Bash what.

--- script ---
#!/bin/bash

echo "a" > a.txt
echo "b" >> a.txt
echo "c" >> a.txt
echo "d" >> a.txt

fun1()
{
    local string="$1"
    local line

    cat a.txt | while read line; do
        if [ "$line" == "$string" ]; then
            echo "$string: found"
            return
        fi
    done

    echo "$string: not found"
}

fun1 123
fun1 b
fun1 456

--- output ---
123: not found
b: found
b: not found
456: not found

--- explanation ---
The pipe ("|") spawns a subshell, a separate bash
process and the "return" inside it exits the
subshell not the function.