When you enter a command line, the operating system examines everything that you typed up to the point at which you pressed Return. If you have not mistyped the name of the command, or any of its parameters, the operating system searches its memory concerning the command that you entered.
The operating system executes a series of tests to determine how to execute the command:
If the command is PURGE, look up the parameters of PURGE, and
If a file is named, look for the file, and
If the file exists, determine whether ,TEMP is specified, and
If ,TEMP is specified, then erase a temporary file; otherwise,
erase a permanent file
This description is an over-simplification, but it illustrates the kind of "reasoning" that the computer follows when it tries to execute a command that you enter.
For every parameter of every command, there is an otherwise that tells the computer what to do if the following occurs:
The user decides to ignore an optional parameter.
The user fails to enter a required parameter.
The user simply mistypes the command or one of its parameters.
In this case, the otherwise of ,TEMP is: "erase a permanent file, if it can be found."
The otherwise (default) for a required parameter that you fail to specify, that you mistype, or that the computer cannot, for some reason, accept is to display an error message.
:PURGE
^
REQUIRES FILE NAME TO BE PURGED. (CIERR 381)
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CIERR 381 identifies the error that the system encountered. It was a CI (command interpreter) error, an error made when the command interpreter attempted to carry out this instruction. It was error number 381 in the catalog of errors that the system recognizes and reports.
