Debugging

A failing lifecycle hook, post_start, pre_stop, or the new pre_start, used to report only its exit code: db hook exited with status 1 What the hook actually printed depended on how it ran. Under a plain docker compose up, the output streamed live to the terminal, mixed into the rest of up’s output, but was never carried into the error message itself. Under restart, run, or any tool driving Compose as a library, there was no listener attached at all, so Compose didn’t even request stdout or stderr from the daemon, and there was nothing to read afterward, live or otherwise. Since Compose 5.5.1, both cases keep a tail of the output and fold it straight into the error on failure. ...

docker compose down looks like a tidy shutdown command. It is, but the flags around it decide whether you walk away with your data intact, with a clean image store, or with a database wiped because you typed -v out of habit. The default behavior Without any flag, down stops and removes: Every container in the project The default network Compose created for the project Anonymous volumes that the stack was using It does not remove: ...

docker compose ps shows the services inside the current project. docker compose ls zooms out: every Compose stack running anywhere on the host, regardless of which directory you happen to be standing in. The basics Run it from any directory: docker compose ls Sample output: NAME STATUS CONFIG FILES api-platform running(4) /home/dev/api-platform/compose.yaml data-pipeline running(2) /home/dev/data-pipeline/compose.yaml old-prototype exited(3) /tmp/prototype/compose.yaml Three columns: the project name, how many services are up, and the path to the Compose file that started it. No need to cd into the project to see what’s running. ...

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. ...