Quick Guide on Docker Utilities, Daemon and its other capabilities

Introduction

Docker is playing very critical role as both a packaging format for the applications and a unifying interface and methodology that enables the application team/s to own the Docker-formatted container images (Note: It is assumed that including all dependencies). This has made Docker as one of the leading front runners in microservices and microservices based adaption and usage.

Docker-Utilities-Daemon

Many of the Docker and Docker based utilities helps us to bring greater efficiencies of scalability and performance by shrinking application footprints through Dockerized containers. In many cases system-level dependencies are reduced to bare minimum, which helps in bringing down memory usage in terms of MB’s (Mega bytes).

All these aspects helped in making Docker as one of the leading container based utility. Docker has many command-line utilities and capabilities. Docker Daemon can provide many of these additional facilities and which makes Docker configuration easier.

Read Also :How to Install Docker (Community Edition) on Ubuntu 16.04 / 18.04 LTS

Hope the reader/user of this article knows how to spin-off docker-container on any given Linux environment. Assuming this, some of the following facilities are described,

Some of these facilities can be used for the following functions,

  • Check Docker server information
  • Download docker images and updates
  • Inspect containers
  • View and monitor logs
  • Monitor various statistics and details

1) How to check present docker version

Knowing the docker version is one of the very important aspects as depending on this, many of the version based decisions are taken.

[email protected]:~$ sudo docker version
Client:
 Version:           18.09.5
 API version:       1.39
 Go version:        go1.10.8
 Git commit:        e8ff056
 Built:             Thu Apr 11 04:43:57 2019
 OS/Arch:           linux/amd64
 Experimental:      false

Server: Docker Engine - Community
 Engine:
  Version:          18.09.5
  API version:      1.39 (minimum version 1.12)
  Go version:       go1.10.8
  Git commit:       e8ff056
  Built:            Thu Apr 11 04:10:53 2019
  OS/Arch:          linux/amd64
  Experimental:     false
[email protected]:~$

This above output provides both server and client API versions as same and OS and Architecture version also should be same. If there is any mismatch in client and server based versions, client-server communication will fail. One has to make sure what the supporting versions are and take decisions accordingly.

2) Capturing and analyzing server information

Using docker info we can also find the following information. Some of the useful information we can gather are something like, which server is running as backend, which kernel version it is, which OS and Docker root directory etc

[email protected]:~$ sudo docker info
Containers: 2
 Running: 1
 Paused: 0
 Stopped: 1
Images: 4
Server Version: 18.09.5
Storage Driver: overlay2
 Backing Filesystem: extfs
 Supports d_type: true
 Native Overlay Diff: true
Logging Driver: json-file
Cgroup Driver: cgroupfs
Plugins:
 Volume: local
 Network: bridge host macvlan null overlay
 Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog
Swarm: inactive
Runtimes: runc
Default Runtime: runc
Init Binary: docker-init
containerd version: bb71b10fd8f58240ca47fbb579b9d1028eea7c84
runc version: 2b18fe1d885ee5083ef9f0838fee39b62d653e30
init version: fec3683
Security Options:
 apparmor
 seccomp
  Profile: default
Kernel Version: 4.15.0-20-generic
Operating System: Ubuntu 18.04 LTS
OSType: linux
Architecture: x86_64
CPUs: 1
Total Memory: 1.947GiB
Name: linuxtechi
ID: VRUY:AWXX:7JWE:YWU7:X4QW:TNKE:6H26:PNRR:QFGI:XYRQ:QUXF:MTXC
Docker Root Dir: /var/lib/docker
Debug Mode (client): false
Debug Mode (server): false
Registry: https://index.docker.io/v1/
Labels:
Experimental: false
Insecure Registries:
 127.0.0.0/8
Live Restore Enabled: false
Product License: Community Engine
WARNING: No swap limit support
[email protected]:~$

All the above information is based on how the Docker Daemon is set-up, underlying OS version and file system type. All these can be captured using the following set-of the commands,

Execute the below command to get OS Name, its version and Code name

[email protected]:~$ cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=18.04
DISTRIB_CODENAME=bionic
DISTRIB_DESCRIPTION="Ubuntu 18.04 LTS"
[email protected]:~$

or

[email protected]:~$ cat /etc/*-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=18.04
DISTRIB_CODENAME=bionic
DISTRIB_DESCRIPTION="Ubuntu 18.04 LTS"
NAME="Ubuntu"
VERSION="18.04 LTS (Bionic Beaver)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 18.04 LTS"
VERSION_ID="18.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=bionic
UBUNTU_CODENAME=bionic
[email protected]:~$

Execute the beneath command to get the file system details :

[email protected]:~$ mount | grep "^/dev"
/dev/sda1 on / type ext4 (rw,relatime,errors=remount-ro,data=ordered)
[email protected]:~$

3) Docker daemon

The docker daemon plays a very crucial role in docker environment as whole.

