HowTo batch replace string in the set of files

This script replaces all occurrences of 'oldString' with 'newString' in all files in the current directory and all subdirectories.

xqx_replace.sh

#!/bin/bash

if test -z "$1" -o -z "$2"; then
    echo Replace all occurrences of 'oldString' with 'newString' in all files in the current directory and all subdirectories.
    echo Usage: $0 oldSting newString
    exit
fi

OLDSTRING=$1
NEWSTRING=$2

replace() {
    echo "$1"
    mv "$1" "$1".bak
    sed s/$OLDSTRING/$NEWSTRING/g "$1".bak >"$1"
}

check() {
    while read; do
        if test -n "`grep -l $OLDSTRING \"$REPLY\"`"; then    
            replace "$REPLY"
        fi
    done
}

find . -type f |check