With 3.3, we already support "prepared" command lines.

$process = new Process('echo $abc && echo $def');
$process->inheritEnvironmentVariables(); // unrelated, just for FC

$process->run(null, array('abc' => 'ABC', 'def' => 'DEF'); // echoes ABCDEF

It works quite simply: it just creates the corresponding env vars and leverages the underlying shell to parse the command line and use the values. The $abc syntax comes from sh of course, nothing we have to deal with fortunately.

BUT this comes with a drawback, namely: portability. Why? Because we rely on the shell to parse the command line :)
On Windows, the same command has to be written as echo !abc! && echo !def!, using the !foo! syntax from Windows' shell (cmd.com) to use env vars.

While creating a portable command line language is out of scope, we could at least provide some portable syntax for env vars injection.

One proposal as already been made, using curly brackets: echo {abc} && echo {def}.
WDYT?