Writing this down, because I'm always getting it wrong.
ln -s /path/to/file symblinkToCreate
Say you're in the
Ghost's themes folder,
Now, you want to add a symlink to a theme that you're working on.
Here's how you do it:
ln -s /path/to/my/theme mytheme
/ghost/content/themes/mytheme -> /path/to/my/theme