Which command in the Linux/UNIX sh shell returns my current directory?

Using the sh shell (not bash), which command in Linux/UNIX prints out my current directory?

$ *showmewhereiam*
/sys/kernel/debug
$

Try pwd.

$ pwd
/home/<username>

While the general answer is pwd, note that this may give different results depending on how you reached a given directory, and whether the route included symbolic links.

For instance, if you have a directory called real and a symbolic link to that directory called virtual, and you cd to the virtual directory, then pwd will show that virtual directory name, even though the actual directory you are in is real.

To show the real underlying directory, use either pwd -P or readlink -f (for a arbitrary path):

$ mkdir real
$ ln -s real virtual
$ cd virtual
$ pwd
/home/username/tmp/virtual
$ pwd -P
/home/username/tmp/real
$ readlink -f .
/home/username/tmp/real

Note that shells often replace the pwd command with their own internal version, so on my system (RHEL6), even though the pwd(1) manual page suggests that --physical will work as well as -P, because I'm running bash, it doesn't:

$ pwd --physical
bash: pwd: --: invalid option
pwd: usage: pwd [-LP]
$ /bin/pwd --physical
/home/username/tmp/real
$ /usr/bin/env pwd --physical
/home/username/tmp/real

As others said, pwd usually does the job well enough. However, I'd like to add an idea which has helped me out.

On all shells in common use today, you're able to customize the appearance of the command prompt. I like to customize mine so that it shows me both the name of the computer I'm on and my working directory. That way, I always know where I am. (The computer name part helps me realize if the terminal window I'm using has been used to SSH into a remote server.) For example, when I open a new terminal window on my laptop, which I call Plastico, I see this:

Plastico ~> cd Desktop/
Plastico ~/Desktop> cd ~/Sites/raygunrobot.com
Plastico ~/Sites/raygunrobot.com> cd /usr
Plastico /usr> 

You can see that it's abbreviating my home directory as ~. I find doing this for my terminal prompts is incredibly handy - I always know at a glance exactly where I am, even more so than I would by looking at a Finder window.

On tcsh, I do this by adding set prompt = 'Plastico %~%# ' to my .tcshrc file in my home directory. I've done it on ksh too by adding export PS1="Plastico $PWD $ " to my .profile file. But being a Linux user, you probably use Bash, which I don't have much experience with (I'm a weirdo like that). But it looks like I might be able to do it by adding PS1="Plastico \w $ " to my .bashrc file.

Edit: Just noticed the OP states you're interested in sh, not bash. Oh well; hope folks find this useful regardless.