Doc/lib/libcmd.tex

   1 \section{\module{cmd} ---
   2          Support for line-oriented command interpreters}
   3
   4 \declaremodule{standard}{cmd}
   5 \sectionauthor{Eric S. Raymond}{esr@snark.thyrsus.com}
   6 \modulesynopsis{Build line-oriented command interpreters.}
   7
   8
   9 The \class{Cmd} class provides a simple framework for writing
  10 line-oriented command interpreters.  These are often useful for
  11 test harnesses, administrative tools, and prototypes that will
  12 later be wrapped in a more sophisticated interface.
  13
  14 \begin{classdesc}{Cmd}{}
  15 A \class{Cmd} instance or subclass instance is a line-oriented
  16 interpreter framework.  There is no good reason to instantiate
  17 \class{Cmd} itself; rather, it's useful as a superclass of an
  18 interpreter class you define yourself in order to inherit
  19 \class{Cmd}'s methods and encapsulate action methods.
  20 \end{classdesc}
  21
  22 \subsection{Cmd Objects}
  23 \label{Cmd-objects}
  24
  25 A \class{Cmd} instance has the following methods:
  26
  27 \begin{methoddesc}{cmdloop}{\optional{intro}}
  28 Repeatedly issue a prompt, accept input, parse an initial prefix off
  29 the received input, and dispatch to action methods, passing them the
  30 remainder of the line as argument.
  31
  32 The optional argument is a banner or intro string to be issued before the
  33 first prompt (this overrides the \member{intro} class member).
  34
  35 If the \module{readline} module is loaded, input will automatically
  36 inherit \program{bash}-like history-list editing (e.g. \kbd{Ctrl-P}
  37 scrolls back to the last command, \kbd{Ctrl-N} forward to the next
  38 one, \kbd{Ctrl-F} moves the cursor to the right non-destructively,
  39 \kbd{Ctrl-B} moves the cursor to the left non-destructively, etc.).
  40
  41 An end-of-file on input is passed back as the string \code{'EOF'}.
  42
  43 An interpreter instance will recognize a command name \samp{foo} if
  44 and only if it has a method \method{do_foo()}.  As a special case,
  45 a line beginning with the character \character{?} is dispatched to
  46 the method \method{do_help()}.  As another special case, a line
  47 beginning with the character \character{!} is dispatched to the
  48 method \method{do_shell} (if such a method is defined).
  49
  50 All subclasses of \class{Cmd} inherit a predefined \method{do_help}.
  51 This method, called with an argument \code{bar}, invokes the
  52 corresponding method \method{help_bar()}.  With no argument,
  53 \method{do_help()} lists all available help topics (that is, all
  54 commands with corresponding \method{help_*()} methods), and also lists
  55 any undocumented commands.
  56 \end{methoddesc}
  57
  58 \begin{methoddesc}{onecmd}{str}
  59 Interpret the argument as though it had been typed in response to the
  60 prompt.  This may be overridden, but should not normally need to be;
  61 see the \method{precmd()} and \method{postcmd()} methods for useful
  62 execution hooks.  The return value is a flag indicating whether
  63 interpretation of commands by the interpreter should stop.
  64 \end{methoddesc}
  65
  66 \begin{methoddesc}{emptyline}{}
  67 Method called when an empty line is entered in response to the prompt.
  68 If this method is not overridden, it repeats the last nonempty command
  69 entered.
  70 \end{methoddesc}
  71
  72 \begin{methoddesc}{default}{line}
  73 Method called on an input line when the command prefix is not
  74 recognized. If this method is not overridden, it prints an
  75 error message and returns.
  76 \end{methoddesc}
  77
  78 \begin{methoddesc}{precmd}{}
  79 Hook method executed just before the command line \var{line} is
  80 interpreted, but after the input prompt is generated and issued.  This
  81 method is a stub in \class{Cmd}; it exists to be overridden by
  82 subclasses.  The return value is used as the command which will be
  83 executed by the \method{onecmd()} method; the \method{precmd()}
  84 implementation may re-write the command or simply return \var{line}
  85 unchanged.
  86 \end{methoddesc}
  87
  88 \begin{methoddesc}{postcmd}{stop, line}
  89 Hook method executed just after a command dispatch is finished.  This
  90 method is a stub in \class{Cmd}; it exists to be overridden by
  91 subclasses.  \var{line} is the command line which was executed, and
  92 \var{stop} is a flag which indicates whether execution will be
  93 terminated after the call to \method{postcmd()}; this will be the
  94 return value of the \method{onecmd()} method.  The return value of
  95 this method will be used as the new value for the internal flag which
  96 corresponds to \var{stop}; returning false will cause interpretation
  97 to continue.
  98 \end{methoddesc}
  99
 100 \begin{methoddesc}{preloop}{}
 101 Hook method executed once when \method{cmdloop()} is called.  This
 102 method is a stub in \class{Cmd}; it exists to be overridden by
 103 subclasses.
 104 \end{methoddesc}
 105
 106 \begin{methoddesc}{postloop}{}
 107 Hook method executed once when \method{cmdloop()} is about to return.
 108 This method is a stub in \class{Cmd}; it exists to be overridden by
 109 subclasses.
 110 \end{methoddesc}
 111
 112 Instances of \class{Cmd} subclasses have some public instance variables:
 113
 114 \begin{memberdesc}{prompt}
 115 The prompt issued to solicit input.
 116 \end{memberdesc}
 117
 118 \begin{memberdesc}{identchars}
 119 The string of characters accepted for the command prefix.
 120 \end{memberdesc}
 121
 122 \begin{memberdesc}{lastcmd}
 123 The last nonempty command prefix seen.
 124 \end{memberdesc}
 125
 126 \begin{memberdesc}{intro}
 127 A string to issue as an intro or banner.  May be overridden by giving
 128 the \method{cmdloop()} method an argument.
 129 \end{memberdesc}
 130
 131 \begin{memberdesc}{doc_header}
 132 The header to issue if the help output has a section for documented
 133 commands.
 134 \end{memberdesc}
 135
 136 \begin{memberdesc}{misc_header}
 137 The header to issue if the help output has a section for miscellaneous
 138 help topics (that is, there are \method{help_*()} methods without
 139 corresponding \method{do_*()} methods).
 140 \end{memberdesc}
 141
 142 \begin{memberdesc}{undoc_header}
 143 The header to issue if the help output has a section for undocumented
 144 commands (that is, there are \method{do_*()} methods without
 145 corresponding \method{help_*()} methods).
 146 \end{memberdesc}
 147
 148 \begin{memberdesc}{ruler}
 149 The character used to draw separator lines under the help-message
 150 headers.  If empty, no ruler line is drawn.  It defaults to
 151 \character{=}.
 152 \end{memberdesc}
 153
 154 \begin{memberdesc}{use_rawinput}
 155 A flag, defaulting to true.  If true, \method{cmdloop()} uses
 156 \function{raw_input()} to display a prompt and read the next command;
 157 if false, \function{sys.stdout.write()} and
 158 \function{sys.stdin.readline()} are used.
 159 \end{memberdesc}