Without a proper daemon, the complete docker system will be useless. One can verify the daemon status using the following command,

Note:- Assuming proper docker installation is done

[email protected]:~$ sudo service docker status

If the docker service is running, then output of above command should be something like below:

docker-service-status-output-ubuntu18

In case docker service is not running then use below command to start it

[email protected]:~$ sudo systemctl start docker
or
[email protected]:~$ sudo service docker start
[email protected]:~$

Use below “docker ps” command to list the running containers

[email protected]:~$ sudo docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES
[email protected]:~$

To list all running and stopped containers use “docker ps -a

[email protected]:~$ sudo docker ps -a
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS                      PORTS               NAMES
497e6733d760        ubuntu              "bash"              19 minutes ago      Exited (0) 2 minutes ago                        goofy_morse
0862fe109f96        hello-world         "/hello"            19 minutes ago      Exited (0) 19 minutes ago                       vibrant_shannon
[email protected]:~$

Docker default root directory  is “/var/lib/docker”

[email protected]:~$ sudo ls -l /var/lib/docker
total 48
drwx------  2 root root 4096 Apr 14 07:00 builder
drwx------  4 root root 4096 Apr 14 07:00 buildkit
drwx------  4 root root 4096 Apr 14 07:09 containers
drwx------  3 root root 4096 Apr 14 07:00 image
drwxr-x---  3 root root 4096 Apr 14 07:00 network
drwx------ 16 root root 4096 Apr 14 07:27 overlay2
drwx------  4 root root 4096 Apr 14 07:00 plugins
drwx------  2 root root 4096 Apr 14 07:27 runtimes
drwx------  2 root root 4096 Apr 14 07:00 swarm
drwx------  2 root root 4096 Apr 14 07:27 tmp
drwx------  2 root root 4096 Apr 14 07:00 trust
drwx------  2 root root 4096 Apr 14 07:00 volumes
[email protected]:~$

The docker daemon if not started can be invoked using the following command,

[email protected]:~$ sudo dockerd

Output of above dockerd command will be something like below:

dockerd-command-output-ubuntu18

Read Also : How to install Docker on CentOS 7

4) Downloading docker container image and inspecting the container

[email protected]:~$ sudo docker pull ubuntu:latest
Using default tag: latest
latest: Pulling from library/ubuntu
898c46f3b1a1: Pull complete
63366dfa0a50: Pull complete
041d4cd74a92: Pull complete
6e1bee0f8701: Pull complete
Digest: sha256:017eef0b616011647b269b5c65826e2e2ebddbe5d1f8c1e56b3599fb14fabec8
Status: Downloaded newer image for ubuntu:latest
[email protected]:~$
latest: Pulling from library/ubuntu898c46f3b1a1: Pull complete
63366dfa0a50: Pull complete
041d4cd74a92: Pull complete6e1bee0f8701: Pull complete
Digest: sha256:017eef0b616011647b269b5c65826e2e2ebddbe5d1f8c1e56b3599fb14fabec8Status: Downloaded newer image for ubuntu:latest

Launching a container, example is shown below

[email protected]:~$ sudo docker run -d -t ubuntu /bin/bash
58c023f0f5689ff08b858221ca10c985936a8c9dd91d08e84213009facb64724
[email protected]:~$
[email protected]:~$ sudo docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES
58c023f0f568        ubuntu              "/bin/bash"         27 seconds ago      Up 26 seconds                           boring_dijkstra
[email protected]:~$

Let’s inspect this container using the following command,

[email protected]:~$ sudo docker inspect 58c023f0f568
[
    {
        "Id": "58c023f0f5689ff08b858221ca10c985936a8c9dd91d08e84213009facb64724",
        "Created": "2019-04-14T06:55:26.289022884Z",
        "Path": "/bin/bash",
        "Args": [],
        "State": {
            "Status": "running",
            "Running": true,
            "Paused": false,
            "Restarting": false,
            "OOMKilled": false,
            "Dead": false,
            "Pid": 15538,
            "ExitCode": 0,
            "Error": "",
            "StartedAt": "2019-04-14T06:55:27.142274111Z",
            "FinishedAt": "0001-01-01T00:00:00Z"
        },
        "Image": "sha256:94e814e2efa8845d95b2112d54497fbad173e45121ce9255b93401392f538499",
        "ResolvConfPath": "/var/lib/docker/containers/58c023f0f5689ff08b858221ca10c985936a8c9dd91d08e84213009facb64724/resolv.conf",
………………………………………………

Note:- The complete output of this command is not shown here, because it’s output is too large

5) Going inside the running container image

As one is aware of Docker used originally everything based on LXC backend, lxc-attach command of linux was used for quite some time. But once docker was built as a standalone package and started using “libcontainer” as the default backend, the “docker exec” command usage became popular

Below set of commands explain on how to login into container using command line,

:~$ sudo docker  exec -t <Container_id>  <Commands>

[email protected]:~$ sudo docker  exec -t 58c023f0f568 ls -a
.   .dockerenv  boot  etc   lib    media  opt   root  sbin  sys  usr
..  bin         dev   home  lib64  mnt    proc  run   srv   tmp  var
[email protected]:~$ sudo docker  exec -t 58c023f0f568 ps -ef
UID        PID  PPID  C STIME TTY          TIME CMD
root         1     0  0 06:55 pts/0    00:00:00 /bin/bash
root        20     0  0 07:17 pts/1    00:00:00 ps -ef
[email protected]:~$

