Strange behaviour of -d conditional expression - shell

Question

My KornShell (ksh) manual says that -d expression returns true if the file exists and it is a directory.
So if [[ -d file ]] should return TRUE if file is a directory. But in my case this is not how it works. It returns TRUE if the file exists and is NOT a directory, but the manual of the shells says "and it is a directory". So does anyone have any idea why it is working in the opposite of what it should be?

Answer 1

It's working fine; it's your expectations that are wrong. In shells, a 0 return value is true, and a non-zero return value is false.
$ true ; echo $?
0
$ false ; echo $?
1

Answer 2

ksh File Operator | True if:
-a | file exists
-d | file is a directory
-f | file is a regular file (i.e., not a directory or other special type of file)
-r | You have read permission on file
-s | file exists and is not empty
-w | You have write permission on file
-x | You have execute permission on file, or directory search permission if it is a directory
-O | file You own file
-G | file Your group ID is the same as that of file
kshFileOperatorsFunction.ksh
#***Function to demo ksh file Operators.***#
fileOperators(){
echo "Entering fileOperators function."
if [[ ! -a $1 ]]; then
print "file $1 does not exist."
return 1
fi
if [[ -d $1 ]]; then
print -n "$1 is a directory that you may "
if [[ ! -x $1 ]]; then
print -n "not "
fi
print "search."
elif [[ -f $1 ]]; then
print "$1 is a regular file."
else
print "$1 is a special type of file."
fi
if [[ -O $1 ]]; then
print 'you own the file.'
else
print 'you do not own the file.'
fi
if [[ -r $1 ]]; then
print 'you have read permission on the file.'
fi
if [[ -w $1 ]]; then
print 'you have write permission on the file.'
fi
if [[ -x $1 && ! -d $1 ]]; then
print 'you have execute permission on the file.'
fi
echo "Exiting fileOperators function."
}
Reference : O'Reilly, Learning the KornShell Volume 1