Welcome to Day 14 of Advent of Docker! Today, we’ll explore essential Docker debugging tools that will help you troubleshoot container issues. We’ll use our Go HTTP server from previous days as an example to demonstrate these tools.
Docker ps - Container Status
The docker ps command is your first stop for debugging. It shows running containers and their basic information:
$ docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
abc123def456 hello-world-go "./main" 20 minutes ago Up 20 minutes 0.0.0.0:8080->8080/tcp myapp
Some useful flags:
-a: Show all containers (including stopped ones)-q: Only display container IDs (useful for scripting)-f status=exited: Filter containers by status
Docker Logs - Container Output
When your container isn’t behaving as expected, checking its logs is crucial:
$ docker logs myapp
Listening on port 8080
Useful flags:
-f: Follow log output (liketail -f)--tail 100: Show last 100 lines--since 5m: Show logs from last 5 minutes
For our Go server, we can watch incoming requests in real-time:
$ docker logs -f myapp
Listening on port 8080
New visit recorded at 2024-12-14T10:00:00Z
New visit recorded at 2024-12-14T10:01:00Z
Docker Inspect - Deep Dive
docker inspect provides detailed information about a container:
$ docker inspect myapp
[
{
"Id": "f915b41c43f7cadb2be97e30b6d7eac9ec0f04beed3ffd1d47b62d7c4f6d649a",
"Created": "2024-12-14T18:07:45.903730046Z",
"Path": "./main",
"Args": [],
"State": {
"Status": "running",
"Running": true,
"Paused": false,
"Restarting": false,
"OOMKilled": false,
"Dead": false,
"Pid": 1564,
"ExitCode": 0,
"Error": "",
"StartedAt": "2024-12-14T18:07:46.19197188Z",
"FinishedAt": "0001-01-01T00:00:00Z"
},
"Image": "sha256:6209fff2e82261192905db51004792b13e688c44729e457299daa5230b2f8c31",
"ResolvConfPath": "/var/lib/docker/containers/f915b41c43f7cadb2be97e30b6d7eac9ec0f04beed3ffd1d47b62d7c4f6d649a/resolv.conf",
"HostnamePath": "/var/lib/docker/containers/f915b41c43f7cadb2be97e30b6d7eac9ec0f04beed3ffd1d47b62d7c4f6d649a/hostname",
"HostsPath": "/var/lib/docker/containers/f915b41c43f7cadb2be97e30b6d7eac9ec0f04beed3ffd1d47b62d7c4f6d649a/hosts",
"LogPath": "/var/lib/docker/containers/f915b41c43f7cadb2be97e30b6d7eac9ec0f04beed3ffd1d47b62d7c4f6d649a/f915b41c43f7cadb2be97e30b6d7eac9ec0f04beed3ffd1d47b62d7c4f6d649a-json.log",
"Name": "/relaxed_mayer",
"RestartCount": 0,
"Driver": "overlay2",
"Platform": "linux",
"MountLabel": "",
"ProcessLabel": "",
"AppArmorProfile": "",
"ExecIDs": null,
"HostConfig": {
"Binds": null,
"ContainerIDFile": "",
"LogConfig": {
"Type": "json-file",
"Config": {}
},
"NetworkMode": "default",
"PortBindings": {},
"RestartPolicy": {
"Name": "no",
"MaximumRetryCount": 0
},
"AutoRemove": false,
"VolumeDriver": "",
"VolumesFrom": null,
"ConsoleSize": [
22,
97
],
"CapAdd": null,
"CapDrop": null,
"CgroupnsMode": "private",
"Dns": [],
"DnsOptions": [],
"DnsSearch": [],
"ExtraHosts": null,
"GroupAdd": null,
"IpcMode": "private",
"Cgroup": "",
"Links": null,
"OomScoreAdj": 0,
"PidMode": "",
"Privileged": false,
"PublishAllPorts": false,
"ReadonlyRootfs": false,
"SecurityOpt": null,
"UTSMode": "",
"UsernsMode": "",
"ShmSize": 67108864,
"Runtime": "runc",
"Isolation": "",
"CpuShares": 0,
"Memory": 0,
"NanoCpus": 0,
"CgroupParent": "",
