Jenkins delete olds dir after deploy

cd /mydir
unlink current
ln -s releases/${BUILD_NUMBER} current

cd /var/www/html
unlink current
ln -s releases/${BUILD_NUMBER} current

num=${BUILD_NUMBER}
x=4
(( num -= x ))
echo $num

for dir in /mydir
do
    dir=${dir%*/}
    if [ "${dir##*/}" -lt "$num" ]; then
        echo ${dir##*/}
        rm -r $dir
    fi
done