For Loops

The for loop iterates over a list of values, executing a block of commands for each item.

Syntax:

for variable in item1 item2 ... itemN
do
    command_sequence
done

Example 1: Iterating through a Numeric Range

To display numbers from 1 to 10:

#!/bin/bash

for num in {1..10}
do
    echo "Current number: $num"
done

Example 2: Batch User Creation

This script reads usernames from a file and creates accounts with a default password.

#!/bin/bash

USER_LIST="/root/users.txt"
DEFAULT_PASS="Password123"

for username in $(cat $USER_LIST)
do
    useradd "$username"
    echo "$DEFAULT_PASS" | passwd --stdin "$username" &> /dev/null
done

echo "User creation process completed."

Example 3: Network Connectivity Checker

Read IP addresses from a file and log their status based on ping results.

#!/bin/bash

TARGET_FILE="/root/ip_list.txt"
RESULT_FILE="/root/network_status.log"

for ip_addr in $(cat $TARGET_FILE)
do
    # Send 2 packets, wait 1 second between pings, 2-second deadline
    if ping -c 2 -i 1 -W 2 "$ip_addr" &> /dev/null; then
        echo "$ip_addr: Online" >> "$RESULT_FILE"
    else
        echo "$ip_addr: Offline" >> "$RESULT_FILE"
    fi
done

Example 4: Sequential IP Scanning

Check connectivity for a specific range of hosts (e.g., 192.168.10.10 through 60).

#!/bin/bash

BASE_IP="192.168.10"

for suffix in 10 20 30 40 50 60
do
    TARGET="$BASE_IP.$suffix"
    if ping -c 1 -W 1 "$TARGET" &> /dev/null; then
        echo "$TARGET is reachable."
    else
        echo "$TARGET is unreachable."
    fi
done

Example 5: Odd and Even Detection

Using a C-style for loop to determine parity.

#!/bin/bash

for (( i=1; i<=10; i++ ))
do
    remainder=$(( i % 2 ))
    if [ $remainder -ne 0 ]; then
        echo "$i is an odd number."
    else
        echo "$i is an even number."
    fi
done

Example 6: Parallel Subnet Scan

Scanning a /24 subnet using background processes for speed.

#!/bin/bash

SUBNET="192.168.80"
OUTPUT_FILE="active_hosts.txt"

for host_suffix in {1..254}
do
    (
        IP="$SUBNET.$host_suffix"
        if ping -c 1 -W 1 "$IP" &> /dev/null; then
            echo "$IP" >> "$OUTPUT_FILE"
        fi
    ) &
done

wait
cat "$OUTPUT_FILE"

Example 7: Calculating a Cumulative Sum

Summing integers from 1 to 10.

#!/bin/bash

total_sum=0
for (( i=1; i<=10; i++ ))
do
    total_sum=$(( total_sum + i ))
done

echo "The sum of 1 to 10 is: $total_sum"

Example 8: Searching Executables in PATH

Iterate through the PATH environment variable to find files starting with a specific prefix.

#!/bin/bash

OLD_IFS=$IFS
IFS=':'

for directory in $PATH
do
    # Silently find files starting with 'py'
    find "$directory" -name "py*" -print 2> /dev/null
done

IFS=$OLD_IFS

Example 9: Generating Fibonacci Sequence

Printing the first 10 numbers of the Fibonacci sequence (1, 1, 2, 3, 5...).

#!/bin/bash

prev=1
curr=1

for (( i=0; i<10; i++ ))
do
    echo "$prev"
    next=$(( prev + curr ))
    prev=$curr
    curr=$next
done

Example 10: Random Password Generator

Generating an 8-character random password using alphanumeric characters.

#!/bin/bash

CHAR_SET="abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
PASS_LEN=8
PASSWORD=""

for (( i=0; i<PASS_LEN; i++ ))
do
    # Get random index based on string length
    index=$(( RANDOM % ${#CHAR_SET} ))
    # Extract character at index
    char="${CHAR_SET:$index:1}"
    PASSWORD+="$char"
done

echo "Generated Password: $PASSWORD"

Example 11: Intelligent User Management

Check if users exist in /etc/passwd. If they exist, verify password status; otherwise, create them.

