Overview

Teaching: 10 min
Exercises: 5 min

Questions

• How are containers run?

• How do you monitor containers?

• How are containers exited?

• How are containers restarted?

Objectives

• Run containers

• Understand container state

• Stop and restart containers

To use a Docker image as a particular instance on a host machine you run it as a container. You can run in either a detached or foreground (interactive) mode.

Run the image we pulled as a container with an interactive bash terminal:

docker run -it sl:7 /bin/bash

The -i option here enables the interactive session, the -t option gives access to a terminal and the /bin/bash command makes the container start up in a bash session.

You are now inside the container in an interactive bash session. Check the file directory

pwd
ls -alh

/
total 56K
drwxr-xr-x   1 root root 4.0K Jun 22 15:47 .
drwxr-xr-x   1 root root 4.0K Jun 22 15:47 ..
-rwxr-xr-x   1 root root    0 Jun 22 15:47 .dockerenv
lrwxrwxrwx   1 root root    7 Jun  1 15:03 bin -> usr/bin
dr-xr-xr-x   2 root root 4.0K Apr 12  2018 boot
drwxr-xr-x   5 root root  360 Jun 22 15:47 dev
drwxr-xr-x   1 root root 4.0K Jun 22 15:47 etc
drwxr-xr-x   2 root root 4.0K Jun  1 15:03 home
lrwxrwxrwx   1 root root    7 Jun  1 15:03 lib -> usr/lib
lrwxrwxrwx   1 root root    9 Jun  1 15:03 lib64 -> usr/lib64
drwxr-xr-x   2 root root 4.0K Apr 12  2018 media
drwxr-xr-x   2 root root 4.0K Apr 12  2018 mnt
drwxr-xr-x   2 root root 4.0K Apr 12  2018 opt
dr-xr-xr-x 215 root root    0 Jun 22 15:47 proc
dr-xr-x---   2 root root 4.0K Jun  1 15:04 root
drwxr-xr-x  11 root root 4.0K Jun  1 15:04 run
lrwxrwxrwx   1 root root    8 Jun  1 15:03 sbin -> usr/sbin
drwxr-xr-x   2 root root 4.0K Apr 12  2018 srv
dr-xr-xr-x  13 root root    0 Jun 22 15:47 sys
drwxrwxrwt   2 root root 4.0K Jun  1 15:04 tmp
drwxr-xr-x  13 root root 4.0K Jun  1 15:03 usr
drwxr-xr-x  18 root root 4.0K Jun  1 15:03 var

and check the host to see that you are not in your local host system

hostname

<generated hostname>

Further, check the os-release to see that you are actually inside a release of Scientific Linux:

cat /etc/os-release

NAME="Scientific Linux"
VERSION="7.9 (Nitrogen)"
ID="scientific"
ID_LIKE="rhel centos fedora"
VERSION_ID="7.9"
PRETTY_NAME="Scientific Linux 7.9 (Nitrogen)"
ANSI_COLOR="0;31"
CPE_NAME="cpe:/o:scientificlinux:scientificlinux:7.9:GA"
HOME_URL="http://www.scientificlinux.org//"
BUG_REPORT_URL="mailto:scientific-linux-devel@listserv.fnal.gov"

REDHAT_BUGZILLA_PRODUCT="Scientific Linux 7"
REDHAT_BUGZILLA_PRODUCT_VERSION=7.9
REDHAT_SUPPORT_PRODUCT="Scientific Linux"
REDHAT_SUPPORT_PRODUCT_VERSION="7.9"

Monitoring Containers

Open up a new terminal tab on the host machine and list the containers that are currently running

docker ps

CONTAINER ID        IMAGE         COMMAND             CREATED             STATUS              PORTS               NAMES
<generated id>      <image:tag>   "/bin/bash"         n minutes ago       Up n minutes                            <generated name>

container command

You can also list the containers by using

docker container ls

Notice that the name of your container is some randomly generated name. To make the name more helpful, rename the running container

docker rename <CONTAINER ID> my-example

and then verify it has been renamed

docker ps

CONTAINER ID        IMAGE         COMMAND             CREATED             STATUS              PORTS               NAMES
<generated id>      <image:tag>   "/bin/bash"         n minutes ago       Up n minutes                            my-example

Renaming by name

You can also identify containers to rename by their current name

docker rename <NAME> my-example

Specifying a name

You can also startup a container with a specific name

docker run -it --name my-example sl:7 /bin/bash

Exiting a container

As a test, go back into the terminal used for your container, and create a file in the container

touch test.txt

In the container exit at the command line

exit

You are returned to your shell. If you list the containers you will notice that none are running

docker ps

CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES

but you can see all containers that have been run and not removed with

docker ps -a

CONTAINER ID        IMAGE         COMMAND             CREATED            STATUS                     PORTS               NAMES
<generated id>      <image:tag>   "/bin/bash"         n minutes ago      Exited (0) t seconds ago                       my-example

Restarting a container

To restart your exited Docker container start it again and then attach it interactively to your shell

docker start <CONTAINER ID>
docker attach <CONTAINER ID>

Attach shortcut

The two commands above (docker start and docker attach) can be combined into a single command as shown below:

docker start -ai <CONTAINER ID>

exec command

The attach command used here is a handy shortcut to interactively access a running container with the same start command (in this case /bin/bash) that it was originally run with.

In case you’d like some more flexibility, the exec command lets you run any command in the container, with options similar to the run command to enable an interactive (-i) session, etc.

For example, the exec equivalent to attaching in our case would look like:

docker start <CONTAINER ID>
docker exec -it <CONTAINER ID> /bin/bash

You can start multiple shells inside the same container using exec.

Starting and attaching by name

You can also start and attach containers by their name

docker start <NAME>
docker attach <NAME>

Notice that your entry point is still / and then check that your test.txt still exists

ls -alh test.txt

-rw-r--r-- 1 root root 0 Sep 25 08:39 test.txt

So this shows us that we can exit Docker containers for arbitrary lengths of time and then return to our working environment inside of them as desired.

Clean up a container

If you want a container to be cleaned up — that is deleted — after you exit it then run with the --rm option flag

docker run --rm -it <IMAGE> /bin/bash

Stopping a container

Sometimes you will exited a container and it won’t stop. Other times your container may crash or enter a bad state, but still be running. In order to stop a container you will exit it (exit) and then enter:

docker stop <CONTAINER ID> # or <NAME>

Stopping the container is a prerequisite for its removal.

Key Points

• Run containers with docker run

• Monitor containers with docker ps

• Exit interactive sessions just as you would a shell, with exit

• Stop a container with docker stop

• Restart stopped containers with docker start