Docker - Pull some public image and play with that container

 

1. You can pull the official Ubuntu image from Docker Hub — it’s public, so no login needed.

[root@devopsvm01 ~]# docker pull ubuntu

Using default tag: latest

latest: Pulling from library/ubuntu

4b3ffd8ccb52: Pull complete

Digest: sha256:66460d557b25769b102175144d538d88219c077c678a49af4afca6fbfc1b5252

Status: Downloaded newer image for ubuntu:latest

docker.io/library/ubuntu:latest

[root@devopsvm01 ~]#

What happens:

• Docker contacts Docker Hub

• Downloads the latest Ubuntu base image

• Stores it locally (you can see it with docker images)

2. Check the image 

[root@devopsvm01 ~]#

[root@devopsvm01 ~]# docker images

REPOSITORY    TAG       IMAGE ID       CREATED        SIZE

ubuntu        latest    97bed23a3497   4 weeks ago    78.1MB

hello-world   latest    1b44b5a3e06a   2 months ago   10.1kB

[root@devopsvm01 ~]#

3. Remove the image

[root@devopsvm01 ~]# docker images

REPOSITORY    TAG       IMAGE ID       CREATED        SIZE

ubuntu        latest    97bed23a3497   4 weeks ago    78.1MB

hello-world   latest    1b44b5a3e06a   2 months ago   10.1kB

[root@devopsvm01 ~]#

[root@devopsvm01 ~]#

[root@devopsvm01 ~]# docker rmi 97bed23a3497

Untagged: ubuntu:latest

Untagged: ubuntu@sha256:66460d557b25769b102175144d538d88219c077c678a49af4afca6fbfc1b5252

Deleted: sha256:97bed23a34971024aa8d254abbe67b7168772340d1f494034773bc464e8dd5b6

Deleted: sha256:073ec47a8c22dcaa4d6e5758799ccefe2f9bde943685830b1bf6fd2395f5eabc

[root@devopsvm01 ~]#

[root@devopsvm01 ~]# docker images

REPOSITORY    TAG       IMAGE ID       CREATED        SIZE

hello-world   latest    1b44b5a3e06a   2 months ago   10.1kB

[root@devopsvm01 ~]#

4. Run ubuntu container

Docker first checks if the Ubuntu image exists locally, If image is found locally, Docker creates a container from that local image. If image is not found Docker automatically pulls the image from Docker Hub (or the specified registry).

[root@devopsvm01 ~]# docker run ubuntu

Unable to find image 'ubuntu:latest' locally

latest: Pulling from library/ubuntu

4b3ffd8ccb52: Pull complete

Digest: sha256:66460d557b25769b102175144d538d88219c077c678a49af4afca6fbfc1b5252

Status: Downloaded newer image for ubuntu:latest

[root@devopsvm01 ~]#

[root@devopsvm01 ~]# docker ps -a

CONTAINER ID   IMAGE         COMMAND       CREATED          STATUS                      PORTS     NAMES

cfcf272bda4b   ubuntu        "/bin/bash"   19 seconds ago   Exited (0) 17 seconds ago             stoic_mcnulty

ff3a9058ef1a   hello-world   "/hello"      21 minutes ago   Exited (0) 21 minutes ago             frosty_wilson

7666d9a0a66a   hello-world   "/hello"      54 minutes ago   Exited (0) 54 minutes ago             naughty_jepsen

[root@devopsvm01 ~]#

The container created from the ubuntu image exited almost immediately after it was run.

When you run docker run ubuntu without specifying a command, Docker uses the default command defined within the ubuntu image, which is typically /bin/bash. Without -it, Docker starts the container, executes /bin/bash, and immediately exits, because /bin/bash ends right away when there’s no terminal attached.

-i -->  Run the shlle in interactive mode

-t -->  Allocate a pseudo-TTY

5. Start the ubuntu container in interactive mode.

[root@devopsvm01 ~]# docker run -it ubuntu

root@965e7a3a24e1:/#

From a different terminal check the container status,

[root@devopsvm01 ~]# docker ps -a

CONTAINER ID   IMAGE         COMMAND       CREATED             STATUS                         PORTS     NAMES

dbea984d618c   ubuntu        "/bin/bash"   5 seconds ago       Up 5 seconds                             serene_jackson

965e7a3a24e1   ubuntu        "/bin/bash"   12 minutes ago      Exited (0) 9 minutes ago                

7666d9a0a66a   hello-world   "/hello"      About an hour ago   Exited (0) About an hour ago             naughty_jepsen

