how does cd command work? (unix)

Question

could anybody explain to me how cd - command work? man cd tells me that An argument of - is equivalent to $OLDPWD. Then I found on the net that $OLDPWD is the previous working directory as set by the cd command. and when I do cd - on my unix I get -bash: cd: OLDPWD not set which is confusing for me.

UPDATE: it works now it gives me the previous working directory. I guess I did not use and cd command before so that is why $OLDPWD was not set.

One more question what cd (type cd and press enter) does? I wouldn't say that nothing. Is there any better technical explanation?

host [~]# cd
host [~]#

Answer 1

OLDPWD is not set, because you have't changed directory

[dave@odessa ~]$ cd -
-bash: cd: OLDPWD not set
[dave@odessa ~]$ cd /tmp
[dave@odessa tmp]$ cd -
/export/home/dave

[dave@odessa ~]$ cd /tmp
[dave@odessa tmp]$ echo $OLDPWD
/export/home/dave

cd without any arguments will chdir to $HOME

[dave@odessa tmp]$ echo $HOME
/export/home/dave
[dave@odessa tmp]$ HOME=/ cd
[dave@odessa /]$ pwd
/

Answer 2

type cd

tells us that

cd is a shell builtin

man sh

tells us what you found out:

If a single dash is specified as the argument, it will be replaced by the value of OLDPWD.

The internal implementation of cd in the shell does a chdir(2) -syscall.

Answer 3

cd by itself changes to your home directory

$ cd /tmp
$ pwd
/tmp
$ cd
$ pwd
/home/username