Category Archives: Docker

All articles related to Docker container, Docker swarm and containerization on Linux.

8 basic Docker container management commands

Learn basic Docker container management with the help of these 8 commands. A useful guide for Docker beginners which includes sample command outputs.

In this article we will walk you through 6 basic Docker container commands which are useful in performing basic activities on Docker containers like run, list, stop, view logs, delete, etc. If you are new to the Docker concept then do check our introduction guide to know what is Docker & how-to guide to install Docker in Linux. Without further delay lets directly jump into commands.

How to run Docker container?

As you know, the Docker container is just an application process running on the host OS. For Docker container, you need a image to run from. Docker image when runs as process called a Docker container. You can have Docker image available locally or you have to download it from Docker hub. Docker hub is a centralized repository that has public and private images stored to pull from. Docker’s official hub is at hub.docker.com. So whenever you instruct the Docker engine to run a container, it looks for image locally, and if not found it pulls it from Docker hub.

Read all docker or containerization related articles here from KernelTalk’s archives.

Let’s run a Docker container for Apache web-server i.e httpd process. You need to run the command docker container run. The old command was just docker run but lately, Docker added sub-command section so new versions support below command –

root@kerneltalks # docker container run -d -p 80:80 httpd
Unable to find image 'httpd:latest' locally
latest: Pulling from library/httpd
3d77ce4481b1: Pull complete
73674f4d9403: Pull complete
d266646f40bd: Pull complete
ce7b0dda0c9f: Pull complete
01729050d692: Pull complete
014246127c67: Pull complete
7cd2e04cf570: Pull complete
Digest: sha256:f4610c3a1a7da35072870625733fd0384515f7e912c6223d4a48c6eb749a8617
Status: Downloaded newer image for httpd:latest
c46f2e9e4690f5c28ee7ad508559ceee0160ac3e2b1688a61561ce9f7d99d682

Docker run command takes image name as a mandatory argument along with many other optional ones. Commonly used arguments are –

-d : Detach container from the current shell
-p X:Y : Bind container port Y with host’s port X
--name : Name your container. If not used, it will be assigned randomly generated name
-e : Pass environmental variables and their values while starting a container

In the above output you can see, we supply httpd as an image name to run a container from. Since the image was not locally found, the Docker engine pulled it from Docker Hub. Now, observe it downloaded image httpd:latest where: is followed by version. That’s the naming convention of Docker container image. If you want a specific version container to run from then you can provide a version name along with image name. If not supplied, the Docker engine will always pull the latest one.

The very last line of output shown a unique container ID of your newly running httpd container.

How to list all running Docker containers?

Now, your container is running, you may want to check it or you want to list all running containers on your machine. You can list all running containers using docker container ls command. In the old Docker version, docker ps does this task for you.

root@kerneltalks # docker container ls
CONTAINER ID        IMAGE               COMMAND              CREATED             STATUS              PORTS                NAMES
c46f2e9e4690        httpd               "httpd-foreground"   11 minutes ago      Up 11 minutes       0.0.0.0:80->80/tcp   cranky_cori

Listing output is presented in column-wise format. Where column-wise values are –

Container ID: First few digits of the unique container ID
Image: Name of the image used to run the container
Command: Command ran by container after it ran
Created: Time created
Status: Current status of the container
Ports: Port binding details with host’s ports
Names: Name of the container (since we haven’t named our container you can see randomly generated name assigned to our container)

How to view logs of Docker container?

Since during the first step we used -d switch to detach container from the current shell once it ran its running in the background. In this case, we are clueless about what’s happening inside the container. So to view logs of the container, Docker provided logs command. It takes a container name or ID as an argument.

root@kerneltalks # docker container logs cranky_cori
AH00558: httpd: Could not reliably determine the server's fully qualified domain name, using 172.17.0.2. Set the 'ServerName' directive globally to suppress this message
AH00558: httpd: Could not reliably determine the server's fully qualified domain name, using 172.17.0.2. Set the 'ServerName' directive globally to suppress this message
[Thu May 31 18:35:07.301158 2018] [mpm_event:notice] [pid 1:tid 139734285989760] AH00489: Apache/2.4.33 (Unix) configured -- resuming normal operations
[Thu May 31 18:35:07.305153 2018] [core:notice] [pid 1:tid 139734285989760] AH00094: Command line: 'httpd -D FOREGROUND'

I used the container name in my command as an argument. You can see the Apache related log within our httpd container.

How to identify Docker container process?

The container is a process that uses host resources to run. If it’s true, then you will be able to locate the container process on the host’s process table. Let’s see how to check the container process on the host.

