Node version (or tell us if you're using electron or some other framework):

8.11.1

npm version (or tell us if you're using electron or some other framework):

5.6.0

ShellJS version (the most recent version/Github branch you see the bug on):

0.7.7 and 0.8.1

Operating system:

ubuntu 14.04

Description of the bug:

On ubuntu the symlink command ln -sf seems to not force the link if the destination exists. Seems to work fine on macOS.

Example ShellJS command to reproduce the error:

On linux:

var shell = require('shelljs');
shell.ln('-sf', "source", "link");