environments, an example of a command-line argument is: rm file.s "file.s" is a command-line argument which tells the program rm to remove the file "file.s". Some programming languages, such as C, C++ and Java, allow a program to interpret the command-line arguments by handling them as string parameters in the main function. Other languages, such as Python, expose operating system specific API (functionality) through sys module, and in particular sys.argv for "command-line arguments". In Unix-like operating systems, a single hyphen used in place of a file name is a special value specifying that a program should handle data coming from the standard input or send data to the standard output. Command-line option A command-line option or simply option (also known as a flag or switch) modifies the operation of a command; the effect is determined by the command's program. Options follow the command name on the command line, separated by spaces. A space before the first option is not always required, such as Dir/? and DIR /? in DOS, which have the same effect of listing the DIR command's available options, whereas dir --help (in many versions of Unix) does require the option to be preceded by at least one space (and is case-sensitive). The format of options varies widely between operating systems. In most cases the syntax is by convention rather than an operating system requirement; the entire command line is simply a string passed to a program, which can process it in any way the programmer wants, so long as the interpreter can tell where the command name ends and its arguments and options begin. A few representative samples of command-line options, all relating to listing files in a directory, to illustrate some conventions: Abbreviating commands In Multics, command-line options and subsystem keywords may be abbreviated. This idea appears to derive from the PL/I programming language, with its shortened keywords (e.g., STRG for STRINGRANGE and DCL for DECLARE). For example, in the Multics "forum" subsystem, the -long_subject parameter can be abbreviated -lgsj. It is also common for Multics commands to be abbreviated, typically corresponding to the initial letters of the words that are strung together with underscores to form command names, such as the use of did for delete_iacl_dir. In some other systems abbreviations are automatic, such as permitting enough of the first characters of a command name to uniquely identify it (such as SU as an abbreviation