🔗 Link to the Room

🔗 Link for further learning

🏷️ Table of Contents

1. Our first simple bash scripts
   
2. Variables
   2.1 Debugging Bash scripts
   2.2 Using multiple variables
   
3. Parameters
   3.1 Command-line parameters
   3.2 Interactive input with read
   3.3 Special Parameters in Bash
   
4. Arrays
   4.1 Indexes
   4.2 Creating an array in Bash
   4.3 Printing all items in an array
   4.4 Accessing a single item
   4.5 Removing an item from an array
   4.6 Changing (replacing) an item
   
5. Conditionals
   5.1 if statement
   5.2 Common comparison operators
   5.3 Using parameters in conditionals
   5.4 Checking files with conditionals
   

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📚 Study Notes

• Bash is a scripting language that runs within the terminal on most Linux distros, as well as MacOS.
• Shell scripts are a sequence of bash commands within a file, combined together to achieve more complex tasks than simple one-liner and are especially useful when it comes to automating sysadmin tasks such as backups.

Our first simple bash scripts

1. Every bash script needs a starting line
   • A bash script must start with this line: #!/bin/bash
   • This tells your system: “Run this file using Bash.”
   • Without it, the system won’t know how to execute your script.
2. Your first example: printing text
   • You can make Bash print text to the terminal using echo.
   • Example: echo "Hello World"
   • When you run the script, it will display: Hello World
   • Think of echo like print in Python — it just shows text on the screen.
3. You can run normal Linux commands in scripts
   • Any command you can run in the terminal can also go inside a bash script.
   • For example: ls
   • When the script runs, it will list the files in the current directory — just like typing ls directly into the terminal.
   • You can even combine commands, like whoami or id = This will show your current username and your user and group IDs
4. Making the script runnable
   • Before you can run your script, you must give it permission: chmod +x yourfile.sh
   • Then run it with ./yourfile.sh
   • The ./ means “run this file from the current directory.”

• Always include #!/bin/bash at the top of your script (even if examples don’t show it)
• Basic Linux command knowledge is assumed
• The best way to learn Bash is by running the commands yourself and experimenting

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❓What piece of code can we insert at the start of a line to comment out our code?

#

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❓What will the following script output to the screen, echo “BishBashBosh”

BishBashBosh

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Variables

• A variable is just a name that stores a value, so you can reuse it later.

• In Bash, you create a variable like this: name="Jammy"
• Important rules: No spaces around = and variable names can’t contain spaces

• To use a variable, you add $ in front of its name.
• Hence name is the variable and $name is the value stored inside it

• Variables help you avoid repeating the same text over and over, change values in one place instead of many and/or make scripts easier to read and fix
• Instead of typing “Jammy” everywhere, you just use $name.

Debugging Bash scripts

• Debugging means finding and fixing mistakes and Bash gives you tools to help.
• From terminal you can run your script in debug mode by bash -x file.sh

• This shows each command as it runs, what works, where errors happen which is great for understanding what your script is actually doing step by step.

• You can turn debugging on and off inside your script:

• This is useful when you only want to debug a specific part.

• When debugging + means the command ran - The output shows what the command produced.
• Errors are easy to spot because you can see exactly where things break
• This makes fixing mistakes much less frustrating.

Using multiple variables

• You’re not limited to just one variable, e.g. echo "Hello $name, welcome back!"
• Bash will replace $name with its value automatically.

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❓What would this code return?

Jammy is 21 years old

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❓How would you print out the city to the screen?

echo $city

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❓How would you print out the country to the screen?

echo $country

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Parameters

• Parameters are one of the most powerful parts of Bash.
• They let your script receive information from the outside, instead of hard-coding everything.
• Think of them as inputs to your script.

Command-line parameters

• When you run a bash script, you can pass values to it like this: ./example.sh Alex
• Inside the script $1 is first argument, $2 second argument, $3 third argument, and so on…

Interactive input with read

• Sometimes you don’t want to pass values in the command line, instead, you want the script to ask the user for input.
• You can do this with read:
  

• When the script runs, it will pause and wait for you to type something.
• Once you press Enter, it stores the input in the variable and prints it.

Special Parameters in Bash

Parameter	**Description	Usage
$0	tells you which script is currently running	writting usage messages; debugging; logging which script ran
$#	tells you how many arguments were passed to the script	useful for checking if the user passed enough arguments
$@	all arguments passed to the script, one by one	commonly used in loops
$*	similar to $@, but it treats everything as one big string	recommended to avoid it unless you know why you need it
$?	exit status of last command, if either succeeded or failed	for error handling, automation, security scripts
$$	gives the PID of the running script	for logging, temporary files, process tracking

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❓How can we get the number of arguments supplied to a script?

$#

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❓How can we get the filename of our current script(aka our first argument)?

$0

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❓How can we get the 4th argument supplied to the script?

$4

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❓If a script asks us for input how can we direct our input into a variable called ‘test’ using “read”

read test

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❓What will the output of “echo $1 $3” if the script was ran with “./script.sh hello hola aloha”

hello aloha

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Arrays

• Arrays let you store multiple values inside one variable instead of creating lots of separate variables.
• Think of an array like a numbered list.
• Most commonly notated as var[index_position]

Indexes

• Every item in an array has a number called an index.
• Indexes always start at 0
• e.g.: ['car', 'train', 'bike', 'bus']

Creating an array in Bash

• Rules:
  • Variable name first
  • Items go inside parentheses ( )
  • Items are separated by spaces
  • Quotes are recommended
• e.g.: transport=('car' 'train' 'bike' 'bus')

Printing all items in an array

• echo "${transport[@]}"
• this means @ is all items and ${} tells bash you’re working with variable
• output: car train bike bus

Accessing a single item

• echo "${transport[1]}"
• output: train
• Because Index 1 = second item

Removing an item from an array

• To delete an item use unset transport[1]
• This removes train from the array.
• If you print the array again, it will be gone.

Changing (replacing) an item

• You can assign a new value to a specific index by transport[1]='trainride'
• Now the array becomes car trainride bike bus

Arrays are great when you need to store multiple values of the same type, loop through data, organize inputs
cleanly, build more advanced scripts (automation, security checks, etc.)

Extend your biography maker:

• Store multiple names in an array
  
• Store multiple facts (age, job, hobby)
  
• Pick and display them dynamically
  
• This is exactly how real scripts evolve — small upgrades over time
  

 

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❓What would be the command to print audi to the screen using indexing.

echo "${cars[1]}"

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❓If we wanted to remove tesla from the array how would we do so?

unset cars[3]

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❓How could we insert a new value called toyota to replace tesla?

cars[3]='toyota'

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Conditionals

• A conditional means that “Only do this if something is true.”
• Bash checks conditions using operators like equal to, greater than, less than

if statement

• Every if statement in Bash follows this pattern:
  

• IMPORTANT: You must have spaces inside [ ] and fi ends the if statement (it’s if backwards)

Example:

What’s happening:

• $count is compared to 10
• -eq means “equal to”
• Since 10 = 10, it prints:

Common comparison operators

operator	definition
-eq	equal to
-ne	not equal to
-gt	greater than
-lt	less than
-ge	greater than or equal to

Using parameters in conditionals

• You can compare user input too.

• Run it like this: ./example.sh guessme (output will be `They are equal)
• Run it with something else like: ./example.sh hi (output will be `They are not equal)

Checking files with conditionals

• Conditionals are very powerful for working with files.
• Two important file checks:
  • -f - does the file exist?
  • -w - is the file writable?

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❓What is the flag to check if we have read access to a file?

-r

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❓What is the flag to check to see if it's a directory?

-d

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