get number of latin letter in alphabet

#!/bin/bash

#get number of latin letter in alphabet
#use nlett <letter>, e.g nlett a

SNUM=1
SFND=`echo $1|grep '^[a-z]$'`
if [ -z "$SFND" ]; then
    echo "Error: in first parameter must be one small latin letter!"
    exit 1
fi

for SYMB in {a..z}; do
    if [[ "$SYMB" == "$SFND" ]]; then
    break
    else
    let "SNUM=SNUM+1"
    fi
done

echo "$SFND is $SNUM"