docker command not found even though installed with apt-get

Question

The Ubuntu package docker actually refers to a GUI application, not the beloved DevOps tool we've come out to look for.

The instructions for docker can be followed per instructions on the docker page here: https://docs.docker.com/engine/install/ubuntu/

=== UPDATED (thanks @Scott Stensland) ===

You now run the following install script to get docker:

`sudo curl -sSL https://get.docker.com/ | sh`

• Note: review the script on the website and make sure you have the right link before continuing since you are running this as sudo.

This will run a script that installs docker. Note the last part of the script:

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 stens`

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

To update Docker run:

`sudo apt-get update && sudo apt-get upgrade`

For more details on what's going on, See the docker install documentation or @Scott Stensland's answer below

.

=== UPDATE: For those uncomfortable w/ sudo | sh ===

Some in the comments have mentioned that it a risk to run an arbitrary script as sudo. The above option is a convenience script from docker to make the task simple. However, for those that are security-focused but don't want to read the script you can do the following:

1. Add Dependencies

sudo apt-get update; \
sudo apt-get install \
 apt-transport-https \
 ca-certificates \
 curl \
 gnupg-agent \
 software-properties-common

2. Add docker gpg key

curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -

(Security check, verify key fingerprint 9DC8 5822 9FC7 DD38 854A E2D8 8D81 803C 0EBF CD88

$ sudo apt-key fingerprint 0EBFCD88

pub   rsa4096 2017-02-22 [SCEA]
      9DC8 5822 9FC7 DD38 854A  E2D8 8D81 803C 0EBF CD88
uid           [ unknown] Docker Release (CE deb) <[email protected]>
sub   rsa4096 2017-02-22 [S]

)

3. Setup Repository

sudo add-apt-repository \
   "deb [arch=amd64] https://download.docker.com/linux/ubuntu \
   $(lsb_release -cs) \
   stable"

4. Install Docker

sudo apt-get update; \
sudo apt-get install docker-ce docker-ce-cli containerd.io

If you want to verify that it worked run: sudo docker run hello-world

---

The following explains why it is named like this: Why install docker on ubuntu should be `sudo apt-get install docker.io`?

IMPORTANT - on ubuntu package docker is something entirely different ( avoid it ) :

issue following to view what if any packages you have mentioning docker

dpkg -l|grep docker

if only match is following then you do NOT have docker installed below is an unrelated package

docker - System tray for KDE3/GNOME2 docklet applications

if you do see above lets remove it since its the wrong docker

sudo apt-get remove docker # remove the wrong docker

if you see something similar to following then you have docker installed

 dpkg -l|grep docker

ii  docker-ce                                  5:19.03.13~3-0~ubuntu-focal         amd64        Docker: the open-source application container engine
ii  docker-ce-cli                              5:19.03.13~3-0~ubuntu-focal         amd64        Docker CLI: the open-source application container engine

NOTE - ubuntu package docker.io is not getting updates ( obsolete do NOT use )

---

Instead do this : install the latest version of docker on linux by executing the following:

sudo apt-get install docker-ce

below is yet another way to issue the install command

  sudo curl -sSL https://get.docker.com/ | sh
# sudo curl -sSL https://test.docker.com | sh  # get dev pipeline version
  

here is a typical output ( ubuntu 16.04 )

