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 $greetings</code></pre><h4 id="59ed2802-fc3c-4a60-bf12-e59adb2b0885" data-toc-id="59ed2802-fc3c-4a60-bf12-e59adb2b0885" collapsed="false" seolevelmigrated="true">Special Example:</h4><ul><li><p><code>$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 👀
Variable
Meaning
$0
Script name
$1, $2
Arguments
#
Number of arguments
*
All arguments
$?
Last command status
$</code></p></td><td colspan="1" rowspan="1"><p>Process ID</p></td></tr></tbody></table><p><span data-name="pushpin" data-type="emoji">📌</span> From page 2 table </p><div data-type="horizontalRule"><hr></div><h4 id="c4d9a593-76f0-4b1b-916a-38de78a6d4be" data-toc-id="c4d9a593-76f0-4b1b-916a-38de78a6d4be" collapsed="false" seolevelmigrated="true"><span data-name="brain" data-type="emoji">🧠</span> Quick Summary:</h4><ul><li><p><code>$ = special access


Used for inputs & system info

👉 Mnemonic:

“ = Special Power”
⌨ 6. Input & Arithmetic
Input:
read name
👉 ginagamit to accept user input 
Arithmetic Operators:
Operator
Meaning
+
Add
-
Subtract
*
Multiply
/
Divide
%
Modulo
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)
Operator
Meaning
-eq
equal
-ne
not equal
-gt
greater than
-lt
less than
-ge
≥
-le
≤
📌 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:
Loop
Behavior
for
fixed iterations
while
runs while true
until
runs until true
📌 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:

Shell → interface


Bash → language


Script → automation


Variables → storage


Input/Operators → processing


Conditionals → decisions


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 💻🔥