fehler im skript

line 31: arithmetic syntax error

# bootup system 60 sec. before timer

safe_margin=60

# modyfy if different location for tvheadend dvr/log path

cd ~hts/.hts/tvheadend/dvr/log

######################

start_date=0

stop_date=0

current_date=`date +%s`

for i in $( ls ); do

            tmp_start=`cat $i | grep '"start":' | cut -f 2 -d " " | cut -f 1 -d ","`

            tmp_stop=`cat $i | grep '"stop":' | cut -f 2 -d " " | cut -f 1 -d ","`

            # check for outdated timer

            if [ $((tmp_stop)) -gt $((current_date)) -a $((tmp_start)) -gt $((current_date)) ]; then

                        # take lower value (tmp_start or start_date)

                        if [ $((start_date)) -eq 0 -o $((tmp_start)) -lt $((start_date)) ]; then

                        start_date=$tmp_start

                        stop_date=$tmp_stop

                        fi

            fi

done

wake_date=$((start_date-safe_margin))

echo $wake_date