Logs, exec and inspect

Logs

Docker captures what the main process writes to stdout and stderr. docker logs NAME shows it; --tail 20 limits lines, -f follows. Applications in containers should log to stdout rather than to files.

exec

docker exec NAME COMMAND runs an extra process in a running container: docker exec -it web sh opens a shell for debugging. Changes you make this way are lost when the container is replaced, so fix problems in the image, not by hand.

inspect

docker inspect returns the container's full configuration and state as JSON; --format extracts fields.

trap 'docker rm -f rc-logs > /dev/null 2>&1' EXIT
docker run -d --name rc-logs alpine:3.24 sh -c 'for i in 1 2 3; do echo "request $i ok"; done; echo "disk almost full" >&2; sleep 300' > /dev/null
sleep 1
echo "--- last 2 log lines (stdout and stderr):"
docker logs --tail 2 rc-logs 2>&1
echo "--- a command inside the running container:"
docker exec rc-logs sh -c 'ps -o pid,args | head -3'
echo "--- restart policy: $(docker inspect rc-logs --format '{{.HostConfig.RestartPolicy.Name}}')"

--- last 2 log lines (stdout and stderr):
request 2 ok
request 3 ok
--- a command inside the running container:
PID   COMMAND
    1 sleep 300
    7 sh -c ps -o pid,args | head -3
--- restart policy: no

See docker logs.