63.Iterative_Statements.md

Iterative Statements in Shell Scripting

If we want to execute a group of commands multiple times, we should use iterative statements. There are 3 types of iterative statements in shell scripting:

1. while loop
2. until loop
3. for loop

1 while Loop

If we don't know the number of iterations in advance, we use the while loop.

Syntax:

while [ condition ]
do
  body
done

As long as the condition is true, the body will execute. Once the condition fails, the loop terminates.

Example 1: Print Numbers from 1 to 10

#!/bin/bash
i=1
while [ $i -le 10 ]
do
  echo $i
  let i++
done

Example 2: Generate Numbers Until Limit Provided by User

#!/bin/bash
read -p "Enter Limit: " n
i=1
while [ $i -le $n ]
do
  echo $i
  sleep 2
  let i++
done

Note: The sleep command pauses execution for the specified number of seconds. Examples:

• sleep 2 → Pause for 2 seconds
• sleep 0.5 → Pause for 0.5 seconds

Example 3: Find the Sum of First n Integers (User Input)

#!/bin/bash
read -p "Enter n value: " n
i=1
sum=0
while [ $i -le $n ]
do
  let sum=sum+i
  let i++
done
echo "The Sum of first $n numbers: $sum"

Output:

1. Input: 3
   Output: The Sum of first 3 numbers: 6
2. Input: 10
   Output: The Sum of first 10 numbers: 55

Example 4: Display a Digital Timer

#!/bin/bash
while [ true ]
do
  clear
  printf "\n\n\n\n\n\n\t\t\t\t$(date +%H:%M:%S)"
  sleep 1
done

Note:

• Use the printf command instead of echo for escape characters like \n and \t.
• The true keyword represents an infinite loop as the condition always evaluates to true.

Concepts Summary:

• sleep Command: Pauses execution for a given time in seconds.
• Escape Characters:
  • \n: New line
  • \t: Tab
• Keywords:
  • true: Represents a boolean value that keeps the loop running indefinitely.
  • false: Represents a boolean value that exits the loop immediately.

---

Example 1: Checking Server Status Continuously

This script continuously checks if a server is reachable.

#!/bin/bash
while [ true ]
do
  ping -c 1 example.com > /dev/null 2>&1
  if [ $? -eq 0 ]; then
    echo "Server is up"
  else
    echo "Server is down"
  fi
  sleep 5
done

Use Case: Monitor server uptime in real-time.

Example 2: Backup Files Periodically

This script creates backups of a directory every 10 seconds.

#!/bin/bash
source_dir="/path/to/source"
backup_dir="/path/to/backup"

while [ true ]
do
  timestamp=$(date +%Y%m%d%H%M%S)
  cp -r $source_dir "$backup_dir/backup_$timestamp"
  echo "Backup created at $timestamp"
  sleep 10
done

Use Case: Automating periodic backups.

Example 3: Monitoring Disk Space

This script monitors disk space usage and alerts when it exceeds a threshold.

#!/bin/bash
threshold=90

while [ true ]
do
  usage=$(df / | grep '/' | awk '{print $5}' | sed 's/%//')
  if [ $usage -gt $threshold ]; then
    echo "Disk usage is above $threshold%: Current usage is $usage%"
  fi
  sleep 30
done

Use Case: Preventing disk space issues on servers.

Example 4: Downloading Files in Chunks

This script downloads a file in chunks until the download is complete.

#!/bin/bash
file_url="https://example.com/largefile.zip"
output_file="largefile.zip"

while [ ! -f "$output_file" ]
do
  wget -c $file_url -O $output_file
  if [ $? -eq 0 ]; then
    echo "Download complete."
    break
  else
    echo "Retrying download..."
  fi
  sleep 5
done

Use Case: Handling unreliable network connections during downloads.

Example 5: User Authentication Retry

This script prompts for a password and allows three attempts for correct input.

#!/bin/bash
correct_password="secret123"
attempts=0
max_attempts=3

while [ $attempts -lt $max_attempts ]
do
  read -sp "Enter your password: " password
  echo
  if [ "$password" == "$correct_password" ]; then
    echo "Access granted."
    break
  else
    echo "Incorrect password."
    attempts=$((attempts + 1))
  fi
done

if [ $attempts -eq $max_attempts ]; then
  echo "Access denied. Too many failed attempts."
fi

Use Case: Login system with retry limits.