Linux Shell Scripting Basics

Problem Explanation

This shell script checks if a file exists at a given path. It uses the -f test operator within an if statement to verify file existence before performing operations.

Step-By-Step Instructions

• Create a shell script file (script.sh)
• Add shebang line #!/bin/bash
• Accept filename as command-line argument
• Use -f test operator to check file existence
• Display appropriate message
• Make script executable (chmod +x)

Code

#!/bin/bash

if [ -f "$1" ]; then
    echo "File $1 exists"
else
    echo "File $1 does not exist"
fi

Output

Key Concept

The [ -f file ] test checks if file exists and is a regular file. Quotes "$1" protect arguments with spaces. if-fi blocks provide conditional execution. This is essential for file management scripts.

Problem Explanation

This script copies a file from source to destination using the cp command. It includes error checking to ensure source file exists before copying.

Step-By-Step Instructions

• Accept source and destination arguments
• Check if source file exists
• Use cp command to copy file
• Display success or error message

Code

#!/bin/bash

if [ $# -ne 2 ]; then
    echo "Usage: $0 source destination"
    exit 1
fi

if [ ! -f "$1" ]; then
    echo "Source file $1 does not exist"
    exit 1
fi

cp "$1" "$2"
echo "File copied successfully from $1 to $2"

Output

Key Concept

$# returns number of arguments. -ne tests inequality. exit 1 signals error. This script validates inputs before executing commands, preventing accidental data loss.

Problem Explanation

This script uses the modulo operator (%) within shell arithmetic to determine if a number is even or odd. Even numbers have remainder 0 when divided by 2.

Step-By-Step Instructions

• Read number from user or command-line
• Use modulo operator (number % 2)
• If remainder is 0, number is even
• Otherwise, number is odd

Code

#!/bin/bash

read -p "Enter a number: " num

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

Output

Key Concept

$((expression)) performs arithmetic evaluation in bash. read command accepts user input with prompt. -eq tests numeric equality. This demonstrates basic arithmetic in shell scripts.

Problem Explanation

This script checks file access permissions using test operators: -r (readable), -w (writable), -x (executable). It helps determine what operations can be performed on a file.

Key Operators

• -r file: File is readable
• -w file: File is writable
• -x file: File is executable
• -f file: Is a regular file
• -d file: Is a directory

Code Example

#!/bin/bash
file="./myfile.txt"
if [ -r "$file" ]; then echo "File is readable"; fi
if [ -w "$file" ]; then echo "File is writable"; fi
if [ -x "$file" ]; then echo "File is executable"; fi

Key Concept

File permissions control access security. Test operators check individual permissions. This is essential for access control and security verification in shell scripts.

Problem Explanation

This script calculates student grade based on marks using conditional statements. Typical grading: A (90+), B (80+), C (70+), D (60+), F (below 60).

Code Example

#!/bin/bash
read -p "Enter marks: " marks

if [ $marks -ge 90 ]; then echo "Grade: A"
elif [ $marks -ge 80 ]; then echo "Grade: B"
elif [ $marks -ge 70 ]; then echo "Grade: C"
elif [ $marks -ge 60 ]; then echo "Grade: D"
else echo "Grade: F"
fi

Key Concept

elif chains multiple conditions. -ge tests greater-than-or-equal. This demonstrates conditional logic for educational grading systems.

Problem Explanation

This script checks if a word contains vowels and determines their case (uppercase or lowercase). It uses pattern matching with grep or case statements.

Key Techniques

• Iterate through each character
• Check if character is a vowel
• Determine case using [[ ]] patterns
• Display results

Key Concept

Pattern matching identifies character types. String manipulation extracts individual characters. Case analysis helps in text processing and validation.

Problem Explanation

Leap year logic: Divisible by 400 OR (Divisible by 4 AND NOT divisible by 100). This script uses complex conditional logic.

Code Example

#!/bin/bash
read -p "Enter year: " year

if [ $((year % 400)) -eq 0 ] || { [ $((year % 4)) -eq 0 ] && [ $((year % 100)) -ne 0 ]; }; then
    echo "$year is a Leap Year"
else
    echo "$year is not a Leap Year"
fi

Key Concept

Complex boolean logic with || (OR) and && (AND). This teaches important date logic and algorithmic thinking in shell scripting.

Problem Explanation

This script uses the date command to get current hour and displays appropriate greeting: "Good Morning" (6-12), "Good Afternoon" (12-18), or "Good Evening" (18-6).

Key Concept

date command retrieves system time. String manipulation extracts hour. Conditional logic selects appropriate greeting. This demonstrates time-based program behavior.

Problem Explanation

Fibonacci series: Each number is sum of previous two (0, 1, 1, 2, 3, 5, 8...). This script uses a while loop to generate the series up to N terms.

Code Example

#!/bin/bash
read -p "Enter number of terms: " n
a=0
b=1
echo "Fibonacci Series:"
for ((i=0; i