"BlkioWeight": 0,
"BlkioWeightDevice": [],
"BlkioDeviceReadBps": [],
"BlkioDeviceWriteBps": [],
"BlkioDeviceReadIOps": [],
"BlkioDeviceWriteIOps": [],
"CpuPeriod": 0,
"CpuQuota": 0,
"CpuRealtimePeriod": 0,
"CpuRealtimeRuntime": 0,
"CpusetCpus": "",
"CpusetMems": "",
"Devices": [],
"DeviceCgroupRules": null,
"DeviceRequests": null,
"MemoryReservation": 0,
"MemorySwap": 0,
"MemorySwappiness": null,
"OomKillDisable": null,
"PidsLimit": null,
"Ulimits": [],
"CpuCount": 0,
"CpuPercent": 0,
"IOMaximumIOps": 0,
"IOMaximumBandwidth": 0,
"MaskedPaths": [
"/proc/asound",
"/proc/acpi",
"/proc/kcore",
"/proc/keys",
"/proc/latency_stats",
"/proc/timer_list",
"/proc/timer_stats",
"/proc/sched_debug",
"/proc/scsi",
"/sys/firmware"
],
"ReadonlyPaths": [
"/proc/bus",
"/proc/fs",
"/proc/irq",
"/proc/sys",
"/proc/sysrq-trigger"
]
},
"GraphDriver": {
"Data": {
"LowerDir": "/var/lib/docker/overlay2/c8b77f1281e2a923debc57ba8ee8a3e61b2eab01c3746707847c56aa504f4549-init/diff:/var/lib/docker/overlay2/plebc717q0dae0vpogboi82uj/diff:/var/lib/docker/overlay2/0vwf87fyclw8wmvgnjngza8ul/diff:/var/lib/docker/overlay2/02a45bf063940c5ed96d43abf699c49e729c0197a8922613a7b1e85454397fd9/diff:/var/lib/docker/overlay2/1f7886b749ec4d3a50ff5a88e082404c777b05cb8af10469eb8d32f283eaa2fd/diff:/var/lib/docker/overlay2/813de0000b1ac87c0ebbc260ab5db42a860e771927966b98a08a6eb942a7478e/diff:/var/lib/docker/overlay2/bb1ea71019466bdd2da7dccf480c104121842b694d6022f96367214f6f623557/diff:/var/lib/docker/overlay2/aa54f03b2bcd787f34371e483e4b14024920790f54d8bdf2dda70c828022d3d0/diff:/var/lib/docker/overlay2/3caedf6970d13d30fd04cb8553c19a1dc1012d1474474f9e38dd992eb5a08e08/diff:/var/lib/docker/overlay2/c2ff2ee7edd0d1efda1493a24508b0bb8132464f0ea3a600572bb823a6f43708/diff",
"MergedDir": "/var/lib/docker/overlay2/c8b77f1281e2a923debc57ba8ee8a3e61b2eab01c3746707847c56aa504f4549/merged",
"UpperDir": "/var/lib/docker/overlay2/c8b77f1281e2a923debc57ba8ee8a3e61b2eab01c3746707847c56aa504f4549/diff",
"WorkDir": "/var/lib/docker/overlay2/c8b77f1281e2a923debc57ba8ee8a3e61b2eab01c3746707847c56aa504f4549/work"
},
"Name": "overlay2"
},
"Mounts": [],
"Config": {
"Hostname": "f915b41c43f7",
"Domainname": "",
"User": "",
"AttachStdin": false,
"AttachStdout": true,
"AttachStderr": true,
"ExposedPorts": {
"8080/tcp": {}
},
"Tty": false,
"OpenStdin": false,
"StdinOnce": false,
"Env": [
"PATH=/go/bin:/usr/local/go/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
"GOLANG_VERSION=1.23.4",
"GOTOOLCHAIN=local",
"GOPATH=/go"
],
"Cmd": [
"./main"
],
"Image": "hello-world-go",
"Volumes": null,
"WorkingDir": "/go",
"Entrypoint": null,
"OnBuild": null,
"Labels": {}
},
"NetworkSettings": {
"Bridge": "",
"SandboxID": "f790e6709650efe5af98f6caf40b365b541b8005f4da2a4a636297c26e2a13d4",
"HairpinMode": false,
"LinkLocalIPv6Address": "",
"LinkLocalIPv6PrefixLen": 0,
"Ports": {
"8080/tcp": null
},
"SandboxKey": "/var/run/docker/netns/f790e6709650",
"SecondaryIPAddresses": null,
"SecondaryIPv6Addresses": null,
"EndpointID": "01cb1873a9821ed1369e79274537ba2234676130a295cbfc5def7a8e0059101a",
"Gateway": "172.17.0.1",
"GlobalIPv6Address": "",
"GlobalIPv6PrefixLen": 0,
"IPAddress": "172.17.0.2",
"IPPrefixLen": 16,
"IPv6Gateway": "",
"MacAddress": "02:42:ac:11:00:02",
"Networks": {
"bridge": {
