Extra Commands#

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

Docker Process Status#

The first command to learn is: docker ps. This displays the running containers on the device. Add --all to see all containers on the device, even if they are not running.

host:~$ docker ps

 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#

Use docker logs <container name> to see the container’s logs. If you want the command to keep following the log, use the --follow parameter:

The log might be empty unless you tested the shellhttpd application with curl or the browser:

$ docker logs --follow shellhttpd

 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 Execute#

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

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

$ docker exec shellhttpd ls /usr/local/bin/

 httpd.sh

To check what processes are running:

$ docker exec shellhttpd ps

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

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

$ docker exec -it shellhttpd sh

 docker:~$ ls
 bin    dev    etc    home   lib    media  mnt    opt    proc   root   run    sbin   srv    sys    tmp    usr    var
 docker:~$ exit

Where:

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

Docker Remove#

To stop and then remove the container, run the commands:

$ docker stop shellhttpd
$ docker rm shellhttpd

During development it is common to make and test changes to the Docker image. Let us give this a try. In httpd.sh we specify the MSG variable with ${MSG-OK}. This means if MSG is not otherwise specified, it is set with the default value “OK”.

Using a text editor, change OK to FoundriesFactory. Rebuild and run:

$ vi 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

 FoundriesFactory

The docker run command can accept 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
$ curl 127.0.0.1:8080

 MyFirstContainer

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

$ docker exec -it shellhttpd sh

 docker:~$ echo $MSG
  MyFirstContainer
 docker:~$ exit

Remove the container:

$ docker stop shellhttpd
$ docker rm shellhttpd

All these commands are important in understanding how Docker containers work. Next we explore how docker-compose works.