Question: Is there any way for me to symlink a particular file so that it appears in every directory on the system?

So if I symlink `/tmp/scratch`

No matter where I am I can type `vi scratch`?

Answer: 
A more robust solution would be to use an alias in your `.bashrc`:

```
alias viscrtch='vi /tmp/scratch'

```

This way `viscrtch` can be used to edit `/tmp/scratch` from anywhere within the filesystem.

If you want the ability to add command line arguments, use a shell function instead:

```
viscrtch(){ vi "$@" /tmp/scratch; }

```

Call it as 

```
viscrtch your_options_here

```

Even if you find a way to symlink one file to every directory in the system, I discourage you from doing it: it's neither maintainable nor practical.
As others noted, it is probably not the best solution, but here is a way to link in all directories beneath the current.

```
#!/bin/bash

lnfile="/tmp/scratch"
for Dir in $(find ./* -type d);
do
    ln -s $lnfile $Dir
done

```