Shell Programming and Scripting

Hi,
I have to replace the values in a file with the values from my variables. For e.g file contains

Code:

Prev_date=’01-01-1999’
Curr_date= ’02-11-2010’

I need to replace the value of this prev_date and curr_date with the value of the variables I create namely

Code:

Last_day=02-10-2010
First_day=01-10-2010

So after replacement the values should look like this

Code:

Prev_date=’02-10-2010’
Curr_date=’01-10-2010’

How can I do this with sed?

The value of the variables Last_day and First_day are calculated and hence changes everytime.
So during replacement I need to use them rather than the values given above. Can we do this with sed?

Declear the Variables

Code:

$Last_day 
$First_day

for PREV_DATE in filename 
do
{
   sed 's/'"$PREV_DATE"'/'"$LAST_DAY"'/g'  FileName > TEST.TXT
   mv TEST.txt FileName
}
done

for CURR_DATE in filename
do
{
    sed 's/'"$CURR_DATE"'/'"$FIRST_DAY"'/g' FileName > TEST.TXT
    mv TEST.TXT FileName
}
done

Code:

sed 's/\(Prev_date=\).*/\1'$Last_day'/g'

How should i assign the value for CURR_DATE and PREV_DATE in the script initially? I got the values for Last_Day and First_Day
in the script. In the same script I am using this variables to replace PREV_DATE and CURR_DATE in another file. So what should i declare
values for PREV_DATE and CURR_DATE in my script?