How to find substring inside a string (or how to grep a variable)? [duplicate]

LIST="some string with a substring you want to match"
SOURCE="substring"
if echo "$LIST" | grep -q "$SOURCE"; then
  echo "matched";
else
  echo "no match";
fi

You can also compare with wildcards:

if [[ "$LIST" == *"$SOURCE"* ]]

This works in Bash without forking external commands:

function has_substring() {
   [[ "$1" != "${2/$1/}" ]]
}

Example usage:

name="hello/world"
if has_substring "$name" "/"
then
   echo "Indeed, $name contains a slash!"
fi

If you're using bash you can just say

if grep -q "$SOURCE" <<< "$LIST" ; then
    ...
fi

You can use "index" if you only want to find a single character, e.g.:

LIST="server1 server2 server3 server4 server5"
SOURCE="3"
if expr index "$LIST" "$SOURCE"; then
    echo "match"
    exit -1
else
    echo "no match"
fi

Output is:

23
match