Docker used famous top command as its sub-commands name to view processes spawned by the container. It takes the container name/ID as an argument. In the old Docker version, only docker top command works. In newer versions, docker top and docker container top both works.

root@kerneltalks # docker container top  cranky_cori
UID                 PID                 PPID                C                   STIME               TTY                 TIME                CMD
root                15702               15690               0                   18:35               ?                   00:00:00            httpd -DFOREGROUND
bin                 15729               15702               0                   18:35               ?                   00:00:00            httpd -DFOREGROUND
bin                 15730               15702               0                   18:35               ?                   00:00:00            httpd -DFOREGROUND
bin                 15731               15702               0                   18:35               ?                   00:00:00            httpd -DFOREGROUND

root@kerneltalks # ps -ef |grep -i 15702
root     15702 15690  0 18:35 ?        00:00:00 httpd -DFOREGROUND
bin      15729 15702  0 18:35 ?        00:00:00 httpd -DFOREGROUND
bin      15730 15702  0 18:35 ?        00:00:00 httpd -DFOREGROUND
bin      15731 15702  0 18:35 ?        00:00:00 httpd -DFOREGROUND
root     15993 15957  0 18:59 pts/0    00:00:00 grep --color=auto -i 15702

In the first output, the list of processes spawned by that container. It has all details like use, PID, PPID, start time, command, etc. All those PID you can search in your host’s process table and you can find them there. That’s what we did in the second command. So, this proves containers are indeed just processes on Host’s OS.

How to stop Docker container?

It’s simple stop command! Again it takes container name /ID as an argument.

root@kerneltalks # docker container stop cranky_cori
cranky_cori

How to list stopped or not running Docker containers?

Now we stopped our container if we try to list container using ls command, we won’t be able to see it.

root@kerneltalks # docker container ls
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES

So, in this case, to view stopped or nonrunning container you need to use -a switch along with ls command.

root@kerneltalks # docker container ls -a
CONTAINER ID        IMAGE               COMMAND              CREATED             STATUS                     PORTS               NAMES
c46f2e9e4690        httpd               "httpd-foreground"   33 minutes ago      Exited (0) 2 minutes ago                       cranky_cori

With -a switch we can see stopped container now. The notice status of this container is mentioned ‘Exited’. Since the container is just a process its termed as ‘exited’ rather than stopped!

How to start Docker container?

Now, we will start this stopped container. There is a difference between running and starting a container. When you run a container, you are starting a command in a fresh container. When you start a container, you are starting an old stopped container which has an old state saved in it. It will start it from that state forward.

root@kerneltalks #  docker container start c46f2e9e4690
c46f2e9e4690

root@kerneltalks # docker container ls -a
CONTAINER ID        IMAGE               COMMAND              CREATED             STATUS              PORTS                NAMES
c46f2e9e4690        httpd               "httpd-foreground"   35 minutes ago      Up 8 seconds        0.0.0.0:80->80/tcp   cranky_cori

How to remove Docker container?

To remove the container from your Docker engine use rm command. You can not remove the running containers. You have to first stop the container and then remove it. You can remove it forcefully using -f switch with rm command but that’s not recommended.

root@kerneltalks # docker container rm cranky_cori
cranky_cori
root@kerneltalks # docker container ls -a
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES

You can see once we remove container, its not visible in ls -a listing too.

What is Docker? Introduction guide to Docker

What is Docker?

Docker is another layer of virtualization where virtualization happens at the operating system level. It’s a software container platform and currently leading this sector globally. You must be familiar with VMware which is virtualization at bare metal level but docker takes one step forward and virtualize things at OS level and hence removing all hardware management, capacity planning, resource management, etc. VMware runs a number of virtual machines (VMs) on single server hardware (refer Figure 1) whereas Docker runs a number of containers on a single Operating System (refer Figure 2). So in simple terms, Docker containers are just processes sharing a host operating system to perform their tasks.

Lets quickly run through the difference between VM and Docker containers. I tabulated the difference for a quick read.

Virtual machine v/s Docker container

Virtual Machine	Docker container
Its a mini version of physical machine	Its just a process
Runs on hypervisor virtualization	Runs on Linux. (HyperV needed if you run on Windows/MAC)
Has its own guest OS	No OS
Can be used only after guest OS boot finishes	Immediately ready to use when launched
Slow	Fast
Uses hardware resources of Host	Uses only OS resources like binaries/libraries of Host
Resource management needed	No resource management
It runs as long as admin/guest OS doesnt power it off	It runs as long as command runs which container executed at startup.
VM stops when you shutdown guest OS	Once the command exits, container stops

Docker engine mainly runs on Linux. So if you are running Docker on Windows or MAC then it’s actually running tiny Linux VM in the background on your Windows or MAC and on top of it, it’s running its own engine to provide you Docker functionalities on non-Linux platform.

