path: root/Doc/lib/libcmd.tex

\section{\module{cmd} ---
         Support for line-oriented command interpreters}

\declaremodule{standard}{cmd}
\sectionauthor{Eric S. Raymond}{esr@snark.thyrsus.com}
\modulesynopsis{Build line-oriented command interpreters.}


The \class{Cmd} class provides a simple framework for writing
line-oriented command interpreters.  These are often useful for
test harnesses, administrative tools, and prototypes that will
later be wrapped in a more sophisticated interface.

\begin{classdesc}{Cmd}{\optional{completekey\optional{,
                       stdin\optional{, stdout}}}}
A \class{Cmd} instance or subclass instance is a line-oriented
interpreter framework.  There is no good reason to instantiate
\class{Cmd} itself; rather, it's useful as a superclass of an
interpreter class you define yourself in order to inherit
\class{Cmd}'s methods and encapsulate action methods.

The optional argument \var{completekey} is the \refmodule{readline} name
of a completion key; it defaults to \kbd{Tab}. If \var{completekey} is
not \constant{None} and \refmodule{readline} is available, command completion
is done automatically.

The optional arguments \var{stdin} and \var{stdout} specify the 
input and output file objects that the Cmd instance or subclass 
instance will use for input and output. If not specified, they
will default to \var{sys.stdin} and \var{sys.stdout}.

\versionchanged[The \var{stdin} and \var{stdout} parameters were added]{2.3}
\end{classdesc}

\subsection{Cmd Objects}
\label{Cmd-objects}

A \class{Cmd} instance has the following methods:

\begin{methoddesc}[Cmd]{cmdloop}{\optional{intro}}
Repeatedly issue a prompt, accept input, parse an initial prefix off
the received input, and dispatch to action methods, passing them the
remainder of the line as argument.

The optional argument is a banner or intro string to be issued before the
first prompt (this overrides the \member{intro} class member).

If the \refmodule{readline} module is loaded, input will automatically
inherit \program{bash}-like history-list editing (e.g. \kbd{Control-P}
scrolls back to the last command, \kbd{Control-N} forward to the next
one, \kbd{Control-F} moves the cursor to the right non-destructively,
\kbd{Control-B} moves the cursor to the left non-destructively, etc.).

An end-of-file on input is passed back as the string \code{'EOF'}.

An interpreter instance will recognize a command name \samp{foo} if
and only if it has a method \method{do_foo()}.  As a special case,
a line beginning with the character \character{?} is dispatched to
the method \method{do_help()}.  As another special case, a line
beginning with the character \character{!} is dispatched to the
method \method{do_shell()} (if such a method is defined).

This method will return when the \method{postcmd()} method returns a
true value.  The \var{stop} argument to \method{postcmd()} is the
return value from the command's corresponding \method{do_*()} method.

If completion is enabled, completing commands will be done
automatically, and completing of commands args is done by calling
\method{complete_foo()} with arguments \var{text}, \var{line},
\var{begidx}, and \var{endidx}.  \var{text} is the string prefix we
are attempting to match: all returned matches must begin with it.
\var{line} is the current input line with leading whitespace removed,
\var{begidx} and \var{endidx} are the beginning and ending indexes
of the prefix text, which could be used to provide different
completion depending upon which position the argument is in.

All subclasses of \class{Cmd} inherit a predefined \method{do_help()}.
This method, called with an argument \code{'bar'}, invokes the
corresponding method \method{help_bar()}.  With no argument,
\method{do_help()} lists all available help topics (that is, all
commands with corresponding \method{help_*()} methods), and also lists
any undocumented commands.
\end{methoddesc}

\begin{methoddesc}[Cmd]{onecmd}{str}
Interpret the argument as though it had been typed in response to the
prompt.  This may be overridden, but should not normally need to be;
see the \method{precmd()} and \method{postcmd()} methods for useful
execution hooks.  The return value is a flag indicating whether
interpretation of commands by the interpreter should stop.  If there
is a \method{do_*()} method for the command \var{str}, the return
value of that method is returned, otherwise the return value from the
\method{default()} method is returned.
\end{methoddesc}

