Currency converter - bash

Question

I'm stuck on this USD to SEK currency converter and the empty row, row 12. The current conversion is 1 SEK = 0.158193 USD 1 USD = 6.32138 SEK.
Row 12 will have something like SEK="(?(USD) )?"
I don't know what to enter in the question marks.
#!/bin/bash
shopt -s -o nounset
declare -i USD # USD
declare -i SEK # SEK
# Title
printf "%s\n" "USD-SEK Currency Convertor"
printf "\n"
# Get the value to convert
read -p "Enter a USD: " USD
# Do the conversion
printf "You will get SEK %d\n" "$SEK"
exit 0

Answer 1

You can do floating point arithmetic using bc like this:
SEK=$( echo " 6.32138 * $USD " | bc -l )
Explanation:
Bash does not have built it support for floating point arithmetic. Thus, we usually handle these operation using the bc program. bc reads an arithmetic expression as a string from the standard input, and prints the result the standard output. Note that the -l option is necessary for keeping the decimal part of the expression.
In order to get the result from bc, and store it in a variable, we use command redirection i.e. the $( ). Note that there are no spaces before and after the = in the previous expression.
Complete Example
#!/bin/bash
printf "%s\n" "USD-SEK Currency Convertor"
# Get the value to convert
read -p "Enter a USD: " USD
SEK=$(echo " 6.32138 * $USD " | bc -l )
printf "You will get SEK %s\n" "$SEK" ;# NOTE THAT I CHANGED THIS TO %s FROM %f DUE TO THE LOCALE SETTINGS
Output
$ ./converter.sh
USD-SEK Currency Convertor
Enter a USD: 10
You will get SEK 63.213800
Note that i removed the declare -i SEK from the script, since the SEK variable is NOT Integer
The harm of declare -i. This code produces:
#!/bin/bash
declare -i SEK ;# WOOOPS I FORGOT THE declare -i
printf "%s\n" "USD-SEK Currency Convertor"
# Get the value to convert
read -p "Enter a USD: " USD
SEK=$(echo " 6.32138 * $USD " | bc -l )
printf "You will get SEK %s\n" "$SEK"
This output:
$ ./converter.sh
USD-SEK Currency Convertor
Enter a USD: 10
./converter.sh: line 6: 63.21380: syntax error: invalid arithmetic operator (error token is ".21380")
You will get SEK 0.000000