Bash - Find substring in a string

Very simple solution to find substring in a string, make script sub.sh:

#!/bin/bash
txt="This is a text"
substr="text"
if [[ $txt == *$substr* ]]
then
echo "String [$txt] contains substring [$substr]"
else
echo "String [$txt] doesn't contains substring [$substr]"
fi

Variable txt is a string which should (or not) contains string from variable substr.

Output:

$ ./sub.sh
String [This is a text] contains substring [text]

Change variable substr:

substr="t1xt"

Output:

$ ./sub.sh
String [This is a text] doesn't contains substring [t1xt]