[root@devopsvm01 ~]#

root@965e7a3a24e1:/# uptime

 22:41:00 up 19:59,  0 user,  load average: 0.00, 0.00, 0.00

root@965e7a3a24e1:/# free -m

               total        used        free      shared  buff/cache   available

Mem:            1511         626         143           2         905         885

Swap:           2095         205        1890

root@965e7a3a24e1:/# cat /proc/cpuinfo

processor       : 0

vendor_id       : GenuineIntel

cpu family      : 6

model           : 142

model name      : Intel(R) Core(TM) i3-7100U CPU @ 2.40GHz

stepping        : 9

cpu MHz         : 2399.998

cache size      : 3072 KB

physical id     : 0

siblings        : 1

core id         : 0

cpu cores       : 1

apicid          : 0

initial apicid  : 0

fpu             : yes

fpu_exception   : yes

cpuid level     : 22

wp              : yes

flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx rdtscp lm constant_tsc rep_good nopl xtopology nonstop_tsc cpuid tsc_known_freq pni pclmulqdq monitor ssse3 cx16 pcid sse4_1 sse4_2 x2apic movbe popcnt aes xsave avx rdrand hypervisor lahf_lm abm 3dnowprefetch invpcid_single pti fsgsbase bmi1 avx2 bmi2 invpcid rdseed clflushopt md_clear flush_l1d

bugs            : bhi cpu_meltdown spectre_v1 spectre_v2 spec_store_bypass l1tf mds swapgs itlb_multihit srbds mmio_stale_data retbleed gds

bogomips        : 4799.99

clflush size    : 64

cache_alignment : 64

address sizes   : 39 bits physical, 48 bits virtual

power management:

root@965e7a3a24e1:/# df -h

Filesystem           Size  Used Avail Use% Mounted on

overlay               37G  7.1G   30G  20% /

tmpfs                 64M     0   64M   0% /dev

shm                   64M     0   64M   0% /dev/shm

/dev/mapper/ol-root   37G  7.1G   30G  20% /etc/hosts

tmpfs                756M     0  756M   0% /proc/asound

tmpfs                756M     0  756M   0% /proc/acpi

tmpfs                756M     0  756M   0% /proc/scsi

tmpfs                756M     0  756M   0% /sys/firmware

root@965e7a3a24e1:/#

Memory Allocation in Docker Containers

The memory is not fully allocated from the host OS; it is shared and managed by the Host OS Kernel, but by default Docker containers can use all the memory available on the host unless you explicitly limit it.

On Linux, unless you set --memory when running the container, it shares the host memory and sees what the kernel exposes.

Example: Limit memory

docker run -it --memory=512m ubuntu

This container now cannot use more than 512 MB, even if the host has more available.

By default, your container is not “taking” all host memory, it just sees what’s available and will allocate as needed.

Docker uses a Linux Kernel feature called cgroups (Control Groups) to manage and restrict resource usage. If you want to limit the container to, say, 512MB,

 you would run the container with the flag: docker run -it --memory 512m ubuntu. Since you did not specify a limit, it sees the Host's total memory.

CPU Allocation

The CPU is also shared and managed by the Host OS Kernel, not exclusively allocated.

By default, Docker containers share the Host's CPU resources and can burst up to 100% of all available cores on the host.

Like memory, CPU limits are set using cgroups. For example, to limit the container to 50% of one CPU core, you'd use 

docker run -it --cpus 0.5 ubuntu.

Filesystem Allocation

The filesystem is allocated using a layered, copy-on-write (CoW) system, which is highly efficient.

Key Concept: The container's filesystem is composed of two main layers:

Read-Only Image Layers: These are the base layers from the ubuntu image. They are shared among all containers created from that image, saving disk space.

Writable Top Layer (overlay): This is a small, dedicated layer created specifically for your running container (965e7a3a24e1). Any changes you make (e.g., creating a file) are stored here.

The large Size and Avail values (37GB and 30GB) you see for the / directory come from the Host OS's underlying filesystem partition (specifically /dev/mapper/ol-root), which Docker uses as its storage location. The container is showing you the total available space on the disk partition used by Docker, not the space allocated just for the container.

