To extract records from file based on the latest date and store in a new file - bash

Question

I have a file with different balances for a particular account. And every day these balance changes. What I want to do is extract balance record of a particular account based on the latest date.
I am following an approach where I am adding the date into the first column of records manually using an awk script but taking date from the file name since I do not get date in my records. Next I want to sort the records based on the Account number and want to extract the record with latest date in another file.
Can anybody help me with this?
Till now I have written this code and I am unable to sort and extract the data into the other file
#!/usr/bin/ksh
f=mainfile_20151201.dat
s=`echo $f | cut -c 16-23`
echo "$f -> $s"
awk -F "~" 'BEGIN { OFS = "~"; ORS = "\n" ; date='$s' ; IFS = "~"} { $1=date"~"$1 ; print }' mainfile_20151201.dat > tempdate
awk -F "~" 'BEGIN { OFS = "~"; ORS = "\n" ; IFS = "~"} { $1 ; print }' tempdate > newfile
Sample data:
AccountNumber~~0~149038.40000000~149038.4~0.00000000~0.00000000~0.00000000
Please note that the data in the 4th field changes everyday

Answer 1

If your main file is always going to be in the format "something_date" this should do the job ok.
#!/bin/bash
f=mainfile_20151201.dat
s=`echo $f | cut -d"_" -f2`
echo "$f -> $s"
awk -F "~" 'BEGIN { OFS = "~"; ORS = "\n" ; date='$s' ; IFS = "~"} { $1=date"~"$1 ; print }' mainfile_20151201.dat > tempdate
awk -F "~" 'BEGIN { OFS = "~"; ORS = "\n" ; IFS = "~"} { $1 ; print }' tempdate > newfile
rm tempdate
sort -u -t~ -k 2 <newfile >newfile.s
#sort by unique arange by field 2 then field 1 (default action)
d=$(cat newfile.s | head -1 | cut -d"~" -f1) #get first date
a=$(cat newfile.s | head -1 | cut -d"~" -f2) #get first account number
while read line; do
d2=`echo "$line" | cut -d"~" -f1` #get date from line
a2=`echo "$line" | cut -d"~" -f2` #get account from line
if [[ $a2 == $a ]] && [[ $d2 > $d ]] || [[ $d2 == $d ]];then #if acount are same but date is 'bigger' OR the same
sed -i '$ d' output.txt #remove last line of file
echo "$line" >> output.txt #append to file
a="$a2" #set new account for later
d="$d2" #set new date for later
else
a="$a2"
d="$d2"
echo "$line" >>output.txt
fi
done <newfile.s #while input declaration
Note: This worked with the sample from your comment but will definitely need to be tweeked for your needs. At any rate, it should be enough to get you going. Hope this helps!