Rebalance your shares

rebalance-shares.sh

#!/bin/bash

# Script to calculate the number of shares to buy/sell for rebalancing.
# Formula: ((market_value * target_percentage / current_percentage) - market_value) / last_price
# Percentages can be provided as decimals (e.g., 0.20 for 20%) or integers (e.g., 20 for 20%).
# The script assumes consistent format for both percentages.

usage() {
    echo "Usage: $0 market_value target_percentage current_percentage last_price"
    echo ""
    echo "Arguments:"
    echo "  last_price         Current price per share (e.g., 50)"
    echo "  market_value       Current market value of the asset (e.g., 10000)"
    echo "  current_percentage Current actual percentage (e.g., 15 or 0.15)"
    echo "  target_percentage  Desired target percentage (e.g., 20 or 0.20)"
    echo ""
    echo "Options:"
    echo "  -h, --help         Display this help message"
    echo ""
    echo "Example:"
    echo "  $0 50 10000 15 20"
    echo "  This calculates shares needed to rebalance from 15% to 20%."
    echo ""
    echo "Note: A positive result means shares to buy; negative means shares to sell."
    exit 0
}

# Check for help flag
if [ "$1" = "-h" ] || [ "$1" = "--help" ]; then
    usage
fi

# Check if exactly 4 arguments are provided
if [ $# -ne 4 ]; then
    echo "Error: Incorrect number of arguments."
    usage
fi

last_price="$1"
market_value="$2"
current_percentage="$3"
target_percentage="$4"

# Validate inputs are numbers (basic check)
if ! [[ "$market_value" =~ ^[0-9]+(\.[0-9]+)?$ ]] ||
   ! [[ "$target_percentage" =~ ^[0-9]+(\.[0-9]+)?$ ]] ||
   ! [[ "$current_percentage" =~ ^[0-9]+(\.[0-9]+)?$ && "$current_percentage" != "0" ]] ||
   ! [[ "$last_price" =~ ^[0-9]+(\.[0-9]+)?$ && "$last_price" != "0" ]]; then
    echo "Error: All arguments must be positive numbers. Current percentage and price cannot be zero."
    exit 1
fi

# Perform the calculation with bc
result=$(echo "scale=6; (($market_value * $target_percentage / $current_percentage) - $market_value) / $last_price" | bc -l)

echo "$result"