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s allows the setting to be changed via setdos w n 30 under dr dos if the setting has been changed from the first directory separator in the display of the prompt parameter g will change to a forward slash which is also a valid directory separator in dos flexos 4680 os 4690 os os 2 and windows thereby serving as a visual clue to indicate the change 23 also the current setting is reflected also in the built in help screens 23 some versions of dr dos command com also support a prompt token to display the current setting command com since dr dos 7 02 also provides a pseudo environment variable named to allow portable batch jobs to be written 31 32 several external dr dos commands additionally support an environment variable switchar to override the system setting however many programs are hardwired to use only rather than retrieving the switch setting before parsing command line arguments a very small number mainly ports from unix like systems are programmed to accept even if the switch character is not set to it for example netstat and ping supplied with microsoft windows will accept the option to list available options and yet the list will specify the convention option conventions in unix like systems edit this section needs more citations please help improve this section by adding citations to reliable sources unsourced material may be challenged and removed july 2021 learn how and when to remove this message in unix like systems the ascii hyphen minus begins options the new and gnu convention is to use two hyphens then a word e g create to identify the option s use while the old convention and still available as an option for frequently used options is to use one hyphen then one letter e g c if one hyphen is followed by two or more letters it may mean two options are being specified or it may mean the second and subsequent letters are a parameter such as filename or date for the first option 33 two hyphen minus characters without following letters may indicate that the remaining arguments should not be treated as options which is useful for example if a file name itself begins with a hyphen or if further arguments are meant for an inner command e g sudo double hyphen minuses are also sometimes used to prefix long options where more descriptive option names are used this is a common feature of gnu software the getopt function and program and the getopts command are usually used for parsing command line options unix command names arguments and options are case sensitive except in a few examples mainly where popular commands from other operating systems have been ported to unix option conventions in other systems edit flexos 4680 os and 4690 os use cp m typically used conversational monitor system cms uses a single left parenthesis to separate options at the end of the command from the other arguments for example in the following command the options indicate that the target file should be replaced if it exists and the date and time of the source file should be retained on the copy copy source file a target file b replace olddate os 360 operator commands use a comma separated list of positional and keyword parameters only documented command and keyword abbreviations are valid keyword values are separated from the keywords with equal signs tso commands use a space separated list of positional and keyword parameters keywords may be truncated to any unique substring only documented command abbreviations are valid keyword values are inside parentheses data general s cli under their rdos aos etc operating systems as well as the version of cli that came with their business basic uses only as the switch character is case insensitive and allows local switches on some arguments to control the way they are interpreted such as mac u lib s a b c lpt l has the global option u to the macro assembler command to append user symbols but two local switches one to specify lib should be skipped on pass 2 and the other to direct listing to the printer lpt built in usage help edit see also help command one of the criticisms of a cli is the lack of cues to the user as to the available actions 34 in contrast guis usually inform the user of available actions with menus icons or other visual cues 35 to overcome this limitation many cli programs display a usage message typically when invoked with no arguments or one of h h h h help help or help 23 36 however entering a program name without parameters in the hope that it will display usage help can be hazardous as programs and scripts for which command line arguments are optional will execute without further notice although desirable at least for the help parameter programs may not support all option lead in characters exemplified above under dos where the default command line option character can be changed from to programs may query the switchar api in order to determine the current setting so if a program is not hardwired to support them all a user may need to know the current setting even to be able to reliably request help if the switchar has been changed to and therefore the character is accepted as an alternative path delimiter at the dos command line programs might misinterpret options like h or h as paths rather than help parameters 23 however if given as the first or only parameter most dos programs will by convention accept it as a request for help regardless of the current switchar setting 23 30 in some cases different levels of help can be selected for a program some programs supporting this allow giving a verbosity level as an optional argument to the help parameter as in h 1 h 2 etc or they give just a short help on help parameters with a question mark and a longer help screen for the other help options 37 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 38 39 36 37 nb 1 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 36 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 the end of the help command output from rt 11sj displayed on a vt100 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 a 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 23 4dos and ndos support the same feature by typing at the prompt 30 which is also accepted by newer versions of dr dos command com internal commands can be individually disabled or reenabled via setdos i 30 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 batch jobs to query the facilities of the underlying command line processor 23 command description syntax edit built in usage help and man pages commonly employ a small syntax to describe the valid command form 40 41 42 nb 2 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 edit 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 