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66.Until_And_For_Loops.md

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until Loop:

  1. Definition: The until loop executes its body as long as the specified condition is false. It stops when the condition becomes true.

  2. Syntax:

    until [ condition ]
do
  # body of the loop
done

  3. Working:

    • The loop runs until the condition becomes true.
    • It is the opposite of the while loop, which runs as long as the condition is true.

  4. Use Case: Suitable when you want to repeat a task until a specific condition is met.

  5. Example:

    # Script to print numbers from 1 to 5
i=1
until [ $i -gt 5 ]
do
  echo $i
  let i++
done

    Output:

    1
2
3
4
5

    Q1 Script to Print 1 to 5 using until Loop:

#! /bin/bash
i=1
until [ $i -gt 5 ]
do
  echo $i
  let i++
done

Output:

1
2
3
4
5

for Loop

  1. Definition: The for loop is used to iterate over a list of items, executing the loop body for each item.

  2. Syntax:

    for variable in item1 item2 item3 ... itemN
do
  # body of the loop
done

  3. Working:

    • The loop iterates over each item in the list.
    • The variable takes the value of each item in each iteration.

  4. Use Case: Ideal for scenarios where you need to perform operations on multiple items or iterate over a fixed range.

  5. Variations:

    • Fixed List: 
      for i in 1 2 3 4 5
do
  echo $i
done
    • Range Using {}: 
      for i in {1..5}
do
  echo $i
done
    • Files in a Directory: 
      for file in *.txt
do
  echo "Processing $file"
done

  6. Example:

    # Script to print numbers from 1 to 5
for i in 1 2 3 4 5
do
  echo "Current Number: $i"
done

    Output:

    Current Number: 1
Current Number: 2
Current Number: 3
Current Number: 4
Current Number: 5

Q2 Script to Display Numbers from 1 to 100 divisible by 10:

#! /bin/bash
count=0
for num in {1..100}
do
  if [ $((num % 10)) -eq 0 ]; then
    echo "$num"
    let count++
  fi
done
echo "The number of values: $count"

Output:

10
20
30
40
50
60
70
80
90
100
The number of values: 10

Q3 Script to Display All File Names in the Current Directory:

#! /bin/bash
for name in *
do
  if [ -f "$name" ]; then
    echo "$name"
  fi
done

Q4 Script to Append Current Date, Time, and Month Calendar to Every .txt File:

#! /bin/bash
for fname in *.txt
do
  date >> "$fname"
  cal >> "$fname"
done
echo "Task Completed"

Q5 Script to Print All Command Line Arguments:

#! /bin/bash
if [ $# -ne 0 ]; then
  count=1
  for arg in "$@"
  do
    echo "Command Line Argument - $count: $arg"
    let count++
  done
else
  echo "Command line arguments are not passed"
fi

Output Examples:

  • If no arguments:

    Command line arguments are not passed

  • With arguments:

    ./script.sh 10 20 30
Command Line Argument - 1: 10
Command Line Argument - 2: 20
Command Line Argument - 3: 30

Case Study

Data: emp.txt

100:durga:1000:Hyderabad
200:bunny:2000:Chennai
300:chinny:3000:Hyderabad
400:vinny:4000:Delhi
500:pinny:5000:Hyderabad

1. Script to Display all Employees Information where Salary is Greater than 2500

#! /bin/bash
for record in $(cat emp.txt)
do
  sal=$(echo $record | cut -d ":" -f 3)
  if [ $sal -gt 2500 ]; then
    echo $record
  fi
done

2. Script to Save all Employees Information where Salary is Greater than 2500 and City is Hyderabad to hyd.txt

#! /bin/bash
for record in $(cat emp.txt)
do
  sal=$(echo $record | cut -d ":" -f 3)
  city=$(echo $record | cut -d ":" -f 4)
  if [ $sal -gt 2500 -a $city = "Hyderabad" ]; then
    echo $record >> hyd.txt
  fi
done
echo "Task Completed"