[root@devopsvm01 ~]# df -h

Filesystem           Size  Used Avail Use% Mounted on

devtmpfs             4.0M     0  4.0M   0% /dev

tmpfs                756M     0  756M   0% /dev/shm

tmpfs                303M   11M  293M   4% /run

/dev/mapper/ol-root   37G  7.1G   30G  20% /

/dev/sda1            960M  536M  425M  56% /boot

tmpfs                152M   56K  152M   1% /run/user/42

tmpfs                152M   36K  152M   1% /run/user/0

overlay               37G  7.1G   30G  20% /var/lib/docker/overlay2/79471e4d2798e8cef93c8610156c0fc253ee07328457671b062f7052eb377eaa/merged

[root@devopsvm01 ~]#

Ie, The container sees its own filesystem structure (like /), but it’s actually stored in /var/lib/docker on the host.

The container will likely exit if your terminal session times out.

root@dbea984d618c:/# exit

exit

[root@devopsvm01 ~]#

[root@devopsvm01 ~]# docker ps -a

CONTAINER ID   IMAGE         COMMAND       CREATED             STATUS                         PORTS     NAMES

dbea984d618c   ubuntu        "/bin/bash"   19 minutes ago      Exited (0) 2 seconds ago                 serene_jackson

965e7a3a24e1   ubuntu        "/bin/bash"   31 minutes ago      Exited (0) 29 minutes ago                eloquent_panini

cfcf272bda4b   ubuntu        "/bin/bash"   41 minutes ago      Exited (0) 41 minutes ago                stoic_mcnulty

ff3a9058ef1a   hello-world   "/hello"      About an hour ago   Exited (0) About an hour ago             frosty_wilson

7666d9a0a66a   hello-world   "/hello"      2 hours ago         Exited (0) 2 hours ago                   naughty_jepsen

[root@devopsvm01 ~]#

The container runs only as long as its main process is active. In your case, when you ran docker run -it ubuntu, the main process is the interactive shell (likely /bin/bash or /bin/sh).

To view the network related information , 

root@6591579e4a8d:/# apt update

root@6591579e4a8d:/# apt install -y iproute2 net-tools

root@6591579e4a8d:/# ip addr show

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000

    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00

    inet 127.0.0.1/8 scope host lo

       valid_lft forever preferred_lft forever

    inet6 ::1/128 scope host

       valid_lft forever preferred_lft forever

2: eth0@if17: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default

    link/ether 2e:0e:7a:74:43:e7 brd ff:ff:ff:ff:ff:ff link-netnsid 0

    inet 172.17.0.2/16 brd 172.17.255.255 scope global eth0

       valid_lft forever preferred_lft forever

root@6591579e4a8d:/#

5. Stop/Start the ubuntu container.

[root@devopsvm01 ~]# docker ps

CONTAINER ID   IMAGE     COMMAND   CREATED   STATUS    PORTS     NAMES

[root@devopsvm01 ~]#

[root@devopsvm01 ~]# docker start f793dfa299d3

f793dfa299d3

[root@devopsvm01 ~]#

[root@devopsvm01 ~]# docker ps

CONTAINER ID   IMAGE     COMMAND       CREATED         STATUS         PORTS     NAMES

f793dfa299d3   ubuntu    "/bin/bash"   4 minutes ago   Up 3 seconds             heuristic_kowalevski

[root@devopsvm01 ~]#

[root@devopsvm01 ~]# docker stop f793dfa299d3

f793dfa299d3

[root@devopsvm01 ~]#

[root@devopsvm01 ~]# docker run ubuntu

[root@devopsvm01 ~]#

[root@devopsvm01 ~]# docker ps

CONTAINER ID   IMAGE     COMMAND   CREATED   STATUS    PORTS     NAMES

Why docker start f793dfa299d3 stays running

That container was originally started with an interactive session — something like:

docker run -it ubuntu

That combination -it means:

• -i = interactive (keep STDIN open)

• -t = allocate a pseudo-TTY (terminal)

So when you docker start that container again, it resumes with a running bash process, which keeps the container alive.

To check the container resource utilization.

Run docker stats from the server machine.

[root@devopsvm01 ~]# docker stats

docker system df

This command is similar to the Linux df (disk free) command — it shows how much disk space Docker is using and how much can be cleaned up.

[root@devopsvm01 ~]# docker system df

TYPE            TOTAL     ACTIVE    SIZE      RECLAIMABLE

Images          2         2         78.13MB   0B (0%)

Containers      10        1         43.18MB   43.18MB (100%)

Local Volumes   0         0         0B        0B

Build Cache     0         0         0B        0B

[root@devopsvm01 ~]#

Docker system prune

docker system prune is a powerful cleanup command used to remove unused Docker resources and reclaim disk space on your machine.

