Bash math expression - bash

Question

I need help with this its been busting my mind.
I have a read with a variable with integers 10 20 -30.
All separated by white space. I try to change the minus to plus and save it onto another variable but it's not saving. If I can't change to plus I would like to remove it so then I can do:
var=$((${num// /+/}))
So it can add all integers.
This is what I have:
read num
echo $num
sum=$num | sed -e 's/-/+/g'
echo $sum

Answer 1

Using standard POSIX variable expansion and arithmetic:
#!/usr/bin/env sh
# Computes the sum of all arguments
sum () {
# Save the IFS value
_OIFS=$IFS
# Set the IFS to + sign
IFS=+
# Expand the arguments with the IFS + sign
# inside an arithmetic expression to get
# the sum of all arguments.
echo "$(($*))"
# Restore the original IFS
IFS=$_OIFS
}
num='10 20 -30'
# shellcheck disable=SC2086 # Intended word splitting of string into arguments
sum $num
More featured version with a join function:
#!/usr/bin/env sh
# Join arguments with the provided delimiter character into a string
# $1: The delimiter character
# $#: The arguments to join
join () {
# Save the IFS value
_OIFS=$IFS
# Set the IFS to the delimiter
IFS=$1
# Shift out the delimiter from the arguments
shift
# Output the joined string
echo "$*"
# Restore the original IFS
IFS=$_OIFS
}
# Computes the sum of all arguments
sum () {
# Join the arguments with a + sign to form a sum expression
sum_expr=$(join + "$#")
# Submit the sum expression to a shell's arithmetic expression
# shellcheck disable=SC2004 # $sum_expr needs its $ to correctly split terms
echo "$(($sum_expr))"
}
num='10 20 -30'
# shellcheck disable=SC2086 # Intended word splitting of string into arguments
sum $num

Answer 2

Simply: whipe last slash:
num="10 20 -30"
echo $((${num// /+}))
0
Some details
*Bash battern substitution has nothing common with so called regular expression. Correct syntax is:
${parameter/pattern/string}
... If pattern begins with /, all matches of pattern are replaced with string.
Normally only the first match is replaced. ...
See: man -Pless\ +/parameter.pattern.string bash
If you try your syntax:
echo ${num// /+/}
10+/20+/-30
Then
echo ${num// /+}
10+20+-30
Or even, to make this pretty:
echo ${num// / + }
10 + 20 + -30
But result will stay same:
echo $(( ${num// / + } ))
0

Answer 3

sum=$num | sed -e 's/-/+/g'
With respect to what's present above, sum=$num and sed become two different commands. It's not grouped together as you wanted, which makes the sed ineffective.
Also, you'd need to echo $num
Solution is to group them together, like:
sum=`echo $num | sed -e 's/-/+/g`
OR
sum=$(echo $num | sed -e 's/-/+/g')
OR Rather, an alternate approach
sum=${num//-/+}