why we use -d in script?

-d can be used for so many things. Unless you show us the code snippet, it is hard to give a definitive answer.

Guessing... look into man test to see about -d.

Where is this -d being used? Is it for a particular command? If so, then check the man page for that command.

Actually my friend using the -d in this code.

if [ -d $LOG_DIR ]
then
cd $LOG_DIR
echo $LOG_DIR >> $LOG_FILE
find . \( -name '*.log*' -type f \) -mtime +30 -exec rm -f {} \;
fi

As I said ealier, look into man test to see the description about -d

i got the solution.thanks

will u tell me what is this meaning?

find . \( -name '*.log*' -type f \) -mtime +30 -exec rm -f {} \;

What it will search the file which is less than 30 minutes?