Escaped References to a Shell Variable

The string \$\$ in the rule for test illustrates how to escape the dollar-sign from interpretation by make. make passes each $ to the shell, which expands the $$ to its process ID. This technique allows each test to write to a unique temporary filename. The set -x command forces the shell to display the commands it runs on the terminal, which allows you to see the actual file name containing the results of the specific test.