Shell Programming and Scripting

daily record stored per month: 
199901.xyz, 199902.xyz, 199903.xyz, 199904.xyz ...199912.xyz

records inside 199901.xyz (original data has no header)
lat lon wind year month day
12.5 121.0 3.4 1999 1 1
12.4 121.1 2.2 1999 1 1
12.1 122.1 1.2 1999 1 2
12.3 124.3 1.0 1999 1 3 
12.5 122.2 1.8 1999 1 3...
12.1 123.7 2.3 1999 1 31

199901_wk01.txt contains the values of lat,lon,wind for days from 1-7
199901_wk02.txt contains the values of lat,lon,wind for days 8-14
199901_wk03.txt contains the values of lat,lon,wind for days 15-21
199901_wk04.txt contains the values of lat,lon,wind for days 22-28

199902_wk01.txt contains the values of lat,lon,wind for days from 1-7
199902_wk02.txt contains the values of lat,lon,wind for days from 8-14
199902_wk03.txt contains the values of lat,lon,wind for days from 15-21
199902_wk04.txt contains the values of lat,lon,wind for days from 22-28

and so on for the rest of the files.

awk 'BEGIN { MIN=-6 }; FNR==1 { MIN+=7 }; ($6 >= MIN) && ($6 <= (MIN+7))' file1 file2 file3 ...

199901_wk01.txt contains
12.5 121.0 3.4 1999 1 1 
12.4 121.1 2.2 1999 1 1 
12.1 122.1 1.2 1999 1 2 
12.3 124.3 1.0 1999 1 3  
12.5 122.2 1.8 1999 1 3
12.5 122.5 1.6 1999 1 5
12.3 121.4 1.2 1999 1 7

#! /bin/bash

for file in *.txt
do
    f=`basename $file .txt`
    while read x
    do
        day=`echo $x | awk '{print $NF}'`
        if [ $day -ge 1 ] && [ $day -le 7 ]
        then
            echo $x >> ${f}_wk01.txt
        elif [ $day -ge 8 ] && [ $day -le 14 ]
        then
            echo $x >> ${f}_wk02.txt
        elif [ $day -ge 15 ] && [ $day -le 21 ]
        then
            echo $x >> ${f}_wk03.txt
        elif [ $day -ge 22 ] && [ $day -le 28 ]
        then
            echo $x >> ${f}_wk04.txt
        elif [ $day -ge 29 ] && [ $day -le 31 ]
        then
            echo $x >> ${f}_wk05.txt
        fi
    done < $file
done