other help options. Depending on the program, additional or more specific help on accepted parameters is sometimes available by either providing the parameter in question as an argument to the help parameter or vice versa (as in /H:W or in /W:? (assuming /W would be another parameter supported by the program)). In a similar fashion to the help parameter, but much less common, some programs provide additional information about themselves (like mode, status, version, author, license or contact information) when invoked with an "about" parameter like -!, /!, -about, or --about. Since the ? and ! characters typically also serve other purposes at the command line, they may not be available in all scenarios, therefore, they should not be the only options to access the corresponding help information. If more detailed help is necessary than provided by a program's built-in internal help, many systems support a dedicated external "help command" command (or similar), which accepts a command name as calling parameter and will invoke an external help system. In the DR-DOS family, typing /? or /H at the COMMAND.COM prompt instead of a command itself will display a dynamically generated list of available internal commands; 4DOS and NDOS support the same feature by typing ? at the prompt (which is also accepted by newer versions of DR-DOS COMMAND.COM); internal commands can be individually disabled or reenabled via SETDOS /I. In addition to this, some newer versions of DR-DOS COMMAND.COM also accept a ?% command to display a list of available built-in pseudo-environment variables. Besides their purpose as quick help reference this can be used in batchjobs to query the facilities of the underlying command-line processor. Command description syntax Built-in usage help and man pages commonly employ a small syntax to describe the valid command form: angle brackets for required parameters: ping <hostname> square brackets for optional parameters: mkdir [-p] <dirname> ellipses for repeated items: cp <source1> [source2…] <dest> vertical bars for choice of items: netstat {-t|-u} Notice that these characters have different meanings than when used directly in the shell. Angle brackets may be omitted when confusing the parameter name with a literal string is not likely. The space character In many areas of computing, but particularly in the command line, the space character can cause problems as it has two distinct and incompatible functions: as part of a command or parameter, or as a parameter or name separator. Ambiguity