3. Script to Display Minimum and Maximum Salaries

#! /bin/bash
max=$(cat emp.txt | head -1 | cut -d ":" -f 3)
min=$(cat emp.txt | head -1 | cut -d ":" -f 3)
max_record=$(cat emp.txt | head -1)
min_record=$(cat emp.txt | head -1)

for record in $(cat emp.txt)
do
  sal=$(echo $record | cut -d ":" -f 3)
  if [ $sal -gt $max ]; then
    max=$sal
    max_record=$record
  fi
  if [ $sal -lt $min ]; then
    min=$sal
    min_record=$record
  fi
done

echo "The Maximum Salary: $max"
echo "The Maximum Salaried Employee Information:"
echo "Employee No: $(echo $max_record | cut -d ":" -f 1)"
echo "Employee Name: $(echo $max_record | cut -d ":" -f 2)"
echo "Employee Salary: $(echo $max_record | cut -d ":" -f 3)"
echo "Employee Address: $(echo $max_record | cut -d ":" -f 4)"
echo
echo "The Minimum Salary: $min"
echo "The Minimum Salaried Employee Information:"
echo "Employee No: $(echo $min_record | cut -d ":" -f 1)"
echo "Employee Name: $(echo $min_record | cut -d ":" -f 2)"
echo "Employee Salary: $(echo $min_record | cut -d ":" -f 3)"
echo "Employee Address: $(echo $min_record | cut -d ":" -f 4)"

OR

#! /bin/bash

# Initialize variables
max=0
min=0
max_record=""
min_record=""

# Read the file line by line
while IFS=":" read -r emp_id name salary city; do
  if [ -z "$max_record" ]; then
    # Initialize with the first record
    max=$salary
    min=$salary
    max_record="$emp_id:$name:$salary:$city"
    min_record="$emp_id:$name:$salary:$city"
  fi

  # Update max record
  if [ "$salary" -gt "$max" ]; then
    max=$salary
    max_record="$emp_id:$name:$salary:$city"
  fi

  # Update min record
  if [ "$salary" -lt "$min" ]; then
    min=$salary
    min_record="$emp_id:$name:$salary:$city"
  fi
done < emp.txt

# Display Maximum Salary Record
echo "===== Maximum Salary ====="
echo "Employee ID   : $(echo $max_record | cut -d ':' -f 1)"
echo "Employee Name : $(echo $max_record | cut -d ':' -f 2)"
echo "Salary        : $(echo $max_record | cut -d ':' -f 3)"
echo "City          : $(echo $max_record | cut -d ':' -f 4)"
echo "=========================="
echo

# Display Minimum Salary Record
echo "===== Minimum Salary ====="
echo "Employee ID   : $(echo $min_record | cut -d ':' -f 1)"
echo "Employee Name : $(echo $min_record | cut -d ':' -f 2)"
echo "Salary        : $(echo $min_record | cut -d ':' -f 3)"
echo "City          : $(echo $min_record | cut -d ':' -f 4)"
echo "=========================="
echo

4. Script to Display Multiple Files Content to the Terminal and all File Names Passed as Command Line Arguments

#!/bin/bash

# Check if arguments are passed
if [ $# -eq 0 ]; then
  echo "Error: Please pass at least one file name."
  exit 1
fi

# Process each file passed as an argument
for fname in "$@"; do
  if [ -f "$fname" ]; then
    echo "===== Content of $fname ====="
    cat "$fname"
    echo "============================="
    echo
  else
    echo "Error: '$fname' does not exist or is not a regular file."
  fi
done

5. Script to Append Multiple Files Content to a Single File result.txt. File Names are Passed as Command Line Arguments

#!/bin/bash

# Check if arguments are passed
if [ $# -eq 0 ]; then
  echo "Error: Please pass at least one file name."
  exit 1
fi

# Process each file and append its content to result.txt
for fname in "$@"; do
  if [ -f "$fname" ]; then
    echo "Appending content of '$fname' to result.txt..."
    cat "$fname" >> result.txt
  else
    echo "Error: '$fname' does not exist or is not a regular file."
  fi
done

echo "Task completed. Content appended to result.txt."