Debugging

Linux isolates containers using kernel namespaces. Sometimes you need the opposite: two containers that can see each other’s processes or share memory. The pid and ipc directives give you that escape hatch. Sharing a PID namespace pid: service:<name> lets a container see and act on processes inside another service: services: app: image: myapp debugger: image: alpine pid: service:app cap_add: - SYS_PTRACE command: sleep infinity The debugger container’s ps, strace, and /proc all reflect app’s processes. Combined with cap_add: SYS_PTRACE, you can attach strace or gdb to a running production-style container without baking debug tools into its image. ...

Need to grab a log file out of a container? Push a quick config change for a one-off test? Don’t redeploy with a bind mount, just use docker compose cp. Basic usage The syntax mirrors docker cp but resolves service names instead of container IDs: # Copy from container to host docker compose cp <service>:<container-path> <host-path> # Copy from host to container docker compose cp <host-path> <service>:<container-path> The direction is determined by which side has the service: prefix. ...

A container is running but your app feels sluggish. Is it CPU-bound? Leaking memory? Stuck on a runaway process? Compose gives you two essential commands to find out: docker compose top and docker compose stats. docker compose top Show running processes inside each service’s container: docker compose top myapp-web-1 UID PID PPID C STIME TTY TIME CMD root 12345 12320 0 09:15 ? 00:00:02 nginx: master process nobody 12400 12345 0 09:15 ? 00:00:00 nginx: worker process myapp-api-1 UID PID PPID C STIME TTY TIME CMD node 12500 12480 2 09:15 ? 00:01:45 node server.js node 12600 12500 0 09:15 ? 00:00:12 node worker.js You see every process running inside each container with their CPU usage (C column) and CPU time. Perfect for spotting runaway workers or unexpected child processes. ...

Most people know docker compose config as “the validate command”. It’s much more than that. The same command can list resources, output JSON, hash services for change detection, and pin image digests for reproducibility. Listing resources Ask Compose what’s actually in your project, after all overrides and interpolation: # List all services docker compose config --services # List all volumes docker compose config --volumes # List all images used docker compose config --images # List all defined profiles docker compose config --profiles These are great in scripts when you need to loop over each service: ...

Not sure what docker compose up will actually do? Add --dry-run to preview every action without executing anything. Basic usage docker compose up --dry-run Compose shows exactly what it would do: DRY-RUN MODE - Container myapp-db-1 Creating DRY-RUN MODE - Container myapp-db-1 Created DRY-RUN MODE - Container myapp-web-1 Creating DRY-RUN MODE - Container myapp-web-1 Created DRY-RUN MODE - Container myapp-db-1 Starting DRY-RUN MODE - Container myapp-db-1 Started DRY-RUN MODE - Container myapp-web-1 Starting DRY-RUN MODE - Container myapp-web-1 Started Nothing is created, started, or modified. You just see the plan. ...