Extra commands¶

Here is a list of useful commands when working with Docker containers.

docker ps¶

The first command is: docker ps. It displays the running containers on the device. Add the --all parameter to see all containers on the device (even if they aren’t running).

docker ps

Example Output:

CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS              PORTS                    NAMES
244a84742697        shellhttpd:1.0       "/usr/local/bin/http…"   6 minutes ago       Up 6 minutes        0.0.0.0:8080->8080/tcp   shellhttpd

docker logs¶

Often, it is useful to watch Docker container logs. Use docker logs <container name> to see the logs of a specific container. If you want the command to keep following the log, use the --follow parameter:

In this case, the log might be empty unless you tested the shellhttpd application with curl or the browser:

docker logs --follow shellhttpd

Example Output:

GET / HTTP/1.1
Host: 127.0.0.1:8080
Connection: keep-alive
Cache-Control: max-age=0
DNT: 1
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/5.0 (X11; Fedora; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/88.0.4324.150 Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
Sec-Fetch-Site: none
Sec-Fetch-Mode: navigate
Sec-Fetch-User: ?1
Sec-Fetch-Dest: document
Accept-Encoding: gzip, deflate, br
Accept-Language: en-US,en;q=0.9,pt-BR;q=0.8,pt;q=0.7

= Thu Mar 18 01:03:14 UTC 2021 =============================
GET /favicon.ico HTTP/1.1
Host: 127.0.0.1:8080
Connection: keep-alive
Pragma: no-cache
Cache-Control: no-cache
User-Agent: Mozilla/5.0 (X11; Fedora; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/88.0.4324.150 Safari/537.36
DNT: 1
Accept: image/avif,image/webp,image/apng,image/svg+xml,image/*,*/*;q=0.8
Sec-Fetch-Site: same-origin
Sec-Fetch-Mode: no-cors
Sec-Fetch-Dest: image
Referer: http://127.0.0.1:8080/
Accept-Encoding: gzip, deflate, br
Accept-Language: en-US,en;q=0.9,pt-BR;q=0.8,pt;q=0.7

= Thu Mar 18 01:03:14 UTC 2021 =============================

docker exec¶

The docker exec command runs a new command in a running container.

To verify the files in root file system of the container, use the following command:

docker exec shellhttpd ls /usr/local/bin/

Example Output:

httpd.sh

To check what processes are running inside the container:

docker exec shellhttpd ps

Example Output:

PID   USER     TIME  COMMAND
root      0:00 {httpd.sh} /bin/sh -e /usr/local/bin/httpd.sh
root      0:00 nc -l -p 8080
root      0:00 ps

Finally, you can start a shell inside the container with:

docker exec -it shellhttpd sh

Example Output:

ls
in    dev    etc    home   lib    media  mnt    opt    proc   root   run    sbin   srv    sys    tmp    usr    var
exit

Where:

-i - keep STDIN open even if not attached.
-t - allocate a pseudo-TTY.
shellhttpd - container name.
sh - shell command.

docker rm¶

To remove the container, run the command below:

docker stop shellhttpd
docker rm shellhttpd

During development, it is very common to change the Docker image and test it again, so let’s give it a try:

In the file httpd.sh, we specify the MSG variable with ${MSG-OK}. This means if MSG is not specified, set it with the default value OK.

Let’s change the OK to FoundriesFactory, then rebuild and run:

gedit httpd.sh

httpd.sh:

#!/bin/sh -e

PORT="${PORT-8080}"
MSG="${MSG-FoundriesFactory}"

RESPONSE="HTTP/1.1 200 OK\r\n\r\n${MSG}\r\n"

while true; do
        echo -en "$RESPONSE" | nc -l -p "${PORT}" || true
        echo "= $(date) ============================="
done

Build and run the container again:

docker build --tag shellhttpd:1.0 .
docker run --name shellhttpd -d -p 8080:8080 shellhttpd:1.0

Test the new change with curl:

curl 127.0.0.1:8080

Example Output:

FoundriesFactory

The docker run command can accept many other parameters. For example, the --env parameter which specifies an environment variable to the container. Remove the previous image and launch it again with: --env MSG=MyFirstContainer

Test the new change with curl:

docker stop shellhttpd
docker rm shellhttpd
docker run --env MSG=MyFirstContainer --name shellhttpd -d -p 8080:8080 shellhttpd:1.0

Testing the new environment variable:

curl 127.0.0.1:8080

Example Output:

MyFirstContainer

Use docker exec to echo the MSG variable inside the container:

docker exec -it shellhttpd sh

Inside the Container:

echo $MSG
MyFirstContainer
exit

Remove the container:

docker stop shellhttpd
docker rm shellhttpd

All these commands are important in understanding how Docker containers work. Now let’s see how docker-compose works.