The Script/Linux

Bash Scripting

Bash is a programming language commonly used to automate tasks in Linux because it's included in nearly every distribution of Linux. In fact any Linux terminal that you use is probably using bash to interpret what you type, but it can also be used non-interactively by writing Bash code into a file and executing it all at once. If you've programmed in other languages before, know that Bash's syntax is very different to other languages. The main reason that it's often better to write code for CyberPatriot in Bash instead of, for example, Python, is because it gives you easy access to many commands that are useful for interacting with the operating system, such as starting and stopping services, and the bash interpreter is preinstalled on almost all distributions of Linux. However Python should probably be used for scripts with more complex logic.

Bash Scripting tutorial

https://www.youtube.com/watch?v=tK9Oc6AEnR4&t=2589s

Command Line tutorial

https://www.youtube.com/watch?v=ZtqBQ68cfJc&t=5139s

• This video is really really long so you can jump around, but it covers pretty much all of the essential commands

More important commands

• sed

https://www.youtube.com/watch?v=nXLnx8ncZyE

• cut

https://www.youtube.com/watch?v=GYP2T34v56E

Useful one liners

These are useful commands or chains of commands to remember or write down for CyberPatriot

grep "sh$" /etc/passwd

• Lists all of the users on the system, plus the root user. It includes extra information, but the usernames are before the first colon.

find /home -name "*\.mp[34]" -o -name "*\.mov" -o -name "*\.webm"

• Finds all files which end in ".mp3", ".mp4", ".mov", or ".webm", which are usually not allowed to be in user's directories.

sudo rm /directory/of/files/to/delete/*

• This deletes all of the files within a certain directory (make sure to include the asterisk at the end). Fore example sudo rm /home/jim/Music/*

Baselining Technique

A good way to get a lot of points with scripts is trying to get the configuration files of the computer that you're working on to be as close as possible to configurations that you know are secure. Some of the most important configuration files to do this for are /etc/pam.d/common-password, /etc/sudoers, /etc/ssh/sshd_config, /etc/login.defs, /etc/apt/sources.list and /etc/sysctl.conf. A secure version of these configuration files can be created by creating a virtual machine of Ubuntu, and then researching how to secure these files. You can then transfer these files to a computer that you're working on during a competition. Before you overwrite the original files on that computer however you should do two things:

• Check the README for specific configurations they may want. For example they may want SSH to be on a different port than default, or give a user special sudo rights. These have to be integrated into your config files before you copy those files into their locations.
• Make a backup of the original file. (this can be scripted). For example to make a backup of the sources.list file:

sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak

Then copy your secure config to its correct location in the computer (cp overwrites files):
sudo cp ./sources.list /etc/apt/sources.list