$shell = $ENV{'SHELL'} || 'csh';
This line is used to define which Unix shell to use to run any Unix
commands. Either we are going to pull the shell from the environment,
or, if that has no value, we are going to use a default. This line is
positioned near the top of the script because we may wish to reconfigure
the default shell someday.
Recall:
- The environment is stored in a special associative array,
%ENV. $ENV{'SHELL'} is going to look
up the value associated with the SHELL variable in
the environment.
|| means "or".
'csh' is the default designated in the goals.