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er 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 also run windows 1 2 and 3 0 qbasic and other development tools 4nt and 4dos the latest release includes several shells namely ms dos 6 22 pc dos 7 dr dos 3 xx and others ibm os 2 and derivatives such as ecomstation and arcaos has the cmd exe processor this copies the command com commands with extensions to rexx implementations of php have a shell for interactive use called php cli standard tcl tk has two interactive shells tclsh and wish the latter being the gui version python ruby lua xlnt and other interpreters also have command shells for interactive use harmonyos openharmony and oniro use the commands from third party toolbox compatibility system attached to linux kernel of the kal subsystem and posix musl libc library with linux abi of hongmeng kernel alongside default shell with its own native exec commands 51 52 routers with cisco ios 53 junos 54 and many others are commonly configured from the command line the plan 9 operating system uses the rc shell which is similar in design to the bourne shell scripting edit most command line interpreters support scripting to various extents they are after all interpreters of an interpreted programming language albeit in many cases the language is unique to the particular command line interpreter they will interpret scripts variously termed shell scripts or batch files written in the language that they interpret some command line interpreters also incorporate the interpreter engines of other languages such as rexx in addition to their own allowing the execution of scripts in those languages directly within the command line interpreter itself conversely scripting programming languages in particular those with an eval function such as rexx perl python ruby or jython can be used to implement command line interpreters and filters for a few operating systems most notably dos such a command interpreter provides a more flexible command line interface than the one supplied in other cases such a command interpreter can present a highly customised user interface employing the user interface and input output facilities of the language other command line interfaces edit the command line provides an interface between programs as well as the user in this sense a command line is an alternative to a dialog box editors and databases present a command line in which alternate command processors might run on the other hand one might have options on the command line which opens a dialog box the latest version of take command has this feature dbase used a dialog box to construct command lines which could be further edited before use programs like basic diskpart edlin and qbasic all provide command line interfaces some of which use the system shell basic is modeled on the default interface for 8 bit intel computers calculators can be run as command line or dialog interfaces emacs provides a command line interface in the form of its minibuffer commands and arguments can be entered using emacs standard text editing support and output is displayed in another buffer there are a number of text mode games like adventure or king s quest 1 3 which relied on the user typing commands at the bottom of the screen one controls the character by typing commands like get ring or look the program returns text that describes how the character sees it or makes the action happen the text adventure the hitchhiker s guide to the galaxy a piece of interactive fiction based on douglas adams s book of the same name is a teletype style command line game the most notable of these interfaces is the standard streams interface which allows the output of one command to be passed to the input of another text f...
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