SHELL SCRIPTING

🧠 SHELL SCRIPTING REVIEWER

(Based on your handout )

💻 1. What is Shell Scripting?

In simple terms:
Shell scripting = automating commands instead of typing them one by one.

Key Points:

• The shell acts as a middleman between:

  • 👤 User

  • ⚙ Kernel (OS core)

• It’s a CLI (Command-Line Interface) — text-based siya, walang GUI.

👉 Think of it like:

Real-life Example:

Instead of typing:

mkdir folder
cd folder
touch file.txt

You write a script, then boom 💥—isang run lang, tapos na.

🧠 Quick Summary:

• 
  Shell = interface
  

• 
  Script = automation
  

• 
  Goal = less effort, more efficiency
  

👉 Mnemonic:

S.H.E.L.L = System Helper for Executing Logical Lines

⚙ 2. Bash & Script Execution

Key Idea:

• Bash = most common shell (Bourne Again SHell)
  

• 
  Script files end in .sh
  

Important Notes:

• 
  Scripts are executed top to bottom
  

• 
  Created using editors like:
  

  • 
    vim
    

  • 
    gedit
    

  • 
    emacs
    

Basic Script Example:

#!/bin/sh
# This is a comment
echo "Hello World"

Breakdown:

• #!/bin/sh → start of script
  

• # → comment
  

• echo → prints output
  

🧠 Quick Summary:

• 
  Bash = scripting language
  

• .sh = script file
  

• 
  Executes line-by-line
  

👉 Mnemonic:

B.A.S.H = Basic Automation Script Handler

📝 3. Names and Strings

Important Rules:

• 
  ❗ Case-sensitive:
  

  • file.txt ≠ File.txt
    

• 
  Allowed:
  

  • 
    letters, numbers, ., _
    

• 
  Avoid:
  

  • > < | : &
    

Why important?

Kasi kahit small mistake (like uppercase), error agad 😭

🧠 Quick Summary:

• 
  Case-sensitive ALWAYS
  

• 
  Avoid special characters
  

👉 Tip:

“If it looks the same to you, hindi ibig sabihin same siya kay Bash 😅”

📦 4. Variables and Operators

What are Variables?

Storage ng data na pwede mong gamitin ulit.

Types of Variables:

📌 Examples:

greetings="Hello"
echo $greetings

Special Example:

• $BASH_VERSION → shows bash version
  

🧠 Quick Summary:

• 
  Variables = storage
  

• $ = ginagamit to access value
  

👉 Mnemonic:

V.A.R = Value Assigned & Reused

🔢 5. Special Variables

Super important ‘to kapag may arguments 👀

📌 From page 2 table

🧠 Quick Summary:

• $ = special access
  

• 
  Used for inputs & system info
  

👉 Mnemonic:

“$ = Special Power”

⌨ 6. Input & Arithmetic

Input:

read name

👉 ginagamit to accept user input

Arithmetic Operators:

Example:

let a=5+3

⚠ Important:

[ $a == $b ] 

✅ [$a==$b] ❌

🧠 Quick Summary:

• read = input
  

• let = math
  

• 
  spaces matter ❗
  

👉 Mnemonic:

R.A.M = Read And Math

⚖ 7. Relational Operators

Used for comparisons (numbers)

📌 From page 3

🧠 Quick Summary:

• 
  Used in conditions
  

• 
  Mostly inside [ ]
  

👉 Mnemonic:

“EQ = Equal, GT = Greater, LT = Lower”

🔀 8. Conditional Statements

IF-ELSE Logic:

if [ condition ]
then
  command
else
  command
fi

👉 Example from handout:

If equal → print OK

Else → ERROR

Case Statement (Cleaner Alternative)

Instead of maraming if-else 😵

case $var in
  option1) ;;
  option2) ;;
  *) default ;;
esac

Parts:

• case → start
  

• ) → option
  

• ;; → end case
  

• * → default
  

🧠 Quick Summary:

• 
  IF = decision
  

• 
  CASE = cleaner decision
  

👉 Mnemonic:

“IF = Think, CASE = Choose”

🔁 9. Loops

For Loop

👉 fixed number of times

for i in 1 2 3
do
  echo $i
done

While Loop

👉 runs while condition is TRUE

while [ condition ]
do
  command
done

Until Loop

👉 opposite ng while

Runs until condition becomes TRUE

Comparison Table:

📌 From page 4

🧠 Quick Summary:

• 
  For = count-based
  

• 
  While = condition TRUE
  

• 
  Until = condition FALSE → TRUE
  

👉 Mnemonic:

F.W.U = For, While, Until

🔥 FINAL RECAP (Memory Tricks)

Alright, eto na yung quick recall cheat sheet mo:

💡 FLOW of Shell Script:

1. 
   Shell → interface
   

2. 
   Bash → language
   

3. 
   Script → automation
   

4. 
   Variables → storage
   

5. 
   Input/Operators → processing
   

6. 
   Conditionals → decisions
   

7. 
   Loops → repetition
   

🧠 SUPER MNEMONIC:

👉 “S.B.S V.I.C.L” (read as: “Sibs Vicel 😂”)

• Shell
  

• Bash
  

• Script
  

• Variables
  

• Input
  

• Conditionals
  

• Loops
  

💬 Final Advice (Gen Z mode ON):

This part is important kasi foundation ito ng automation + Linux skills mo.

Pag gets mo ‘to, next level ka na sa scripting 💻🔥