Question

For example:

s1="my_foo"
s2="not_my_bar"

the desired result would be my_o. How do I do this in bash?

Answer 1

My solution below uses fold to break the string into one character per line, sort to sort the lists, comm to compare the two strings and finally tr to delete the new line characters

comm -12 <(fold -w1 <<< $s1 | sort -u) <(fold -w1 <<< $s2 | sort -u) | tr -d '\n'

Alternatively, here is a pure Bash solution (which also maintains the order of the characters). It iterates over the first string and checks if each character is present in the second string.

s="temp_foo_bar"
t="temp_bar"
i=0
while [ $i -ne ${#s} ]
do
    c=${s:$i:1}
    if [[ $result != *$c* && $t == *$c* ]]
    then
      result=$result$c
    fi
    ((i++))
done
echo $result

prints: temp_bar