Nephology Exercises#

Cloud Computing Basics#

Exercise 22

Write a bash script that takes a file path as an argument and copies that file to a designated folder on your server.
Find a file online that changes periodically, then write a program to download that file every time it changes.
Try creating your own Twitter or GitHub bot with the Twitter API or the GitHub API.

Solution to

Listing 11 cloud-backup.sh#

[[ $# -ne 1 ]] && echo Usage: $0 FILE-OR-DIR && exit 1
FILE_OR_DIR_SAVE_AS=$(date -I)_$(basename $1)
# `basename` gets rid of parent directories, otherwise `scp` cannot store it as
# `scp` cannot create many levels of directories. `rsync` is better but must be
# installed both on server and the client
# e.g., if the argument is `dir1/file`, then without `basename` `scp` will fail.
SSH_SERVER=aydos.de
BACKUP_FOLDER=backups
scp -r $1 $SSH_SERVER:$BACKUP_FOLDER/$FILE_OR_DIR_SAVE_AS

For instance the index page of https://lsi.th-deg.de/the-unix-workbench-notes changes frequently. Using [curl -z] we can download only if a file has changed. Probably this flag makes much more sense if we are checking a very large file periodically.

Listing 12 download-if-modified.sh#

[[ $# -ne 1 ]] && echo Usage: $0 URL && exit 1
URL=$1
curl --remote-name -z $(basename $URL) $URL
# `--remote-name` uses the remote name of the downloaded file to save the file 
# as. For instance https://aydos.de/dir1/dir2/file.jpg will be installed as
# `file.jpg`.
# `basename` used for getting the remote name.

To check if curl -z really works, you can check

the modification date of the downloaded file
or use curl -v to make curl more verbose.

A Twitter bot that posts jokes:
Listing 13 tweet-a-joke.sh#
```
USER_AGENT='Joke Tweeter https://aydos.de/uni'
JOKE=$(curl -H 'User-Agent: $USER_AGENT' -H 'Accept: text/plain' https://icanhazdadjoke.com/)
TWEET="$JOKE Credit: icanhazdadjoke.com"
echo Tweeting
echo $TWEET
echo ...
twurl -d "status=$TWEET" /1.1/statuses/update.json
```
Steps:
1. Register for a Twitter developer account. Note that you have to describe what you plan to do with your developer account. Then create an app.
2. Your app will use a Twitter account to tweet. The Twitter developer account and the Twitter user account are different from eachother. Create a new account or use your existing account which your app will use to post tweets.
3. Install twurl which is a special version of curl tailored for the Twitter API.
4. Authorize the app using your user account.
```
twurl authorize \
	--consumer-key=NvrPvtYAWeiJSAdGpHRD \
	--consumer-secret=OXtcqpmahpgcjDZDuVOkHPaIFuJwROvyLybZSsWvCzPOyxnqzpolBdmmSnsz
```
5. Try the script by running it manually.
6. Deploy the script on a computer or server so that it runs periodically. There are multiple ways to achieve this.
  - You can use cron to run a program preiodically.
  - If you have root access on the server you can use systemctl timers.
  - If you only have user access, you can create a persistent session — in other words a shell which will not be killed even you disconnect from the server. Some programs for creating a persistent session are:
    - screen
    - tmux
    - xpra
For example using tmux:
1. Run tmux, a new shell will be opened.
2. To run your script every 24h assuming that the script is stored under $HOME/.scripts
```
while true; do $HOME/.scripts/tweet-a-joke.sh; sleep 1d; done
```
3. You can exit your persistent sessions by pressing: <CTRL>B then d (stands for detach)
4. You can reenter your session by tmux attach
This bot works similar to @icanhazdadjoke

Nephology Exercises

Contents

Nephology Exercises#

Cloud Computing Basics#