This is the Lab1c from Yanxi Wang (604249838) and Pengyuan Du (204100962)

 Our implementation parse the basic operators of shell script, including "||", "&&", "|", ";", "<>" and "()".
 We also output syntax error for invalid script.
 This program can successfully go through the "test-p-ok.sh" and "test-p-bad.sh".
 We declare structure "Node" in "command.h" to implement the stack.