How to read symbol separated strings from a variable, convert to numbers, and perform arithmetic?

Question

In a bash script, I wish to pass in a string which is contained in a variable, and contains up to 3 substrings separated by the '+' symbol. Each substring will have a specific numeric value, sort of like an octal mode.

I'd like to split the string into the substrings, convert them to their numeric value, and add the values together.

For instance if my substrings and values are:

• hat = 1
• shirt = 2
• trousers = 4

And my variable containing these in a string is:

my_outfit=shirt+trousers

I'd like a variable called outfit_value to have a value of 6. How should I go about doing this?

I've thought about setting IFS to '+' and reading the variable into an array, then looping through the array and converting each element to its value. Unfortunately, my head turns to custard when I think about retrieving these numeric value elements and doing an arithmetic expression.

EDIT:

This is what I have so far, it seems to work, but I'd like to know if there are any problems, or if it could be done more safely/efficiently:

my_outfit=hat+shirt+trousers

oIFS=$IFS
IFS=+
read -a clothes <<< "$my_outfit"
IFS=$oIFS

outfit_value=0

for string in ${clothes[@]}
do
   if [[ $string = "hat" ]]
   then
      add_value=1
   elif [[ $string = "shirt" ]]
   then
      add_value=2
   elif [[ $string = "trousers" ]]
   then
      add_value=4
   fi
   let outfit_value="$outfit_value"+"$add_value"
done

echo "OUTFIT VALUE is $outfit_value"

Answer 1

With bash, in an arithmetic evaluation context, variable names do not require the $ prefix. That means this is possible:

# set up the variables
hat=1 shirt=2 trousers=4
string="my_outfit=shirt+trousers"

# evaluate the equation 
(( $string ))

echo $my_outfit

outputs

6

Answer 2

Here's a variant of your original method, but using bash arrays. Note that there's no need to save an restore the field separator - you can simply assign a temporary value when you read the string

#!/bin/bash

# create a map (lookup table) from items to values
declare -A values=( [hat]=1 [shirt]=2 [trousers]=4 )
# (an ASSOCIATIVE array)

my_outfit='hat+shirt+trousers'

# convert the string to a simple (INDEXED) array
IFS=+ read -r -a my_items <<< "$my_outfit"

# loop over the array of items, looking up and summing the values 
outfit_value=0
for item in "${my_items[@]}"; do
  ((outfit_value += values[$item]))
done

printf 'OUTFIT VALUE is %s\n' "$outfit_value"