In Unix script, how to get a "date - 1" ie, yesterday?

#!/bin/bash

# figure out what yesterday was.
date '+%Y %m %d' |
{
read year month day

day=`expr "$day" - 1`
case "$day" in
0)
month=`expr "$month" - 1`
case "$month" in
0)
month=12
year=`expr "$year" - 1`
;;
esac

day=`cal $month $year | grep . | fmt -1 | tail -1`
esac

echo "Yeseterday was: $day $month $year"
}

or if you are using GNU date, simply do:

date --date=yesterday

It works. Thank you.

in Perl there is really handy way to manipulate Date

print scalar localtime (time() - 86400 * n);
it will substract n from current date and print the date

print scalar localtime (time() - 86400 * 1); #yesterday
print scalar localtime (time() - 86400 * 30); #Date of 30 days back

To get date four days from now
print scalar localtime (time() + 86400 * 4);

and so on....