title: $(shell) references in bash
body: <p>Given the sample code form the <a href="https://github.com/google/simian" rel="nofollow">Simian project</a> Makefile:</p>
  
  <pre><code>OSX_VERSION=$(shell sw_vers -productVersion 2&gt;/dev/null | cut -d. -f1-2)
  SWIG=$(shell type -p swig 2&gt;/dev/null)
  SIMIAN_VERSION=2.4
  SIMIAN=simian-${SIMIAN_VERSION}
  SDIST_TAR=dist/simian-${SIMIAN_VERSION}.tar
  SDIST=${SDIST_TAR}.gz
  MUNKI_VERSION=2.3.0.2519
  MUNKI=munkitools-${MUNKI_VERSION}
  MUNKIFILE=${MUNKI}.pkg
  PYTHON_VERSION=2.6
  PYTHON=$(shell type -p python${PYTHON_VERSION})
  TS=$(shell date '+%s')
  </code></pre>
  
  <p>I've tried to follow the makefile steps manually, but 'shell' is not referenced in the <a href="https://github.com/google/simian/blob/master/Makefile" rel="nofollow">Makefile</a>, or in my path.  'shell' is also absent from inside the VE created by the project.</p>
  
  <p>Where is $(shell) referenced from?</p>