bash : "while :"

bonjour,

while :
do
        _end=`wc -l < "${_file}"`
        _end="${_end##* }"
        if (( ${_end} > ${_start} ))
        then
                _start=$(($_start + 1 ))
                sed -n "${_start},${_end}p" "${_file}" | grep "${_pat}" >> 
$LOG
                _start=${_end}
        fi
        sleep 1
done

Je n'arrive pas à comprendre ce que cela fait ce "while :"
Le ":" correspond à quoi?

merci
anne