UNIX for Dummies Questions & Answers

#!/bin/bash

STR="kitten"
GUESS="${STR//[a-z]/_}" # Replace all [a-z] with _

while [ "$GUESS" != "$STR" ]
do
        IN=""
        while [ "${#IN}" -ne 1 ] || [ ! -z "${IN/[a-z]}" ]
        do
                printf "${GUESS} Enter a character, then ENTER: "
                read IN
        done

        # Loop through every character in $STR.
        for((N=0; N<${#STR}; N++))
        do
                if [ "${STR:$N:1}" = "$IN" ]
                then
                        if [ "${GUESS:$N:1}" = "$IN" ]
                        then
                                echo "You already guessed that!"
                                break;
                        fi
                        # i.e. replacing the first 't' in kitten.
                        # ${GUESS:0:$N} gets 'ki'.
                        # ${GUESS:$((N+1))} gets 'ten'.
                        # We stick our guess, $IN, in the middle.
                        GUESS="${GUESS:0:$N}${IN}${GUESS:$((N+1))}"
                fi
        done
done

echo "You got it!  $GUESS"