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

7.7.4

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

0.7.7

Operating system:

Ubuntu 16.04

Description of the bug:

Trying to execute the source command using shell.exec() but it isn't working. I get source not found error.

/bin/sh : 1: source: not found

Example ShellJS command to reproduce the error:

var shell = require('shelljs');
shell.exec('source somefile.sh');