How to export a runtime container and import to use it as new image with Podman

Container

How to export a runtime container and import to use it as new image with Podman

By novicejava1 0

Test Environment

Fedora 32 installed
Podman installed

Podman provides two command line utilities (ie. export and import).

The ‘export’ utility helps in capturing the current snapshot of the container that has been instantiated from an image into a tar archival format and store locally.

The ‘import’ utility helps in creating a filesystem using the tar file of the container snapshot that we created using the export utility.

The main difference between save/load and export/import utility are as below. – save/load work on the container images taking the snaphost of complete layers of the image and storing them into a tar file which can further be loaded using the load utility – In export/import, export utility work on the container runtime instance to create a snaphost and save it to tar file. While import utility works on create a new image out of the new snapshot in the tar file created from export utility

In this article we try to see how we can use export and import utility and some important difference to note between save/load and export/import. We will use a httpd instance to create a archive of the container runtime using export utility and then try to import it as an image using the import utility and run the container from the new imported image. If you are interested in watchin the video, here is the youtube video on the same step by step procedure below.

Procedure –

Step1: Create a DockerFile based on the httpd base image and a hello.html file in current working directory as shown below

As a first lets create a Dockerfile based on httpd base image. We will use this image to create the archive as shown in below steps.

Dockerfile and html code 
[admin@fed32 httpd]$ pwd
/home/admin/httpd

[admin@fed32 httpd]$ cat Dockerfile 
FROM httpd:latest
WORKDIR /usr/local/apache2/htdocs
COPY hello.html .

[admin@fed32 httpd]$ cat hello.html 
Hello Podman

Step2: Build new image from the above Dockerfile

Create a custom image based on the Dockerfile and html page that we created in the Step1.

Build Custom image hellohttpd 
[admin@fed32 httpd]$ podman build -t hellohttpd .
STEP 1: FROM httpd:latest
STEP 2: WORKDIR /usr/local/apache2/htdocs
--> a504dc9981a
STEP 3: COPY hello.html .
STEP 4: COMMIT hellohttpd
--> a91c3d71227
a91c3d71227af5e16aeac2e5b9cfab9de0aa62b024c90f62f4421b3b2b341de5

The new custom image has been created with our hello.html page and working directory set /usr/local/apache2/htdocs

List podman images 
[admin@fed32 httpd]$ podman images
REPOSITORY               TAG     IMAGE ID      CREATED         SIZE
localhost/hellohttpd     latest  a91c3d71227a  21 seconds ago  142 MB

Step3: Launch a container from this new image ‘hellohttpd’ and verify the hello.html page

Now, we will instantiate a new container from the custom image that we prepared and validate the default apache html page and hello.html page by running the container.

Launch container from hellohttpd image 
[admin@fed32 httpd]$ podman run -d -p 9999:80 --name hellopodmancontainer hellohttpd
74357fa07391bda368ad863ed819951d405ffe1d3c9dfaaf292ea0ae879e5fa6
[admin@fed32 httpd]$ podman ps -a
CONTAINER ID  IMAGE                        COMMAND           CREATED        STATUS            PORTS                 NAMES
74357fa07391  localhost/hellohttpd:latest  httpd-foreground  7 seconds ago  Up 6 seconds ago  0.0.0.0:9999->80/tcp  hellopodmancontainer

Validate the below URL URL – http://localhost:9999/hello.html

Step4: Inspect the current hellohttpd image to capture the CMD and WorkingDir

Lets inspect the custom image for the below configuration details which are need to launch the container and access the html content from the apache instance.

Inspect the custom image 
[admin@fed32 httpd]$ podman inspect --format "{{.Config.Cmd}} {{.Config.WorkingDir}}" hellohttpd
[httpd-foreground] /usr/local/apache2/htdocs

Step5: Create a snapshot of the current container hellopodmancontainer

Now that we have our custom image ready, lets create a snapshot of the container using the export utility as shown below.

Stop the container hellopodmancontainer.

Stop container hellopodmancontainer 
[admin@fed32 httpd]$ podman stop hellopodmancontainer
74357fa07391bda368ad863ed819951d405ffe1d3c9dfaaf292ea0ae879e5fa6

Export the hellopodmancontainer as hellopodmancontainer.tar and verify the tar file.

