In part 1 of this series, we saw how to pull Docker images from Docker Hub and launch Docker containers. We interacted with a running Docker container by running some bash commands, in this tutorial we will see how to use Dockerfile to automate image building for quicker deployment of applications in a container.

Dockerfile is a text file containing a set of instructions or commands in order to build a Docker image.

Prerequisites

1. Complete the tutorial on part 1 before proceeding. You will need a Docker engine running and the latest official Ubuntu Docker images locally hosted.

2. Create directories

$mkdir ~/docker-flask 
$cd ~/docker-flask

3. Add Dockerfile : ~/docker-flask/Dockerfile
The commands below will be used to create the Docker image. It will pull the latest Ubuntu official Docker image as a first layer or base. Then it will resynchronize the apt package index files from their sources.

A /flask directory will be created in the image, followed by installing Flask and running our Flask app, which we will write in next step.

FROM ubuntu:latest
RUN apt-get update && apt-get install -y python-pip python-dev
COPY . /flask
WORKDIR /flask
RUN pip install -r requirements.txt
EXPOSE 80
ENTRYPOINT ["python"]
CMD ["app.py"]

4. Write flask app : ~/docker-flask/app.py
Let us write a practical app, rather than just printing hello world. The flask app will return the user agent information of the visitor if the index page is visited.

We will also have a URL under /status/ followed by a valid HTTP status code. Given this HTTP status code by the visitor, the flask web server will generate the same HTTP status code header. For instance, if the user visits http://localhost/status/502, the flask server will respond with ‘502 BAD GATEWAY’ HTTP header.

Let us write it under ~/docker-flask/app.py

from flask import Flask
from flask import request, jsonify

app = Flask(__name__)

@app.route('/')
def user_agent():
    user_agent = request.headers.get('User-Agent')
    return 'Your browser is %s.' % user_agent

@app.route('/status/<int:httpcode>')
def get_status(httpcode):
    httpcode = int(httpcode)
    if httpcode < 100 or httpcode >= 600:
        return jsonify({'Status': 'Invalid HTTP status code'})
    elif httpcode >= 100 and httpcode < 500:
        return jsonify({'Status': 'UP'}) , httpcode
    else:
        return jsonify({'Status': 'DOWN'}) , httpcode

if __name__ == '__main__':
    app.run(debug=True, host='0.0.0.0', port=80)

5. requirements.txt : ~/docker-flask/requirements.txt

cat ~/docker-flask/requirements.txt
Flask==0.12

By now, your directory structure should look similar to this –

daniel@lindell:~/docker-flask$ pwd
/home/daniel/docker-flask

daniel@lindell:~/docker-flask$ ls
app.py  Dockerfile  requirements.txt

Time to build the Docker image –

sudo docker build -t flaskweb:latest .

This will execute the series of commands under Dockerfile. If successful, you will end up with a Docker image named flaskweb and tagged latest –

root@lindell:~# docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
flaskweb            latest              6b45443b6380        22 minutes ago      440.4 MB
ubuntu              latest              104bec311bcd        2 weeks ago         129 MB

If you encounter any errors, validate you don’t have any syntax errors on Dockerfile.

It is time to run the container –

daniel@lindell:~/docker-flask$ sudo docker run -d -p 80:80 flaskweb
d9af9a1c92bff45b56fc97d13935972b65e3554bfe22ec2f3c102fd26bd20e4c

daniel@lindell:~/docker-flask$ sudo docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED              STATUS              PORTS                NAMES
d9af9a1c92bf        flaskweb            "python app.py"     About a minute ago   Up About a minute   0.0.0.0:80->80/tcp   drunk_mcnulty

In this case, both the host and container will be listening on port 80, feel free to modify this according to your setup.

Test it, we will use httpie to query the web server, if you don’t have httpie installed, you can use ‘curl -I’ to get the full header –

daniel@lindell:~/blog/docker-flask$ http http://localhost/
HTTP/1.0 200 OK
Content-Length: 29
Content-Type: text/html; charset=utf-8
Date: Fri, 30 Dec 2016 14:35:53 GMT
Server: Werkzeug/0.11.13 Python/2.7.12

Your browser is HTTPie/0.9.2.

daniel@lindell:~/blog/docker-flask$ http http://localhost/status/404
HTTP/1.0 404 NOT FOUND
Content-Length: 21
Content-Type: application/json
Date: Fri, 30 Dec 2016 14:35:56 GMT
Server: Werkzeug/0.11.13 Python/2.7.12

{
    "Status": "UP"
}

daniel@lindell:~/blog/docker-flask$ http http://localhost/status/502
HTTP/1.0 502 BAD GATEWAY
Content-Length: 23
Content-Type: application/json
Date: Fri, 30 Dec 2016 14:35:58 GMT
Server: Werkzeug/0.11.13 Python/2.7.12

{
    "Status": "DOWN"
}

Full clean up – if you want to start all over again or want to delete the container and images we have created, i have outlined the steps below. The first step is to stop the running container using ‘docker stop’ command, pass it the first few digits of the container ID.

