Docker Cheatsheet 14 — Debugging Containers

docker exec -it web bash         # or sh
docker exec -it web sh -c "ls /app"
docker exec --user root web bash # as root

docker run --rm -it --pid=container:web --net=container:web nicolaka/netshoot

docker inspect web | jq
docker inspect --format='{{.State.Status}}' web
docker inspect --format='{{.NetworkSettings.IPAddress}}' web
docker inspect --format='{{json .Config.Env}}' web | jq
docker inspect --format='{{range .Mounts}}{{.Source}} -> {{.Destination}}{{println}}{{end}}' web

docker logs web
docker logs -f --tail 100 web
docker logs --since 10m web
docker logs web 2>&1 | grep ERROR
docker logs -t web                # add timestamps

docker run --rm myimage           # see the error
docker inspect <id> --format='{{.State.Error}}'
docker logs <id>
docker events --filter container=<id>

# DNS
docker exec web nslookup db
docker exec web getent hosts db

# Connectivity
docker exec web ping -c 3 db
docker exec web nc -zv db 5432
docker exec web curl -v http://api/

# Port listen
docker exec web netstat -tlnp        # if available
docker exec web ss -tlnp

docker top web
docker exec web ps auxf
docker exec web pstree

docker exec web cat /etc/hosts
docker diff web                  # changes since image started

docker exec web ls -la /proc/1/fd/

docker exec web cat /proc/1/limits

docker exec --cap-add SYS_PTRACE web strace -p 1

docker run --rm --net container:web nicolaka/netshoot \
  tcpdump -i any -w - | wireshark -k -i -

docker stats web
docker stats --no-stream

# Inside container
docker exec web top
docker exec web free -h

docker system df
docker system df -v
du -sh /var/lib/docker/volumes/*/_data | sort -h

docker inspect --format='{{.State.ExitCode}}' web

docker run --ulimit core=-1 --security-opt seccomp=unconfined app
# Inside container, core dump on crash

docker inspect --format='{{.State.RestartCount}}' web

docker logs --tail 100 web        # what's the error?

docker history --no-trunc myimage
docker run --rm -it myimage sh   # inspect contents
docker save myimage > image.tar
tar tvf image.tar                # list layers

# Python
docker exec web py-spy top --pid 1

# Node
docker exec web kill -SIGUSR1 1   # signals inspector start
# Then chrome://inspect

# Go
docker exec web go tool pprof http://localhost:6060/debug/pprof/profile

docker compose logs -f
docker compose ps
docker compose config            # render final compose
docker compose top

docker build --progress=plain .          # verbose output
docker build --no-cache .                # bypass cache
docker buildx build --target builder .   # stop at stage

# Inspect intermediate stage
docker build -t debug --target builder .
docker run --rm -it debug bash

docker debug web                 # attach debug shell with tools