apparmor is enabled in the kernel and apparmor utils were already installed
+ sudo -E sh -c apt-key adv --keyserver hkp://ha.pool.sks-keyservers.net:80 --recv-keys 58118E89F3A912897C070ADBF76221572C52609D
Executing: /tmp/tmp.rAAGu0P85R/gpg.1.sh --keyserver
hkp://ha.pool.sks-keyservers.net:80
--recv-keys
58118E89F3A912897C070ADBF76221572C52609D
gpg: requesting key 2C52609D from hkp server ha.pool.sks-keyservers.net
gpg: key 2C52609D: "Docker Release Tool (releasedocker) <[email protected]>" 1 new signature
gpg: Total number processed: 1
gpg:         new signatures: 1
+ break
+ sudo -E sh -c apt-key adv -k 58118E89F3A912897C070ADBF76221572C52609D >/dev/null
+ sudo -E sh -c mkdir -p /etc/apt/sources.list.d
+ dpkg --print-architecture
+ sudo -E sh -c echo deb [arch=amd64] https://apt.dockerproject.org/repo ubuntu-xenial main > /etc/apt/sources.list.d/docker.list
+ sudo -E sh -c sleep 3; apt-get update; apt-get install -y -q docker-engine
Hit:1 http://repo.steampowered.com/steam precise InRelease
Hit:2 http://download.virtualbox.org/virtualbox/debian xenial InRelease                                                           
Ign:3 http://dl.google.com/linux/chrome/deb stable InRelease                                                                      
Hit:4 http://dl.google.com/linux/chrome/deb stable Release                                                                        
Hit:5 http://archive.canonical.com/ubuntu xenial InRelease                                                                        
Hit:6 http://mirror.cc.columbia.edu/pub/linux/ubuntu/archive xenial InRelease                                                     
Hit:7 http://mirror.cc.columbia.edu/pub/linux/ubuntu/archive xenial-updates InRelease                                             
Hit:8 http://ppa.launchpad.net/me-davidsansome/clementine/ubuntu xenial InRelease                                                 
Ign:9 http://repo.mongodb.org/apt/debian wheezy/mongodb-org/3.2 InRelease                                                         
Hit:10 http://mirror.cc.columbia.edu/pub/linux/ubuntu/archive xenial-backports InRelease                                          
Hit:11 http://repo.mongodb.org/apt/debian wheezy/mongodb-org/3.2 Release                                                          
Hit:12 http://mirror.cc.columbia.edu/pub/linux/ubuntu/archive xenial-security InRelease                                           
Hit:14 http://ppa.launchpad.net/numix/ppa/ubuntu xenial InRelease                                                                 
Ign:15 http://linux.dropbox.com/ubuntu wily InRelease                                                                             
Ign:16 http://repo.vivaldi.com/stable/deb stable InRelease                                                                        
Hit:17 http://repo.vivaldi.com/stable/deb stable Release                                                                          
Get:18 http://linux.dropbox.com/ubuntu wily Release [6,596 B]            
Get:19 https://apt.dockerproject.org/repo ubuntu-xenial InRelease [20.6 kB]    
Ign:20 http://packages.amplify.nginx.com/ubuntu xenial InRelease                      
Hit:22 http://packages.amplify.nginx.com/ubuntu xenial Release
Hit:23 https://deb.opera.com/opera-beta stable InRelease
Hit:26 https://deb.opera.com/opera-developer stable InRelease
Get:28 https://apt.dockerproject.org/repo ubuntu-xenial/main amd64 Packages [1,719 B]
Hit:29 https://packagecloud.io/slacktechnologies/slack/debian jessie InRelease
Fetched 28.9 kB in 1s (17.2 kB/s)
Reading package lists... Done
W: http://repo.mongodb.org/apt/debian/dists/wheezy/mongodb-org/3.2/Release.gpg: Signature by key 42F3E95A2C4F08279C4960ADD68FA50FEA312927 uses weak digest algorithm (SHA1)
Reading package lists...
Building dependency tree...
Reading state information...
The following additional packages will be installed:
  aufs-tools cgroupfs-mount
The following NEW packages will be installed:
  aufs-tools cgroupfs-mount docker-engine
0 upgraded, 3 newly installed, 0 to remove and 17 not upgraded.
Need to get 14.6 MB of archives.
After this operation, 73.7 MB of additional disk space will be used.
Get:1 http://mirror.cc.columbia.edu/pub/linux/ubuntu/archive xenial/universe amd64 aufs-tools amd64 1:3.2+20130722-1.1ubuntu1 [92.9 kB]
Get:2 http://mirror.cc.columbia.edu/pub/linux/ubuntu/archive xenial/universe amd64 cgroupfs-mount all 1.2 [4,970 B]
Get:3 https://apt.dockerproject.org/repo ubuntu-xenial/main amd64 docker-engine amd64 1.11.2-0~xenial [14.5 MB]
Fetched 14.6 MB in 7s (2,047 kB/s)
Selecting previously unselected package aufs-tools.
(Reading database ... 427978 files and directories currently installed.)
Preparing to unpack .../aufs-tools_1%3a3.2+20130722-1.1ubuntu1_amd64.deb ...
Unpacking aufs-tools (1:3.2+20130722-1.1ubuntu1) ...
Selecting previously unselected package cgroupfs-mount.
Preparing to unpack .../cgroupfs-mount_1.2_all.deb ...
Unpacking cgroupfs-mount (1.2) ...
Selecting previously unselected package docker-engine.
Preparing to unpack .../docker-engine_1.11.2-0~xenial_amd64.deb ...
Unpacking docker-engine (1.11.2-0~xenial) ...
Processing triggers for libc-bin (2.23-0ubuntu3) ...
Processing triggers for man-db (2.7.5-1) ...
Processing triggers for ureadahead (0.100.0-19) ...
Processing triggers for systemd (229-4ubuntu6) ...
Setting up aufs-tools (1:3.2+20130722-1.1ubuntu1) ...
Setting up cgroupfs-mount (1.2) ...
Setting up docker-engine (1.11.2-0~xenial) ...
Processing triggers for libc-bin (2.23-0ubuntu3) ...
Processing triggers for systemd (229-4ubuntu6) ...
Processing triggers for ureadahead (0.100.0-19) ...
+ sudo -E sh -c docker version
Client:
 Version:      1.11.2
 API version:  1.23
 Go version:   go1.5.4
 Git commit:   b9f10c9
 Built:        Wed Jun  1 22:00:43 2016
 OS/Arch:      linux/amd64