Once the container is stopped, use ‘docker rm’ to delete the container. At this point, we can proceed with deleting the image as the image is not attached to any running container. Use ‘docker rmi’ to delete the image. We will keep the base Ubuntu image for future use.

daniel@lindell:/tmp$ sudo docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS                NAMES
d9af9a1c92bf        flaskweb            "python app.py"     12 minutes ago      Up 12 minutes       0.0.0.0:80->80/tcp   drunk_mcnulty

daniel@lindell:/tmp$ sudo docker stop d9a
d9a

daniel@lindell:/tmp$ sudo docker rm d9a
d9a

daniel@lindell:/tmp$ sudo docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES

daniel@lindell:/tmp$ sudo docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
flaskweb            latest              6b45443b6380        39 minutes ago      440.4 MB
ubuntu              latest              104bec311bcd        2 weeks ago         129 MB

daniel@lindell:/tmp$ sudo docker rmi 6b45
Untagged: flaskweb:latest
Deleted: sha256:6b45443b63805583f41fbf60aaf5cf746b871fdcfa8fe1c6d5adfb52870e7c89
Deleted: sha256:02062a8ea251d993f54e15f9e5654e40894449430acd045476000cd9ebbdf459
Deleted: sha256:fa2439cd5bc8a53152877c1dc3b12a60ab808bcfe5078549ea5e945f462330da
Deleted: sha256:3bac38b223d80a4db6c4283fd56275fe05ceeab6a1dfd81871aa14c6cda387df
Deleted: sha256:d97357dc5d7454e3b7757f2c348323c84d1902dd806792c53d1fd0ca7813b091
Deleted: sha256:b55dd5bd3326ec4657dc389f4aae69c34a7ba222872f7b868eb8de69d7f69dab
Deleted: sha256:eab59ae84eb136339d08fbacd2905a1ee80a0c875e8e14a4d5184fac30445714
Deleted: sha256:588253a9066c49786fcd0121353e7f0f2cea05cebbc6b9cef67f0c823d23dce8
Deleted: sha256:fe9f27a1cb9165531a1f5149c16ebcd522422e4ac2610035bbbcada7fd0b7551
Deleted: sha256:18ca1bc40895f6f97cae28fa5707bde537ac27023762303f98912c11549431ae

daniel@lindell:/tmp$ sudo docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
ubuntu              latest              104bec311bcd        2 weeks ago         129 MB

References –
https://docs.docker.com/engine/reference/builder/
https://docs.docker.com/engine/userguide/eng-image/dockerfile_best-practices/
http://flask.pocoo.org/docs/0.12/quickstart/

The scheduling format for the Linux scheduling daemon cron are not easy to remember, especially if you don’t work with cron that frequently. The first reaction for most Linux sys admins when they can’t remember the ordering of fields is to type ‘man crontab’, and unfortunately this man page section does not contain the schedule format information. If you are like me, you will immediately start Googling it.

What is the best way to locate the man page for crontab scheduling format then? For one thing, you can search the man page for the key work ‘crontab’ using the command below –

daniel@linubuvma:/tmp$ man -k crontab
anacrontab (5)       - configuration file for anacron
crontab (1)          - maintain crontab files for individual users (Vixie Cron)
crontab (5)          - tables for driving cron

You see, there are two sections for crontab – section 1 describes the command usage and section 5 shows the tables we are looking for. If you are familiar with how man page section numbers are assigned, you would have immediately jumped to section 5 of the man page for crontab –


1. General commands
2. System calls
3. C library functions
4. Special files (usually devices, those found in /dev) and drivers
5. File formats and conventions
6. Games and screensavers
7. Miscellanea
8. System administration commands and daemons

Short answer to how do i see the crontab schedule format is – run

 man 5 crontab 

Per the man page, the time and date fields in order are –

field allowed values
----- --------------
minute 0-59
hour 0-23
day of month 1-31
month 1-12 (or names, see below)
day of week 0-7 (0 or 7 is Sun, or use names)

One of the most commonly used Linux system administration tools is chown, which is part of the coreutils package. It is used to change the user and/or group ownership of a given file or directory. Something to be aware of this tool is, it doesn’t change the ownership of symbolic links, as shown below –

root@linubuvma:/tmp# touch test
root@linubuvma:/tmp# ls -l test
-rw-r--r-- 1 root root 12 Dec 20 08:01 test
root@linubuvma:/tmp# ln -s test sltest
root@linubuvma:/tmp# ls -l sltest
lrwxrwxrwx 1 root root 4 Dec 20 08:01 sltest -> test
root@linubuvma:/tmp# chown daniel:daniel sltest
root@linubuvma:/tmp# ls -l sltest
lrwxrwxrwx 1 root root 4 Dec 20 08:01 sltest -> test

The reason this doesn’t work is in the man page for chown – symbolic links named by arguments are silently left unchanged unless -h is used.” By simply running chown on symbolic link without ‘-h’ option, you are changing the ownership of the target. The ‘-h’ option affects symbolic links instead of any referenced file.

root@linubuvma:/tmp# chown -h daniel:daniel sltest

root@linubuvma:/tmp# ls -l sltest
lrwxrwxrwx 1 daniel daniel 4 Dec 20 08:01 sltest -> test

Though not portable, in some distros

 chown -R 

will recursively change the owernship of all files, including symbolic link files and directories. In my case, ‘chown -R /path/to/file’ works for GNU chown which is part of the ‘GNU coreutils 8.21’ package on Ubuntu 14.04.