<p>In VIM in command line mode a "%" denotes the current file, "cword" denotes the current word under the cursor. I want to create a shortcut where I need the current line number. What is the symbol which denotes this?</p>

. (dot) stands for the current line.
To clarify:
This is meant for stuff like
```
:1,.s/foo/bar/g
```
which will transform every
```
foo
```
to
```
bar
```
from the beginning of the file up to the current line.
I don't know know of a way to get the current line number expanded for a shell command, which is what you are trying to do by doing
```
:!echo .
```
You can find out about the expansions that are done (like
```
%
```
and
```
#
```
for example) in
```
:he cmdline-special
```
.