The alternate way to run commands inside the container is, first login to container using “docker attach” and then execute commands

:~$ sudo docker attach <Container_id>

[email protected]:~$ sudo docker attach 58c023f0f568
[email protected]:/#

There are some situations where you want to launch a container, attaching a volume to it and when we exit from container it should deleted automatically, example is shown below

[email protected]:~$ sudo docker run -it --rm -v /usr/local/bin:/target jpetazzo/nsenter bash
[email protected]:/src# df -h /target/
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda1        18G  5.6G   12G  34% /target
[email protected]:/src# exit
exit
[email protected]:~$

Now verify whether docker is removed or not automatically when we exit from the container, run “docker ps”

[email protected]:~$ sudo docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES
58c023f0f568        ubuntu              "/bin/bash"         About an hour ago   Up About an hour                        boring_dijkstra
[email protected]:~$

6) Monitoring Docker

Use “docker stats” command to display resource utilization of all the containers,

[email protected]:~$ sudo docker stats
CONTAINER ID        NAME                CPU %               MEM USAGE / LIMIT     MEM %               NET I/O             BLOCK I/O           PIDS
58c023f0f568        boring_dijkstra     0.00%               1.059MiB / 1.947GiB   0.05%               4.75kB / 0B         4.74MB / 0B         1
CONTAINER ID        NAME                CPU %               MEM USAGE / LIMIT     MEM %               NET I/O             BLOCK I/O           PIDS
58c023f0f568        boring_dijkstra     0.00%               1.059MiB / 1.947GiB   0.05%               4.75kB / 0B         4.74MB / 0B         1
[email protected]:~$

7) Docker Events

Many a times capturing docker events will be crucial as this will provide information on various docker operations and events taking place. Below is the snapshot of the same.

[email protected]:~$ sudo docker events
2019-04-14T09:29:07.636990738+01:00 image pull wordpress:latest (name=wordpress)
2019-04-14T09:29:46.936676431+01:00 volume create 36187e0a44d277439fea0df2446fc44987fa814c52744091929e5c81bd8134e5 (driver=local)
2019-04-14T09:29:46.998798935+01:00 container create b97f6a565c518eb1464cf81e6e09db1acfd84a0fdcbaea94255f1f182d79c058 (image=wordpress, name=friendly_heisenberg)
2019-04-14T09:29:47.000202026+01:00 container attach b97f6a565c518eb1464cf81e6e09db1acfd84a0fdcbaea94255f1f182d79c058 (image=wordpress, name=friendly_heisenberg)
2019-04-14T09:29:47.209257002+01:00 network connect 18dd93c3c6fc9ce51a98f7d2359b319db251efcae6b991157965ef727a580702 (container=b97f6a565c518eb1464cf81e6e09db1acfd84a0fdcbaea94255f1f182d79c058, name=bridge, type=bridge)
2019-04-14T09:29:47.239846902+01:00 volume mount 36187e0a44d277439fea0df2446fc44987fa814c52744091929e5c81bd8134e5 (container=b97f6a565c518eb1464cf81e6e09db1acfd84a0fdcbaea94255f1f182d79c058, destination=/var/www/html, driver=local, propagation=, read/write=true)
2019-04-14T09:29:47.942997316+01:00 container start b97f6a565c518eb1464cf81e6e09db1acfd84a0fdcbaea94255f1f182d79c058 (image=wordpress, name=friendly_heisenberg)
2019-04-14T09:29:47.944521098+01:00 container resize b97f6a565c518eb1464cf81e6e09db1acfd84a0fdcbaea94255f1f182d79c058 (height=39, image=wordpress, name=friendly_heisenberg, width=130)
2019-04-14T09:29:59.829378089+01:00 container die b97f6a565c518eb1464cf81e6e09db1acfd84a0fdcbaea94255f1f182d79c058 (exitCode=0, image=wordpress, name=friendly_heisenberg)
2019-04-14T09:30:00.147435896+01:00 network disconnect 18dd93c3c6fc9ce51a98f7d2359b319db251efcae6b991157965ef727a580702 (container=b97f6a565c518eb1464cf81e6e09db1acfd84a0fdcbaea94255f1f182d79c058, name=bridge, type=bridge)
2019-04-14T09:30:00.845336887+01:00 volume unmount 36187e0a44d277439fea0df2446fc44987fa814c52744091929e5c81bd8134e5 (container=b97f6a565c518eb1464cf81e6e09db1acfd84a0fdcbaea94255f1f182d79c058, driver=local)
………………
Conclusion

The combination of all these commands and utilities are very important in making docker and container a successful microservices based environment. As many of the microservices architectures need these kinds of utilities to debug, understand and learn more about them for every day usage, I hope this article will play role in helping such cases.

Read Also : How to install and use docker-compose to deploy containers in CentOS 7

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