Shell Games

CLASS FILES:

Why SHELL Scripting?

Why Automate?

• reduce errors.
• Save Time.
• Eliminate 'Toil'.
• ...Fun.

Shell SCRIPTING VS the World

• *NIX-native Way to perform Actions
• Extends Naturally from shell commands
• 'Just Enough' Programming

Limitations

• BASH Syntax is Kinda Garbage
• Higher-Order Data Structures Are Cumbersome
• painful to debug/troubleshoot

OVERALL

• quick and dirty way to leverage unix tool philosophy
• Not a programming replacement

The Philosophy of Unix Tools

It's  All About the Pipe

EXERCISE 0

Swiss Army Knives VS ToolbOxes

• At Shell prompt: info coreutils
• /toolbox
• on 'Opening the Software Toolbox', hit enter.

Swiss Army Knives Vs Toolboxes, cont.

• Scroll down to Toolbox introduction
• Read until I/O Redirection
• Done!
• Let's Discuss

BAsh Scripting

Foundations

Basics of Shell Scripting

• Variables (user, environment, Built-In)
• operators
• Control Structures (if/THEN, While, For)
• functions

Dassit!

EXERCISE 1

Encapsulating the CLI (SETUP)

• sudo yum install -y git
• cd ~/
• git clone https://github.com/gamefiend/shell-games.git
• cd shell-games/skels
• sudo mv ./* /etc/

Encapsulating the CLI

• cd ~/shell-games/exercises
• vi ./useradd-1.sh

Encapsulating the CLI

• this script is 'static'
• We have to change the script for each new user..
• Or do we?

Variables

Encapsulating the CLI

• $0 , $1, $X ...
• MYVARIABLE='', ${MYVARIABLE}

Encapsulating the CLI

• Variables add flexibility to procedures
• Makes one off commands repeatable and generally useful

Exercise 2

Altering the Flow

• conditionals and control structures
• if this, then That
• while this happens... do this
• for each X in Y...do this
• In Case X...do this

Altering the Flow

• the power to respond to different input differently.

Altering the Flow

• We can prevent or enable actions based on conditions
• We can check for problems...

#1 REason to script (IMO)

altering the Flow

• $? - get exit status code of previous command
• you can now make your own skeleton directory for home dir templates.
• mkdir /etc/random_skel
• We can take one or two parameters as needed.

Exercise 3

Adding More Structure

• needing to remember the order and types of the arguments...
• Some reminders would help.

Adding More Structure

• use $#
• use while...do
• use case
• et voila!

adding more structure

• This makes the script easier to come back to
• you can ask it for help and it will tell you
• seems like a bother, but worth the time.

Adding More Structure

• Functions help you avoid repetition
• also help organization

Wrapping UP

Bash is Suprisingly intricate

Keep Expanding Tricks

Know when to Quit

Have Fun

Thank YOU!