How can I calculate the date preceding a given date in unix?

Question

I have two variables: X and Y.

The value of X will be a date given in the format mmddyy and I want to calculate the date preceding that date

Answer 1

~$ date -d "20090101 -1 day"
Wed Dec 31 00:00:00 CET 2008

And if you want to retrieve the date in a custom format you just throw in some format strings.

~$ date -d "2009-09-15 -1 day" +%Y%m%d
20090914

As for your conversion you may use bash substring extraction (assuming you use bash of course). This also assumes that your input is consistent and "safe".

X="091509"
Y=`date -d "${X:4:2}${X:0:2}${X:2:2} -1 day" +%Y%m%d`
echo $Y

See http://www.walkernews.net/2007/06/03/date-arithmetic-in-linux-shell-scripts/