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fundamentals

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Concepts​

Architecture​

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  • dockerd: Long-running background process
  • containerd: Lower-level runtime
  • runc: Executes syscalls

Overview​

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Commands​

# Create and start a container from an image
# clone() with namespace flags, set cgroups, exec the entrypoint
docker run  

# List running containers (-a for stopped too)
# Reads daemon state
docker ps

# Run a new process inside an existing container's namespaces
# setns() into target's namespaces, then exec
docker exec

# Show stdout/stderr captured by the daemon
# Reads the log driver's file
docker logs

# Send SIGTERM, wait --time, then SIGKILL
# Signal to PID 1 of the container
docker stop

# Remove a stopped container
# Frees the OverlayFS upper dir
docker rm

Best practices​

  • Pin tags: Pin specific versions like redis:7.2-alpine instead of redis:latest
  • Container naming: Name containers using --name
  • Explicit port: Map ports using -p HOST:CONTAINER as EXPOSE is only documentation
  • Execution modes: -d for background services, -it for interactive terminal commands
  • Restart policy: --restart unless-stopped prevents downtime from crashes or reboots
  • Lifecycle: docker rm deletes the container writable layer

Examples​

docker run --rm -it alpine:3.20 sh
/ #

docker run --rm python:3.12-slim python -c "import sys; print(sys.version)"

Implementation​

docker run -d \
  --name dev-redis \
  -p 127.0.0.1:6379:6379 \
  --restart unless-stopped \
  redis:7.2-alpine

docker ps -f name=dev-redis
docker port dev-redis
docker exec dev-redis redis-cli PING
docker logs dev-redis

Comparison​

Image base variations

  • redis:latest: Worst choice
  • redis:7.2: Debian based, heavier
  • redis:7.2-alpine: Alpine based, lightweight, best for Dev/CI
  • redis@sha256:...: Digest pin, best for production

Restart policy behaviors

  • none: Default, stays dead on crash or daemon restarts
  • on-failure: Restarts only when exiting with error code
  • always: Forces restart, even after manual stop
  • unless-stopped: Restarts on crash or machine reboot

Alpine​

Concepts​

  • Alpine image: Built on Alpine Linux instead of heavy distributions like Ubuntu or Debian
  • Philosophy: Only includes files strictly necessary to run the process
  • Base size: Approximately 5 MB

Components​

  • BusyBox: Combines common UNIX utilities into a single optimized executable file
  • musl libc: Replaces glibc as interface to the Linux kernel
  • Characteristics: Lightweight, fast, designed for embedded systems and minimal environments
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