\begin{methoddesc}[Cmd]{emptyline}{}
Method called when an empty line is entered in response to the prompt.
If this method is not overridden, it repeats the last nonempty command
entered.  
\end{methoddesc}

\begin{methoddesc}[Cmd]{default}{line}
Method called on an input line when the command prefix is not
recognized. If this method is not overridden, it prints an
error message and returns.
\end{methoddesc}

\begin{methoddesc}[Cmd]{completedefault}{text, line, begidx, endidx}
Method called to complete an input line when no command-specific
\method{complete_*()} method is available.  By default, it returns an
empty list.
\end{methoddesc}

\begin{methoddesc}[Cmd]{precmd}{line}
Hook method executed just before the command line \var{line} is
interpreted, but after the input prompt is generated and issued.  This
method is a stub in \class{Cmd}; it exists to be overridden by
subclasses.  The return value is used as the command which will be
executed by the \method{onecmd()} method; the \method{precmd()}
implementation may re-write the command or simply return \var{line}
unchanged.
\end{methoddesc}

\begin{methoddesc}[Cmd]{postcmd}{stop, line}
Hook method executed just after a command dispatch is finished.  This
method is a stub in \class{Cmd}; it exists to be overridden by
subclasses.  \var{line} is the command line which was executed, and
\var{stop} is a flag which indicates whether execution will be
terminated after the call to \method{postcmd()}; this will be the
return value of the \method{onecmd()} method.  The return value of
this method will be used as the new value for the internal flag which
corresponds to \var{stop}; returning false will cause interpretation
to continue.
\end{methoddesc}

\begin{methoddesc}[Cmd]{preloop}{}
Hook method executed once when \method{cmdloop()} is called.  This
method is a stub in \class{Cmd}; it exists to be overridden by
subclasses.
\end{methoddesc}

\begin{methoddesc}[Cmd]{postloop}{}
Hook method executed once when \method{cmdloop()} is about to return.
This method is a stub in \class{Cmd}; it exists to be overridden by
subclasses.
\end{methoddesc}

Instances of \class{Cmd} subclasses have some public instance variables:

\begin{memberdesc}[Cmd]{prompt}
The prompt issued to solicit input.
\end{memberdesc}

\begin{memberdesc}[Cmd]{identchars}
The string of characters accepted for the command prefix.
\end{memberdesc}

\begin{memberdesc}[Cmd]{lastcmd}
The last nonempty command prefix seen. 
\end{memberdesc}

\begin{memberdesc}[Cmd]{intro}
A string to issue as an intro or banner.  May be overridden by giving
the \method{cmdloop()} method an argument.
\end{memberdesc}

\begin{memberdesc}[Cmd]{doc_header}
The header to issue if the help output has a section for documented
commands.
\end{memberdesc}

\begin{memberdesc}[Cmd]{misc_header}
The header to issue if the help output has a section for miscellaneous 
help topics (that is, there are \method{help_*()} methods without
corresponding \method{do_*()} methods).
\end{memberdesc}

\begin{memberdesc}[Cmd]{undoc_header}
The header to issue if the help output has a section for undocumented 
commands (that is, there are \method{do_*()} methods without
corresponding \method{help_*()} methods).
\end{memberdesc}

\begin{memberdesc}[Cmd]{ruler}
The character used to draw separator lines under the help-message
headers.  If empty, no ruler line is drawn.  It defaults to
\character{=}.
\end{memberdesc}

\begin{memberdesc}[Cmd]{use_rawinput}
A flag, defaulting to true.  If true, \method{cmdloop()} uses
\function{input()} to display a prompt and read the next command;
if false, \method{sys.stdout.write()} and
\method{sys.stdin.readline()} are used. (This means that by
importing \refmodule{readline}, on systems that support it, the
interpreter will automatically support \program{Emacs}-like line editing 
and command-history keystrokes.)
\end{memberdesc}