📘 PAPER 5 – OPERATING SYSTEM (UNIT 5 – SHELL PROGRAMMING & FILTER COMMANDS) university of allahabad

 

🔴 UNIT 5 – SHELL PROGRAMMING & FILTER COMMANDS

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🟢 1. Introduction to Shell

✅ What is a Shell?

A Shell is a command-line interpreter that:

• Accepts user commands

• Executes them

• Acts as an interface between user and OS

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🔹 Types of Shells in Linux

Shell	Description
sh	Bourne shell
bash	Bourne Again Shell (most used)
csh	C shell
ksh	Korn shell
zsh	Advanced shell

👉 Bash is most commonly used.

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🟢 2. Shell Programming

✅ Definition

Shell programming is writing a script to automate tasks using shell commands.

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🔹 Advantages

✔ Saves time
✔ Automates tasks
✔ Easy to write
✔ Used in system administration

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🟢 3. Structure of Shell Script

#!/bin/bash
echo "Hello World"

Explanation:

• #!/bin/bash → Shebang line

• echo → Print output

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🔹 Running a Shell Script

chmod +x file.sh
./file.sh

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🟢 4. Variables in Shell

User Defined Variable

name="Amit"
echo $name

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System Variables

$HOME
$USER
$PATH

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🟢 5. Read Command

echo "Enter name:"
read name
echo "Hello $name"

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🟢 6. Conditional Statements

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🔹 if Statement

if [ $a -gt $b ]
then
  echo "A is greater"
fi

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🔹 if-else

if [ $a -eq $b ]
then
  echo "Equal"
else
  echo "Not equal"
fi

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🔹 Case Statement

case $choice in
1) echo "One";;
2) echo "Two";;
*) echo "Invalid";;
esac

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🟢 7. Looping Statements

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🔹 for Loop

for i in 1 2 3
do
  echo $i
done

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🔹 while Loop

i=1
while [ $i -le 5 ]
do
  echo $i
  i=$((i+1))
done

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🟢 8. Command Line Arguments

echo "File name: $0"
echo "First arg: $1"
echo "Second arg: $2"

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🟢 9. Filter Commands

Filter commands process text input and give output.

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🔹 1. grep

Search text in file

grep "hello" file.txt

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🔹 2. sed

Stream editor (search & replace)

sed 's/old/new/' file.txt

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🔹 3. awk

Used for report generation

awk '{print $1}' file.txt

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🔹 4. cut

Extract columns

cut -d ":" -f1 file.txt

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🔹 5. sort

Sort data

sort file.txt

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🔹 6. uniq

Remove duplicate lines

uniq file.txt

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🟢 10. Pipes

Used to connect commands.

ls | grep ".txt"

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🟢 11. Shell Keywords

Keyword	Use
if	Condition
then	Execution
fi	End if
for	Loop
do	Loop start
done	Loop end

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🟢 12. Shell Script Example

Program: Check Even or Odd

echo "Enter number:"
read n

if [ $((n % 2)) -eq 0 ]
then
  echo "Even"
else
  echo "Odd"
fi

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📌 IMPORTANT EXAM QUESTIONS (UNIT 5)

✔ Explain shell scripting
✔ Write shell program for even/odd
✔ Explain grep, sed, awk
✔ Explain pipes & filters
✔ Write notes on shell variables
✔ Explain case statement