Suppose the ordinary user 'user1' has a file 'ABC' in the current directory and he wants to execute this file. But when type 'ABC' at the '$' prompt. The system reports an error. So the possible reasons for such error are: (1) the environment parameter PATH of user1 does not contain the current directory, so the system cannot find this file. (2) user1 does not have executable permissions of 'ABC'. (3) the current directory does not have executable permissions.