Add a new built-in path command in self-made Shell

 Add a new built-in path command that allows users to show the current pathname list, append one pathname, or remove one pathname. In your shell implementation, you may keep a data structure to deal with the pathname list. If you do not use execle() or execve() that allows you to execute with your own environment variables, you will need to add it to the “real” PATH environment variable for executables in the path to work correctly. The initial value of path within your shell shall be the pathname list contained in the PATH environment variable. Implement the path command as follows: • path (without arguments) displays the pathnames currently set. It should show pathnames separated by colons. For example, "/bin:/user/bin". • path + ./bin appends the pathname to the path variable. You may assume that only one pathname is added at a time. • path - ./bin removes the pathname to the path variable. You may assume that only one pathname is removed at a time