Server:
 Version:      1.11.2
 API version:  1.23
 Go version:   go1.5.4
 Git commit:   b9f10c9
 Built:        Wed Jun  1 22:00:43 2016
 OS/Arch:      linux/amd64

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 stens

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

Here is the underlying detailed install instructions which as you can see comes bundled into above technique ... Above one liner gives you same as :

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

Once installed you can see what docker packages were installed by issuing

dpkg -l|grep docker
ii  docker-ce                                  5:19.03.13~3-0~ubuntu-focal         amd64        Docker: the open-source application container engine
ii  docker-ce-cli                              5:19.03.13~3-0~ubuntu-focal         amd64        Docker CLI: the open-source application container engine

now Docker updates will get installed going forward when you issue

sudo apt-get update
sudo apt-get upgrade

take a look at

 ls -latr /etc/apt/sources.list.d/*docker*
-rw-r--r-- 1 root root 202 Jun 23 10:01 /etc/apt/sources.list.d/docker.list.save
-rw-r--r-- 1 root root  71 Jul  4 11:32 /etc/apt/sources.list.d/docker.list


cat /etc/apt/sources.list.d/docker.list

deb [arch=amd64] https://apt.dockerproject.org/repo ubuntu-xenial main

or more generally

cd /etc/apt
grep -r docker *
sources.list.d/docker.list:deb [arch=amd64] https://download.docker.com/linux/ubuntu focal test

sudo apt-get install docker # DO NOT do this

is a different library on ubuntu.

Use sudo apt-get install docker-ce to install the correct docker.

SET UP THE REPOSITORY

For Ubuntu 14.04/16.04/16.10/17.04:

sudo add-apt-repository "deb [arch=amd64] \
     https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"

For Ubuntu 17.10:

sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu zesty stable"

Add Docker’s official GPG key:

$ curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -

Then install

$ sudo apt-get update && sudo apt-get -y install docker-ce