Basic bash script example to calculate addition, subtraction, multiplication and division of two numbers

calculate.sh

#!/bin/bash

# Make the script to abort if any command fails. Use set +e to change the behaviour and ignore failed command.
set -e

# Uncomment below line to print the commands as it is executed. Useful for debugging. Use set +x to disable debugging.
# set -x

# Function to print usage of the script with different options
function print_usage() 
{
  echo "Usage: calculate.sh --operation OPERATION --lhs VALUE --rhs VALUE"
  echo "or"
  echo "Usage: calculate.sh -o OPERATION -l VALUE -r VALUE"
  echo ""
  echo "-o,--operation: add, subtract, multiply, divide"
}

# Check if the arguments is empty
if [[ $# -eq 0 ]]
then
    print_usage
    exit 1
fi

function validate_value() {
  if [[ $# -eq 1 ]]
  then
      echo "Value not provided for $1" >&2
      exit 1
  fi
}

# Parse the positional arguments
VARARGS=()
DISPLAY_HELP=no
while [[ $# -gt 0 ]]
do
  KEY="$1"
  case $KEY in
      -o|--operation)
        shift # past argument
        validate_value "$KEY" "$@"
        OPERATION="$1"
        shift # past value
        ;;
      -l|--lhs)
        shift # past argument
        validate_value "$KEY" "$@"
        LHS="$1"
        shift # past value
        ;;
      -r|--rhs)
        shift # past argument
        validate_value "$KEY" "$@"
        RHS="$1"
        shift # past value
        ;;
      -h|--help)
        DISPLAY_HELP=yes
        shift # past argument
        ;;
      *)    # unknown option
        VARARGS+=("$1") # save it in an array for later
        shift # past argument
        ;;
  esac
done

# Uncomment below line to restore positional parameters with only the unrecognised values
# set -- "${VARARGS[@]}"

# Display usage of the script
if [[ "$DISPLAY_HELP" == "yes" ]]
then
    print_usage
    exit 0
fi

# Error out for unexpected parameters
if [[ ${#VARARGS[@]} -ne 0 ]] 
then
  # get length of an array
  VARARGS_LENGTH=${#VARARGS[@]}
  VARARGS_INDEX=0
  # iterate through the array
  while [ $VARARGS_INDEX -lt $VARARGS_LENGTH ]
  do
    echo "Unknown option: ${VARARGS[$VARARGS_INDEX]}" >&2
    VARARGS_INDEX=$(( $VARARGS_INDEX + 1 ))
  done
  exit 1
fi

function validate_number()
{
  reg_ex_for_number='^[+-]?[0-9]+([.][0-9]+)?$'
  if ! [[ $2 =~ $reg_ex_for_number ]] ; then
    echo "error: $1 value '$2' is missing or not a valid number" >&2
    exit 1
  fi
}

validate_number "LHS" "$LHS"
validate_number "RHS" "$RHS"

function add()
{
  echo "$1 + $2" | bc -l
}

function subtract()
{
  echo "$1 - $2" | bc -l
}

function multiply()
{
  echo "$1 * $2" | bc -l
}

function divide()
{
  echo "$1 / $2" | bc -l
}

case $OPERATION in
  add)
    RESULT=$(add $LHS $RHS)
    ;;
  subtract)
    RESULT=$(subtract $LHS $RHS)
    ;;
  multiply)
    RESULT=$(multiply $LHS $RHS)
    ;;
  divide)
    RESULT=$(divide $LHS $RHS)
    ;;
  *)
    echo "error: '$OPERATION' is not a valid operation" >&2
    exit 1
    ;;
esac

echo "$RESULT"