#!/bin/bash

INPUT_FILE="/root/users.txt"

for user in $(cat "$INPUT_FILE")
do
    # Check if user exists
    if id "$user" &> /dev/null; then
        echo "User '$user' already exists."
        # Check if password is set (checking shadow file)
        pass_status=$(grep "^$user:" /etc/shadow | cut -d: -f2)
        if [ -z "$pass_status" ] || [ "$pass_status" == "!" ] || [ "$pass_status" == "!!" ]; then
            echo "User '$user' has no password."
            read -p "Set password for $user: " -s pass1
            echo
            read -p "Confirm password: " -s pass2
            echo
            if [ "$pass1" == "$pass2" ]; then
                echo "$pass1" | passwd --stdin "$user" &> /dev/null
                echo "Password updated."
            else
                echo "Passwords do not match."
            fi
        fi
    else
        echo "User '$user' does not exist. Creating..."
        read -p "Enter password for new user: " -s new_pass
        echo
        useradd "$user"
        echo "$new_pass" | passwd --stdin "$user" &> /dev/null
        echo "User created."
    fi
done

While Loops

The while loop executes commands as long as the control condition remains true.

Syntax:

while [ condition ]
do
    command_sequence
done

Example 1: Summing User Input

Calculate the sum of numbers from 1 to a user-provided value (N).

#!/bin/bash

read -p "Enter an integer (1-100): " limit

counter=1
sum=0

if [[ $limit -ge 1 && $limit -le 100 ]]; then
    while [ $counter -le $limit ]
    do
        sum=$(( sum + counter ))
        (( counter++ ))
    done
    echo "Sum from 1 to $limit is: $sum"
else
    echo "Input out of range."
fi

Example 2: Separating Odd and Even Sums

Calculate the sum of all odd and even numbers between 1 and 100.

#!/bin/bash

i=1
odd_total=0
even_total=0

while [ $i -le 100 ]
do
    if [ $(( i % 2 )) -eq 0 ]; then
        even_total=$(( even_total + i ))
    else
        odd_total=$(( odd_total + i ))
    fi
    (( i++ ))
done

echo "Odd Sum: $odd_total"
echo "Even Sum: $even_total"

Example 3: Host Discovery Loop

Check IP addresses ending in 100 through 104.

#!/bin/bash

host_id=100
LOG_FILE="/root/active_hosts.log"

while [ $host_id -le 104 ]
do
    IP_ADDR="192.168.10.$host_id"
    ping -c 1 -W 1 "$IP_ADDR" &> /dev/null
    if [ $? -eq 0 ]; then
        echo "$IP_ADDR is up" >> "$LOG_FILE"
    fi
    (( host_id++ ))
done

echo "Scan complete. Results in $LOG_FILE."

Example 4: Login Attempt Limiter

Allow a user 3 attempts to enter the correct password.

#!/bin/bash

CORRECT_PASS="secret123"
attempts=0
max_attempts=3

while [ $attempts -lt $max_attempts ]
do
    read -p "Enter password: " user_input
    if [ "$user_input" == "$CORRECT_PASS" ]; then
        echo "Access granted."
        exit 0
    else
        echo "Incorrect password."
    fi
    (( attempts++ ))
done

echo "Maximum attempts reached. Exiting."

Example 5: Decimal to Binary Conversion

Convert a decimal number (0-255) to an 8-bit binary string.

#!/bin/bash

read -p "Enter a number (0-255): " dec_num
binary_str=""

for weight in 128 64 32 16 8 4 2 1
do
    if [ $dec_num -ge $weight ]; then
        binary_str+="1"
        dec_num=$(( dec_num - weight ))
    else
        binary_str+="0"
    fi
done

echo "Binary: $binary_str"

Example 6: Splitting Files by Line Count

Split a large file into smaller files containing 50 lines each.

#!/bin/bash

SOURCE_FILE="/root/large_log.txt"
line_count=0
file_part=1

# Ensure output directory exists
mkdir -p "/root/split_files/"

while IFS= read -r line
do
    echo "$line" >> "/root/split_files/part_${file_part}.txt"
    (( line_count++ ))
    
    if [ $(( line_count % 50 )) -eq 0 ]; then
        (( file_part++ ))
    fi
done < "$SOURCE_FILE"

Until Loops

The until loop operates conversely to the while loop; it executes until the condition becomes true.

Syntax:

until [ condition ]
do
    command_sequence
done

Example 1: Countdown Execution

Print numbers starting from 1 until the number exceeds 10.

#!/bin/bash

val=1

until [ $val -gt 10 ]
do
    echo "Number: $val"
    (( val++ ))
done