Since Docker engine runs containers it also termed as containerization!

Why use Docker?

Docker containers are portable. They can be stored as an image which can be copied to any other machine and can be launched there. This ensures even if host OS parameters, version changes containers still functions the same across the different OS.

Containers use the host operating system, they don’t have their own OS to boot when containers are launched. It means they are almost available for use immediately as there is not booting of OS of anything that sort which takes time to prepare the container for use. Docker containers are fast to use!

They use resources from host OS, there is no resource management like adding/removing CPU, memory, storage, etc tasks on containers!

There are lots of functionality, flexibility being added to Docker every month. Its fast-evolving virtualization concept and gives you more ease of managing IT infra.

What are Docker variants available to use?

Docker Editions

At present, there are two editions available. CE and EE. CE stands for Community Edition and EE stands for Enterprise Edition. Let’s see the difference between Docker CE and Docker EE.

Docker CE	Docker EE
Community Edition	Enterprise Edition
It’s free	It’s paid
Primarily for development use	Use this edition for Production environment
Do it yourself. No support	Support subscription from Docker
For personal use	For enterprise/big/production use

Docker releases

Docker also releases in two forms. Stable and Edge. Let’s see the difference between Docker stable release and Docker edge release.

Docker stable release	Docker edge release
Its tested final release	Its kind of beta release
Stable version.	Includes upcoming features/functionalities
For dev/prod use	For experimental use only
New release every quarter	New release every month
Support available	No support for issues faced

Where to get Docker?

Docker can be downloaded from Docker’s official store. For each platform, related instructions are included. Detailed installation steps and other information on Docker can be found on Docker’s official documentation portal. You can also refer to our article to install Docker on Linux.

I believe that should be enough for an introductory article on Docker. If you have any questions/feedback, please leave us to comment below or reach us using the contact form.

How to install docker in Linux

Install docker using package

If your package manager has a Docker package available to install then it’s an easy way to get Docker on your system.

Before going got Docker installation you should install below packages on your system to use the full flexible functionality of Docker. These packages are not dependencies but its good to have them pre-installed so that all Docker functions/drivers you can use.

For CenOs, Redhat etc YUM based systems – yum-utils device-mapper-persistent-data lvm2
For Debian, Ubuntu etc apt based systems – apt-transport-https ca-certificates curl software-properties-common

But you may not be getting the latest version of Docker in this case. You can install a package simply using yum or apt-get command. Below sample output for your reference from the OpenSuse server.

root@kerneltalks # zypper in docker
Building repository 'openSUSE-13.2-Update' cache .................................................................................................................[done]
Retrieving repository 'openSUSE-13.2-Update-Non-Oss' metadata ....................................................................................................[done]
Building repository 'openSUSE-13.2-Update-Non-Oss' cache .........................................................................................................[done]
Loading repository data...
Reading installed packages...
Resolving package dependencies...

The following NEW package is going to be installed:
  docker

1 new package to install.
Overall download size: 6.2 MiB. Already cached: 0 B  After the operation, additional 22.9 MiB will be used.
Continue? [y/n/? shows all options] (y): y
Retrieving package docker-1.9.1-56.1.x86_64                                                                                        (1/1),   6.2 MiB ( 22.9 MiB unpacked)
Retrieving: docker-1.9.1-56.1.x86_64.rpm .............................................................................................................[done (2.5 MiB/s)]
Checking for file conflicts: .....................................................................................................................................[done]
(1/1) Installing: docker-1.9.1-56.1 ..............................................................................................................................[done]
Additional rpm output:
creating group docker...
Updating /etc/sysconfig/docker...

Install docker using the script

In the below procedure, we will be using the script from Docker’s official website which will scan your system for details and automatically fetch the latest and compatible docker version for your system and installs it. We will be fetching script from this docker URL and using it to install the latest Docker on the list of Linux distros.

Fetch the latest script from docker official website using curl. If you read this script, SUPPORT_MAP variable shows the list of Linux distros this script support. If you are running any other Linux version than listed here then this method won’t be useful for you.

root@kerneltalks # curl -fsSL get.docker.com -o get-docker.sh
root@kerneltalks # ls -lrt
-rw-r--r--. 1 root root 13847 May 30 18:59 get-docker.sh

Now we have latest get-docker.sh script from docker official website on our server. Now, you just have to run the script and it will do the rest!

