Linux Shell Scripting Tutorial – Module 4: Loops (for, while, until)

Module 4: Loops (for, while, until)

Loops allow you to execute commands repeatedly until a condition is met.
In Bash scripting, the main loops are for, while, and until.
Understanding these helps you automate repetitive tasks effectively.

✅ The for Loop

• Used to iterate over a list of values, files, or ranges.
• Syntax: for variable in list; do ...; done

#!/bin/bash
for i in 1 2 3 4 5
do
  echo "Number: $i"
done
  

Output:

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

✅ For Loop with Range

#!/bin/bash
for i in {1..5}
do
  echo "Counting: $i"
done
  

✅ For Loop with Files

#!/bin/bash
for file in *.sh
do
  echo "Script file: $file"
done
  

✅ The while Loop

• Executes commands as long as a condition is true.
• Good for reading input or waiting for events.

#!/bin/bash
count=1
while [ $count -le 5 ]
do
  echo "Count: $count"
  ((count++))
done
  

✅ Reading a File with While Loop

#!/bin/bash
while read line
do
  echo "Line: $line"
done < sample.txt
  

✅ The until Loop

• Similar to while loop, but runs until the condition becomes true.
• Syntax: until [ condition ]; do ...; done

#!/bin/bash
num=1
until [ $num -gt 5 ]
do
  echo "Number: $num"
  ((num++))
done
  

✅ Loop Control (break & continue)

• break → exit the loop immediately.
• continue → skip current iteration and move to next.

#!/bin/bash
for i in {1..10}
do
  if [ $i -eq 5 ]; then
    continue
  fi
  if [ $i -eq 8 ]; then
    break
  fi
  echo "Value: $i"
done
  

✅ Summary

• for loops iterate over lists, ranges, and files.
• while loops run as long as a condition is true.
• until loops run until a condition becomes true.
• break and continue give fine control over loops.

✅ By the end of this module, you can use loops to repeat tasks,
process files, and control execution flow efficiently.

Module 4: Bash Loop Assignments

These assignments help practice for, while, and until loops, including loop control with break and continue. They range from simple to medium-level complexity.

✅ Assignment 1: Print Multiples of a Number

Use a for loop to print all multiples of 3 between 1 and 50, skipping others.

for i in {1..50}
do
  if (( i % 3 != 0 )); then
    continue
  fi
  echo "Multiple of 3: $i"
done

✅ Assignment 2: Sum of Numbers in a Range

Calculate the sum of numbers from 1 to 100 using a for loop.

sum=0
for i in {1..100}
do
  sum=$((sum + i))
done
echo "Sum of 1 to 100 is: $sum"

✅ Assignment 3: File Count and List

List all .txt files in a directory and count them.

count=0
for file in *.txt
do
  if [ -f "$file" ]; then
    echo "File: $file"
    ((count++))
  fi
done
echo "Total .txt files: $count"

✅ Assignment 4: Reverse Counting

Count down from 10 to 1 using a while loop.

count=10
while [ $count -ge 1 ]
do
  echo "Count: $count"
  ((count--))
done
echo "Countdown Complete!"

✅ Assignment 5: Read and Number a File

Read students.txt and print each name with a line number.

line_no=1
while read student
do
  echo "$line_no. $student"
  ((line_no++))
done < students.txt

✅ Assignment 6: Login Attempt Simulation

Use an until loop to simulate a login system that allows 3 attempts.

attempts=0
until [ $attempts -ge 3 ]
do
  read -p "Enter password: " pass
  if [ "$pass" == "secret123" ]; then
    echo "Login Successful"
    break
  else
    echo "Incorrect password"
  fi
  ((attempts++))
done

✅ Assignment 7: Print Odd Numbers Only

Use a for loop to print odd numbers between 1 and 20.

for i in {1..20}
do
  if (( i % 2 == 0 )); then
    continue
  fi
  echo "Odd Number: $i"
done

✅ Assignment 8: Infinite Loop with Exit Condition

Use a while true loop to repeatedly ask for a number until the user enters 0.

while true
do
  read -p "Enter a number (0 to exit): " num
  if [ $num -eq 0 ]; then
    echo "Exiting loop."
    break
  fi
  echo "You entered: $num"
done

✅ Assignment 9: Nested Loops Table

Use nested for loops to print a multiplication table from 1 to 5.

for i in {1..5}
do
  for j in {1..5}
  do
    echo -n "$((i*j)) "
  done
  echo
done

✅ Assignment 10: Skip Specific Numbers

Use a for loop from 1 to 15, skip numbers 5, 10 using continue, and stop the loop if the number reaches 13 using break.

for i in {1..15}
do
  if [ $i -eq 5 ] || [ $i -eq 10 ]; then
    continue
  fi
  if [ $i -eq 13 ]; then
    echo "Reached 13, exiting loop."
    break
  fi
  echo "Value: $i"
done