"IPAMConfig": null,
"Links": null,
"Aliases": null,
"NetworkID": "ee1f25dbee96b37d4acbdbaee780065e892468e515104b6e7c590be39365c3e5",
"EndpointID": "01cb1873a9821ed1369e79274537ba2234676130a295cbfc5def7a8e0059101a",
"Gateway": "172.17.0.1",
"IPAddress": "172.17.0.2",
"IPPrefixLen": 16,
"IPv6Gateway": "",
"GlobalIPv6Address": "",
"GlobalIPv6PrefixLen": 0,
"MacAddress": "02:42:ac:11:00:02",
"DriverOpts": null
}
}
}
}
]
Now this might be a bit overwhelming, but it’s a good starting point to understand what’s going on with your container! Once you know what exactly you’re looking for, you can use Go templates to extract specific information:
# Get IP address
$ docker inspect -f '{{range .NetworkSettings.Networks}}{{.IPAddress}}{{end}}' myapp
# Get environment variables
$ docker inspect -f '{{range .Config.Env}}{{println .}}{{end}}' myapp
Container Dumps - Advanced Debugging
For more serious issues, you can create a container dump:
$ docker container export myapp > container.tar
This creates a tarball containing the container’s entire filesystem, which you can then examine:
$ tar -tvf container.tar | grep main
-rwxr-xr-x 0/0 1839488 2024-12-14 10:00 ./main
Resource Usage and Cleanup
Sometimes your apps might run out of disk space. You can check how much space Docker is using with docker system df:
$ docker system df
TYPE TOTAL ACTIVE SIZE RECLAIMABLE
Images 5 3 1.92GB 1.2GB
Containers 3 1 125MB 125MB
Local Volumes 2 2 256MB 0B
Clean Up Resources
When debugging leads to many unused containers and images, clean up with:
$ docker system prune
WARNING! This will remove:
- all stopped containers
- all networks not used by at least one container
- all dangling images
- all dangling build cache
Are you sure you want to continue? [y/N]
Add -a to remove all unused images, not just dangling ones:
$ docker system prune -a
Real-World Example
Let’s debug a common issue with our Go server. Imagine it’s not responding on port 8080:
- Check if container is running:
$ docker ps
CONTAINER ID IMAGE STATUS PORTS
# No container shown - it's not running
- Check stopped containers:
$ docker ps -a
CONTAINER ID IMAGE STATUS PORTS
def456abc789 hello-world-go Exited (1) 2 minutes ago
- Check logs for error:
$ docker logs def456abc789
bind: address already in use
- Inspect container for port bindings:
$ docker inspect def456abc789 -f '{{json .HostConfig.PortBindings}}'
{"8080/tcp":[{"HostIp":"","HostPort":"8080"}]}
Now we know the issue: port 8080 is already in use on the host. We can fix this by using a different port:
$ docker run -p 8081:8080 hello-world-go
Best Practices
- Always check logs first (
docker logs) - Use meaningful container names for easier debugging
- Regularly clean up unused resources
- Monitor resource usage to prevent disk space issues
- Keep your Docker daemon and tools updated
Next Steps
Practice these debugging techniques with your own containers. Try to:
- Simulate different failure scenarios
- Extract specific information using
docker inspect - Monitor resource usage over time
Tomorrow, we’ll have our second quiz! Make sure you are comfortable with:
- Debugging Techniques
- Multi-Stage Builds
- Docker Compose
- Docker Networking
- Environment Variables & Configuration
Until then, happy debugging! 🐳
Jonas