# sh get-docker.sh
# Executing docker install script, commit: 36b78b2
+ sh -c 'yum install -y -q yum-utils'
Package yum-utils-1.1.31-45.el7.noarch already installed and latest version
+ sh -c 'yum-config-manager --add-repo https://download.docker.com/linux/centos/docker-ce.repo'
Loaded plugins: fastestmirror
adding repo from: https://download.docker.com/linux/centos/docker-ce.repo
grabbing file https://download.docker.com/linux/centos/docker-ce.repo to /etc/yum.repos.d/docker-ce.repo
repo saved to /etc/yum.repos.d/docker-ce.repo
+ '[' edge '!=' stable ']'
+ sh -c 'yum-config-manager --enable docker-ce-edge'
Loaded plugins: fastestmirror
========================================================================= repo: docker-ce-edge =========================================================================
[docker-ce-edge]
async = True
bandwidth = 0
base_persistdir = /var/lib/yum/repos/x86_64/7
baseurl = https://download.docker.com/linux/centos/7/x86_64/edge
cache = 0
cachedir = /var/cache/yum/x86_64/7/docker-ce-edge
check_config_file_age = True
compare_providers_priority = 80
cost = 1000
deltarpm_metadata_percentage = 100
deltarpm_percentage =
enabled = 1
enablegroups = True
exclude =
failovermethod = priority
ftp_disable_epsv = False
gpgcadir = /var/lib/yum/repos/x86_64/7/docker-ce-edge/gpgcadir
gpgcakey =
gpgcheck = True
gpgdir = /var/lib/yum/repos/x86_64/7/docker-ce-edge/gpgdir
gpgkey = https://download.docker.com/linux/centos/gpg
hdrdir = /var/cache/yum/x86_64/7/docker-ce-edge/headers
http_caching = all
includepkgs =
ip_resolve =
keepalive = True
keepcache = False
mddownloadpolicy = sqlite
mdpolicy = group:small
mediaid =
metadata_expire = 21600
metadata_expire_filter = read-only:present
metalink =
minrate = 0
mirrorlist =
mirrorlist_expire = 86400
name = Docker CE Edge - x86_64
old_base_cache_dir =
password =
persistdir = /var/lib/yum/repos/x86_64/7/docker-ce-edge
pkgdir = /var/cache/yum/x86_64/7/docker-ce-edge/packages
proxy = False
proxy_dict =
proxy_password =
proxy_username =
repo_gpgcheck = False
retries = 10
skip_if_unavailable = False
ssl_check_cert_permissions = True
sslcacert =
sslclientcert =
sslclientkey =
sslverify = True
throttle = 0
timeout = 30.0
ui_id = docker-ce-edge/x86_64
ui_repoid_vars = releasever,
   basearch
username =

+ sh -c 'yum makecache'
Loaded plugins: fastestmirror
base                                                                                                                                             | 3.6 kB  00:00:00
docker-ce-edge                                                                                                                                   | 2.9 kB  00:00:00
docker-ce-stable                                                                                                                                 | 2.9 kB  00:00:00
epel/x86_64/metalink                                                                                                                             |  21 kB  00:00:00
extras                                                                                                                                           | 3.4 kB  00:00:00
updates                                                                                                                                          | 3.4 kB  00:00:00
(1/15): docker-ce-stable/x86_64/filelists_db                                                                                                     | 7.7 kB  00:00:03
(2/15): base/7/x86_64/other_db                                                                                                                   | 2.5 MB  00:00:04
(3/15): docker-ce-edge/x86_64/filelists_db                                                                                                       | 9.6 kB  00:00:04
(4/15): docker-ce-edge/x86_64/other_db                                                                                                           |  62 kB  00:00:04
(5/15): docker-ce-stable/x86_64/other_db                                                                                                         |  66 kB  00:00:00
(6/15): base/7/x86_64/filelists_db                                                                                                               | 6.9 MB  00:00:05
(7/15): epel/x86_64/filelists_db                                                                                                                 |  10 MB  00:00:01
(8/15): epel/x86_64/prestodelta                                                                                                                  | 2.8 kB  00:00:00
(9/15): epel/x86_64/other_db                                                                                                                     | 3.1 MB  00:00:01
(10/15): extras/7/x86_64/prestodelta                                                                                                             |  48 kB  00:00:02
(11/15): extras/7/x86_64/other_db                                                                                                                |  95 kB  00:00:02
(12/15): extras/7/x86_64/filelists_db                                                                                                            | 519 kB  00:00:02
(13/15): updates/7/x86_64/filelists_db                                                                                                           | 1.3 MB  00:00:02
(14/15): updates/7/x86_64/prestodelta                                                                                                            | 231 kB  00:00:00
(15/15): updates/7/x86_64/other_db                                                                                                               | 228 kB  00:00:00
Loading mirror speeds from cached hostfile
 * base: mirror.genesisadaptive.com
 * epel: s3-mirror-us-east-1.fedoraproject.org
 * extras: mirror.math.princeton.edu
 * updates: mirror.metrocast.net
Metadata Cache Created
+ sh -c 'yum install -y -q docker-ce'
If you would like to use Docker as a non-root user, you should now consider
adding your user to the "docker" group with something like:

  sudo usermod -aG docker your-user

Remember that you will have to log out and back in for this to take effect!