What docker system prune Removes

When you run docker system prune, Docker will ask for confirmation and then delete the following items:

• All Stopped Containers: Containers that are not currently running (docker ps -a shows them as Exited).

• All Networks Not Used: Networks that are not attached to any running or stopped containers.

• All Dangling Images: Images that do not have a tag and are not associated with any container (often left over after a build).

[root@devopsvm01 ~]# docker system prune --help

Usage:  docker system prune [OPTIONS]

Remove unused data

Options:

  -a, --all             Remove all unused images not just dangling ones

      --filter filter   Provide filter values (e.g. "label=<key>=<value>")

  -f, --force           Do not prompt for confirmation

      --volumes         Prune anonymous volumes

[root@devopsvm01 ~]#

[root@devopsvm01 ~]# docker images

REPOSITORY    TAG       IMAGE ID       CREATED        SIZE

ubuntu        latest    97bed23a3497   4 weeks ago    78.1MB

hello-world   latest    1b44b5a3e06a   2 months ago   10.1kB

[root@devopsvm01 ~]#

[root@devopsvm01 ~]# docker ps -a

CONTAINER ID   IMAGE         COMMAND       CREATED             STATUS                         PORTS     NAMES

403a136be406   ubuntu        "/bin/bash"   9 minutes ago       Up 9 minutes                             awesome_payne

954830c65252   ubuntu        "/bin/bash"   17 minutes ago      Exited (0) 17 minutes ago                wizardly_brattain

f793dfa299d3   ubuntu        "/bin/bash"   24 minutes ago      Exited (137) 19 minutes ago              heuristic_kowalevski

6591579e4a8d   ubuntu        "/bin/bash"   54 minutes ago      Exited (129) 30 minutes ago              amazing_pascal

eff7184828ce   ubuntu        "/bin/bash"   54 minutes ago      Exited (127) 54 minutes ago              hungry_heyrovsky

dbea984d618c   ubuntu        "/bin/bash"   About an hour ago   Exited (0) 59 minutes ago                serene_jackson

965e7a3a24e1   ubuntu        "/bin/bash"   2 hours ago         Exited (0) About an hour ago             eloquent_panini

cfcf272bda4b   ubuntu        "/bin/bash"   2 hours ago         Exited (0) 2 hours ago                   stoic_mcnulty

ff3a9058ef1a   hello-world   "/hello"      2 hours ago         Exited (0) 2 hours ago                   frosty_wilson

7666d9a0a66a   hello-world   "/hello"      3 hours ago         Exited (0) 3 hours ago                   naughty_jepsen

[root@devopsvm01 ~]#

[root@devopsvm01 ~]# docker system prune

WARNING! This will remove:

  - all stopped containers

  - all networks not used by at least one container

  - all dangling images

  - unused build cache

Are you sure you want to continue? [y/N] y

Deleted Containers:

954830c652526c75e1b14a451b331d1a05b5b104e9b3b22308aef601349e599c

f793dfa299d38f57b974938d8170e77afd6cf2f19d970c2cc54bb0b9d9aa9010

6591579e4a8d6d887000611020d229ed5832ca21df837271e7a454b1803c8e8b

eff7184828cebc5ce5412eae901b0370c41b75d051573ea330e5ca95616df9b1

dbea984d618c7aac7e01b7e07257ba3bb9fc542e3efb0b211a88500f7a873ecf

965e7a3a24e161a87f3600bac45050c8ca2e952732f81dd98c778f7a5c77b9d6

cfcf272bda4b0f21f310deab9a22bf3c01383d4c804ef42e09084ac8887ce34a

ff3a9058ef1aed89b0309092ef7c777780cac1ae30e90c88bc8b77d150af914c

7666d9a0a66a40e3dac54282f6bd3704df25e5cec2583b27f1b1237955bf21b2

Total reclaimed space: 43.18MB

[root@devopsvm01 ~]#

[root@devopsvm01 ~]# docker ps -a

CONTAINER ID   IMAGE     COMMAND       CREATED         STATUS         PORTS     NAMES

403a136be406   ubuntu    "/bin/bash"   9 minutes ago   Up 9 minutes             awesome_payne

[root@devopsvm01 ~]#

[root@devopsvm01 ~]#

[root@devopsvm01 ~]# docker images

REPOSITORY    TAG       IMAGE ID       CREATED        SIZE

ubuntu        latest    97bed23a3497   4 weeks ago    78.1MB

hello-world   latest    1b44b5a3e06a   2 months ago   10.1kB

[root@devopsvm01 ~]#