can be prevented either by prohibiting embedded spaces in file and directory names in the first place for example by substituting them with underscores _ or by enclosing a name with embedded spaces between quote characters or using an escape character before the space usually a backslash for example long path long program name parameter one parameter two is ambiguous is program name part of the program name or two parameters however long_path long_program_name parameter_one parameter_two longpath longprogramname parameterone parametertwo long path long program name parameter one parameter two and long path long program name parameter one parameter two are not ambiguous unix based operating systems minimize the use of embedded spaces to minimize the need for quotes in microsoft windows one often has to use quotes because embedded spaces such as in directory names are common command line interpreter edit see also list of command line interpreters although most users think of the shell as an interactive command interpreter it is really a programming language in which each statement runs a command because it must satisfy both the interactive and programming aspects of command execution it is a strange language shaped as much by history as by design brian w kernighan rob pike 43 the term command line interpreter is applied to computer programs designed to interpret a sequence of lines of text which may be entered by a user read from a file or another kind of data stream the context of interpretation is usually one of a given operating system or programming language command line interpreters allow users to issue various commands in a very efficient and often terse way this requires the user to know the names of the commands and their parameters and the syntax of the language that is interpreted the unix mechanism and os 2 extproc command facilitate the passing of batch files to external processors one can use these mechanisms to write specific command processors for dedicated uses and process external data files which reside in batch files many graphical interfaces such as the os 2 presentation manager and early versions of microsoft windows use command lines to call helper programs to open documents and programs the commands are stored in the graphical shell clarification needed or in files like the registry or the os 2 os2user ini file early history edit a teletype model 33 asr teleprinter keyboard with punched tape reader and punch dec vt52 terminal the earliest computers did not support interactive input output devices often relying on sense switches and lights to communicate with the computer operator this was adequate for batch systems that ran one program at a time often with the programmer acting as operator this also had the advantage of low overhead since lights and switches could be tested and set with one machine instruction later a single system console was added to allow the operator to communicate with the system from the 1960s onwards user interaction with computers was primarily by means of command line interfaces initially on machines like the teletype model 33 asr but then on early crt based computer terminals such as the vt52 all of these devices were purely text based with no ability to display graphics or pictures nb 3 for business application programs text based menus were used but for more general interaction the command line was the interface around 1964 louis pouzin introduced the concept and the name shell in multics building on earlier simpler facilities in the compatible time sharing system ctss 44 better source needed from the early 1970s the unix operating system adapted the concept of a powerful command line environment and introduced the ability to pipe the output of one command in as input to another unix also had the capability to save and re run strings of commands as shell scripts which acted like custom commands the command line was also the main interface for the early home computers such as the commodore pet apple ii and bbc micro almost always in the form of a basic interpreter when more powerful business oriented microcomputers arrived with cp m and later dos computers such as the ibm pc the command line began to borrow some of the syntax and features of the unix shells such as globbing and piping of output the command line was first seriously challenged by the parc gui approach used in the 1983 apple lisa and the 1984 apple macintosh a few computer users used guis such as geos and windows 3 1 but the majority of ibm pc users did not replace their command com shell with a gui until windows 95 was released in 1995 45 46 modern usage as an operating system shell edit while most non expert computer users now use a gui almost exclusively more advanced users have access to powerful command line environments unix like systems include the bourne shell and compatible shells such as bash and other unix shells freebsd uses tcsh as its default interactive shell for the superuser and ash as default scripting shell many linux distributions have bash apple macos and some linux distributions use zsh previously macos used tcsh and bash embedded linux and other embedded unix like systems often use ash as part of busybox android uses the mksh shell 47 which replaces a shell derived from ash 48 that was used in older android versions supplemented with commands from the separate toolbox 49 binary openvms uses the digital command language dcl unix command shells have been ported to openvms dos and windows have several command line environments command com is the command line interpreter of ms dos ibm pc dos and windows 9x as well as clones such as dr dos sisne plus pts dos rom dos and freedos windows nt has cmd exe and later versions have powershell the windows resource kit and windows services for unix include korn and the bourne shells along with a perl interpreter services for unix contains activestate activeperl in later versions and interix for versions 1 and 2 and a shell compiled by microsoft windows users might use the cscript interface to alternate programs from the command line implementations of the unix shell are also available as part of the posix sub system 50 cygwin mks toolkit uwin hamilton c shell and other software packages available shells for these interoperability tools include csh ksh sh bash rsh tclsh and less commonly zsh psh dcl has been ported to windows systems at least three times including pc dcl and acceler8 dcl lite yet another cmd exe is a stripped down shell for windows ce 3 0 an ms dos type interpreter called pocketdos has been ported to windows ce machines the most recent release is almost identical to ms dos 6 22 and can a...
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