WARNING: Adding a user to the "docker" group will grant the ability to run
         containers which can be used to obtain root privileges on the
         docker host.
         Refer to https://docs.docker.com/engine/security/security/#docker-daemon-attack-surface
         for more information.

If you observe the above output then you will get to know that script will detect your OS and will download, configure, and use supported repo to install Docker on your machine. It also notifies you to add non-root user to group docker so that he/she can run docker commands with root privileges.

You can download and run the script this in a single command as well like below –

root@kerneltalks # curl -fsSL get.docker.com -o - get-docker.sh | bash -s

If you are running the script on un-supported Linux version (which is not mentioned in SUPPORT_MAP list) then you will see below error.

root@kerneltalks # sh get-docker.sh
Executing docker install script, commit: 36b78b2

Either your platform is not easily detectable or is not supported by this
installer script.
Please visit the following URL for more detailed installation instructions:

https://docs.docker.com/engine/installation/

If you are on RHEL, SLES (basically Enterprise Linux editions) then only Docker EE i.e. Enterprise Edition (paid) is supported on them. You will need to purchase appropriate subscriptions to use them. You will see below message –

# sh get-docker.sh
# Executing docker install script, commit: 36b78b2


  WARNING: rhel is now only supported by Docker EE
           Check https://store.docker.com for information on Docker EE

Install with help from docker store

If both above methods are not suitable for you then you can always opt for the last method. Head to Docker online store. Goto Docker CE i.e. Community Edition (the free one) and choose your Linux distro. Currently, they have listed AWS, Azure, Fedora, CentOS, Ubuntu & Debian. Click on your choice, head to Resources tab, and click Detailed installation instructions. You will be redirected to appropriate documents on Docker documents which have detailed step by step commands to perform a clean install of Docker on Linux of your choice! Or you can always head to this home page of installation and choose your host.

Check if Docker is installed

Finally, you have to check if Docker is installed on the system. To check if docker is installed, simply run the command docker version

root@kerneltalks # docker version
Client:
 Version:      18.05.0-ce
 API version:  1.37
 Go version:   go1.9.5
 Git commit:   f150324
 Built:        Wed May  9 22:14:54 2018
 OS/Arch:      linux/amd64
 Experimental: false
 Orchestrator: swarm
Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?

The last line in the above output shows that the Docker service is not yet running on the server. You can start the service and then the output will show your Docker server details as well.

root@kerneltalks # service docker start
root@kerneltalks # docker version
Client:
 Version:      18.05.0-ce
 API version:  1.37
 Go version:   go1.9.5
 Git commit:   f150324
 Built:        Wed May  9 22:14:54 2018
 OS/Arch:      linux/amd64
 Experimental: false
 Orchestrator: swarm

Server:
 Engine:
  Version:      18.05.0-ce
  API version:  1.37 (minimum version 1.12)
  Go version:   go1.9.5
  Git commit:   f150324
  Built:        Wed May  9 22:18:36 2018
  OS/Arch:      linux/amd64
  Experimental: false

So, now you have successfully installed Docker on your machine and started the Docker server. You are yet to create containers in it!

Setting up docker for non-root user

For non-root user to use Docker, you need to add the user into a group called docker. This group is automatically gets created when you install Docker.

root@kerneltalks # usermod -aG <user> docker

Run above command to add non-root user in docker group and then that user will be able to run all docker commands without root privileges.

Also, you need to make sure that docker services start automatically when the server reboots. Since system control systemctl is becoming standard on all latest Linux versions, below command will suit on nearly major Linux distros

root@kerneltalks # systemctl enable docker

This command will enable docker to run with system boot and hence no root intervention needed when the system reboots. Non-root users will continue to use docker even after a reboot.

Try Docker without installing!

If you want to try Docker without installing it on your machine then just head to Play with Docker website and you will be able to spin up machines having Docker in it. You can try Docker commands in it from your web browser!

The only limitation they have is your session will be auto closed after 4 hours. You have a clock ticking in your browser window set to 4 hours once you log in.

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