a specific time
docker logs --tail 100 Show only the last 100 lines of logs
docker logs --details CONTAINER Show detailed log information including timestamps and stream type

Example: Viewing logs from a running container

docker logs my-app-container

Example: Following logs in real-time

docker logs -f my-app-container

Example: Viewing logs since a specific time

docker logs --since "2024-01-01T00:00:00" my-app-container

Example: Viewing last 50 lines of logs

docker logs --tail 50 my-app-container

2. Following Log Output

Option Values Description
docker logs CONTAINER_ID Display logs from a container
docker logs -f CONTAINER_ID Follow log output in real-time
docker logs --since 2024-01-01T00:00:00Z Show logs since a specific timestamp
docker logs --until 2024-01-02T00:00:00Z Show logs until a specific timestamp
docker logs -t CONTAINER_ID Include timestamps in output
docker logs --details CONTAINER_ID Show additional details (available in newer versions)

Example: Following Logs in Real-Time

docker logs -f <container_id>

Example: Filtering Logs by Time

docker logs --since "2024-01-01T00:00:00Z" --until "2024-01-02T00:00:00Z" <container_id>
Note: The -f flag allows you to follow logs in real-time, which is useful for monitoring application behavior during runtime.

3. Limiting Log Output (--tail, --since)

Option Description Usage
--tail Limits the number of lines from the end of the logs to display docker logs --tail 50 container_name
--since Shows logs only from a specific time or duration docker logs --since "2024-01-01T00:00:00" container_name

Example: Viewing Recent Logs

docker logs --tail 100 my_container

Example: Filtering Logs by Time

docker logs --since "1h" my_container

4. Inspecting Container Details

Command Description Example
docker inspect Get detailed configuration and status information about a container docker inspect my-container
docker inspect --format Format output using Go template syntax docker inspect --format '{{.State.Status}}' my-container

Example: Inspecting Container Configuration

docker inspect --format '{{.Config.Image}}' my-container
Note: The inspect command returns a JSON object with extensive container details including network settings, mount points, and environment variables.

5. Viewing Container Processes

Command Description Options Example
docker top Display the running processes inside a container -a - Show all processes, including those not attached to a TTY docker top my-container
docker ps List running containers; includes process info in the output -a - Show all containers, not just running ones docker ps --format "table {{.Names}}\t{{.Command}}\t{{.Status}}"
docker exec Execute a command inside a running container -it - Allocate a TTY and keep stdin open for interactive use docker exec -it my-container ps aux
Note: The docker top command provides a view similar to the Unix ps command, showing process details like PID, USER, and COMMAND.

Example: Inspecting a container's processes

docker top my-container

Example: Running ps inside a container

docker exec -it my-container ps aux

6. Monitoring Container Stats

Command Description Options Example
docker stats Displays live stream of container resource usage statistics --no-stream, --format, --all docker stats --no-stream
docker stats --format Formats output using Go template --format with JSON or Go template syntax docker stats --format "table {{.Name}}\t{{.CPUPerc}}\t{{.MemUsage}}"
Note: Stats are updated every second by default. Use --no-stream to get a single snapshot.

Key Stats Fields

  • CPU % - Percentage of CPU used
  • MEM USAGE / LIMIT - Memory usage and limit
  • NET I/O - Network input/output
  • BLOCK I/O - Block read/write operations
  • PIDS - Number of processes inside container

7. Executing Commands in Container

Command Description Example
docker exec Execute a command in a running container docker exec -it container_name /bin/bash
docker exec -d Execute a command in the background docker exec -d container_name echo "Hello"
docker exec -it Execute a command with TTY and interactive mode docker exec -it container_name sh

Example: Running a shell in a container

docker exec -it my_container /bin/bash

Example: Running a command in the background

docker exec -d my_container /bin/echo "Background task completed"
Note: The -it flags are commonly used together to provide an interactive terminal session inside the container.

8. Attaching to Running Container

Command Description Options Example
docker attach Attach local standard input, output, and error streams to a running container --no-stdin, --detach-keys docker attach --no-stdin my-container
Note: The container must be running for docker attach to work. It does not support interactive mode like docker exec.
Warning: Attaching to a container can interfere with its normal operation if the container's main process is not designed to handle terminal input/output.

9. Copying Files To/From Container

Command Description Example
docker cp [OPTIONS] SRC_PATH DEST_PATH Copies files or directories between a container and the local filesystem docker cp container:/app/data.txt ./local-data.txt

Example: Copying a file from container to host

docker cp my-container:/usr/local/bin/app ./app-binary

Example: Copying a directory from host to container

docker cp ./config-dir my-container:/etc/config
Note: The docker cp command requires the container to be running.

10. Viewing Container Changes

Command Description Example
docker diff Shows changes to files in the container's filesystem docker diff container-name
docker inspect Displays low-level information about a container docker inspect container-name

Example: Using docker diff

docker diff container-name

Output:

A /app/log.txt
C /app/config.json

Where:

  • A = Added file
  • C = Changed file
  • D = Deleted file
Note: docker diff only shows changes to the filesystem, not environment variables or runtime state.

11. Exporting Container Filesystem

Use the docker export command to export a container's filesystem as a tar archive.

Example: Exporting Container Filesystem

docker export <container-id> -o filesystem.tar
Option Values Description
--output, -o filesystem.tar Path to the output tar archive file
Note: The exported filesystem does not include any layers or metadata from the original image. It's a flat archive of the container's filesystem at the time of export.

12. Inspecting Container Exit Codes

Command Description Example
docker wait Blocks until container stops and prints its exit code docker wait my-container
docker inspect Shows detailed container configuration and state including exit code docker inspect --format='{{.State.ExitCode}}' my-container
docker logs Displays container logs, useful for diagnosing exit causes docker logs my-container

Example: Exit Code Analysis

docker run --rm ubuntu:20.04 /bin/sh -c 'exit 1'
docker wait $(docker ps -q --filter &#34;ancestor=ubuntu:20.04&#34;)
Note: Exit code 0 indicates success, any non-zero value indicates an error or failure.

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