Export the container to tar file and validate the tar file 
[admin@fed32 httpd]$ podman export hellopodmancontainer > hellopodmancontainer.tar
[admin@fed32 httpd]$ ls -ltr hellopodmancontainer.tar 
-rw-rw-r--. 1 admin admin 138093568 Jan  5 05:52 hellopodmancontainer.tar
[admin@fed32 httpd]$ tar -tvf hellopodmancontainer.tar 
drwxr-xr-x root/root         0 2020-12-11 17:50 bin/
-rwxr-xr-x root/root   1168776 2019-04-18 09:42 bin/bash
-rwxr-xr-x root/root     43744 2019-02-28 21:00 bin/cat
-rwxr-xr-x root/root     64320 2019-02-28 21:00 bin/chgrp
-rwxr-xr-x root/root     64288 2019-02-28 21:00 bin/chmod
-rwxr-xr-x root/root     72512 2019-02-28 21:00 bin/chown
-rwxr-xr-x root/root    146880 2019-02-28 21:00 bin/cp
-rwxr-xr-x root/root    121464 2019-01-18 00:38 bin/dash
-rwxr-xr-x root/root    109408 2019-02-28 21:00 bin/date
...
-rw-r--r-- root/root      1467 2020-12-11 17:49 usr/local/apache2/conf/extra/httpd-vhosts.conf
-rw-r--r-- root/root      3161 2020-12-11 17:49 usr/local/apache2/conf/extra/proxy-html.conf
-rw-r--r-- root/root     20823 2020-12-11 17:49 usr/local/apache2/conf/httpd.conf
-rw-r--r-- root/root     13064 2020-12-11 17:49 usr/local/apache2/conf/magic
-rw-r--r-- root/root     60847 2020-12-11 17:49 usr/local/apache2/conf/mime.types

Step6: Import the hellopodmancontainer.tar as new container image

Lets try to import the tar file that we created in previous step to create a new container image.

Import the tar file to create new custom image newhellopodman 
[admin@fed32 httpd]$ podman import hellopodmancontainer.tar newhellopodman
Getting image source signatures
Copying blob 7998582ca78d done  
Copying config 446a991661 done  
Writing manifest to image destination
Storing signatures
446a99166173de851753ae01b47baa0123d1a14b73c864a551249bab87a15d2d
[admin@fed32 httpd]$ podman images
REPOSITORY                        TAG     IMAGE ID      CREATED         SIZE
docker.io/library/newhellopodman  latest  446a99166173  5 minutes ago   138 MB

Step7: Launch container from new image ‘newhellopodman’

Now, we can try to launch the container from this new image created from the tar file and see whether we will be able to access the html code in the apache instance.

Launch container from new custom image newhellopodman 
[admin@fed32 httpd]$ podman run -d -p 7777:80 --name newhellopodmancontainer newhellopodman
Error: no command or entrypoint provided, and no CMD or ENTRYPOINT from image

It shows the error ENTRYPOINT or CMD not provided

Step8: Modify the image to incorporate the CMD, EXPOSE, WORKDIR, ENV and STOPSIGNAL definition as in the original image

Lets try to change the image to incorporate the CMD, EXPOSE, WORKDIR, ENV and STOPSIGNAL definition as in the original image as shown below.

Modify custom image to Configuration 
[admin@fed32 httpd]$ podman import hellopodmancontainer.tar newhellopodman2 -c CMD=httpd-foreground -c EXPOSE=80 -c WORKDIR=/usr/local/apache2/htdocs -c ENV=PATH=/usr/local/apache2/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin -c ENV=HTTPD_PREFIX=/usr/local/apache2 -c ENV=HTTPD_VERSION=2.4.46 -c STOPSIGNAL=SIGWINCH
Getting image source signatures
Copying blob 7998582ca78d [--------------------------------------] 0.0b / 0.0b
Copying config aa06e92c70 done  
Writing manifest to image destination
Storing signatures
aa06e92c7009149e02ba9fb8b57b6d1ef246671eb833fd27938b44d85890f018

[admin@fed32 httpd]$ podman inspect --format "{{.Config.Cmd}} {{.Config.WorkingDir}} {{.Config.ExposedPorts}} {{.Config.Env}}" newhellopodman2
[/bin/sh -c httpd-foreground] /usr/local/apache2/htdocs map[80/tcp:{}] [PATH=/usr/local/apache2/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin HTTPD_PREFIX=/usr/local/apache2 HTTPD_VERSION=2.4.46]

Relaunch the container from the new modified image ‘newhellopodman2’

Relaunch the container from the new custom image newhellopodman2 
[admin@fed32 httpd]$ podman images
REPOSITORY                         TAG     IMAGE ID      CREATED      SIZE
docker.io/library/newhellopodman   latest  446a99166173  2 hours ago  138 MB
docker.io/library/newhellopodman2  latest  aa06e92c7009  2 hours ago  138 MB

[admin@fed32 httpd]$ podman run -d -p 7777:80 --name newhellopodmancontainer newhellopodman2
788459b72856dacb16091d8cba5450ffa1685bc8fb2f3903099b4b37860e3adf
[admin@fed32 httpd]$ 
[admin@fed32 httpd]$ podman ps -a
CONTAINER ID  IMAGE                                     COMMAND               CREATED            STATUS                          PORTS                 NAMES
788459b72856  docker.io/library/newhellopodman2:latest  /bin/sh -c httpd-...  7 seconds ago      Up 6 seconds ago                0.0.0.0:7777->80/tcp  newhellopodmancontainer

Validate the below URL on port 7777 URL – http://localhost:7777/hello.html

As you can by updating the configuration to the custom image we are able to launch the container and access our html code content.

Hope you learned something in this article. Thank you..