Instalando o grunhido globalmente :
$npm install grunt-cli -g
tentando verificar um:
$grunt -v
grunt-cli: The grunt command line interface (v1.2.0)
Fatal error: Unable to find local grunt.
If you're seeing this message, grunt hasn't been installed locally to
your project. For more information about installing and configuring grunt,
please see the Getting Started guide:
http://gruntjs.com/getting-started
Mais detalhes:
$ which grunt
/usr/bin/grunt