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<span id="cmd-support-for-line-oriented-command-interpreters"></span><h1>cmd — Support for line-oriented command interpreters</h1> <p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/3.12/Lib/cmd.py">Lib/cmd.py</a></p> <p>The <a class="reference internal" href="#cmd.Cmd" title="cmd.Cmd"><code>Cmd</code></a> 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.</p> <dl class="py class"> <dt class="sig sig-object py" id="cmd.Cmd">
<code>class cmd.Cmd(completekey='tab', stdin=None, stdout=None)</code> </dt> <dd>
<p>A <a class="reference internal" href="#cmd.Cmd" title="cmd.Cmd"><code>Cmd</code></a> instance or subclass instance is a line-oriented interpreter framework. There is no good reason to instantiate <a class="reference internal" href="#cmd.Cmd" title="cmd.Cmd"><code>Cmd</code></a> itself; rather, it’s useful as a superclass of an interpreter class you define yourself in order to inherit <a class="reference internal" href="#cmd.Cmd" title="cmd.Cmd"><code>Cmd</code></a>’s methods and encapsulate action methods.</p> <p>The optional argument <em>completekey</em> is the <a class="reference internal" href="readline#module-readline" title="readline: GNU readline support for Python. (Unix)"><code>readline</code></a> name of a completion key; it defaults to <kbd class="kbd docutils literal notranslate">Tab</kbd>. If <em>completekey</em> is not <a class="reference internal" href="constants#None" title="None"><code>None</code></a> and <a class="reference internal" href="readline#module-readline" title="readline: GNU readline support for Python. (Unix)"><code>readline</code></a> is available, command completion is done automatically.</p> <p>The optional arguments <em>stdin</em> and <em>stdout</em> 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 <a class="reference internal" href="sys#sys.stdin" title="sys.stdin"><code>sys.stdin</code></a> and <a class="reference internal" href="sys#sys.stdout" title="sys.stdout"><code>sys.stdout</code></a>.</p> <p>If you want a given <em>stdin</em> to be used, make sure to set the instance’s <a class="reference internal" href="#cmd.Cmd.use_rawinput" title="cmd.Cmd.use_rawinput"><code>use_rawinput</code></a> attribute to <code>False</code>, otherwise <em>stdin</em> will be ignored.</p> </dd>
</dl> <section id="cmd-objects"> <span id="id1"></span><h2>Cmd Objects</h2> <p>A <a class="reference internal" href="#cmd.Cmd" title="cmd.Cmd"><code>Cmd</code></a> instance has the following methods:</p> <dl class="py method"> <dt class="sig sig-object py" id="cmd.Cmd.cmdloop">
<code>Cmd.cmdloop(intro=None)</code> </dt> <dd>
<p>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.</p> <p>The optional argument is a banner or intro string to be issued before the first prompt (this overrides the <a class="reference internal" href="#cmd.Cmd.intro" title="cmd.Cmd.intro"><code>intro</code></a> class attribute).</p> <p>If the <a class="reference internal" href="readline#module-readline" title="readline: GNU readline support for Python. (Unix)"><code>readline</code></a> module is loaded, input will automatically inherit <strong class="program">bash</strong>-like history-list editing (e.g. <kbd class="kbd compound docutils literal notranslate"><kbd class="kbd docutils literal notranslate">Control</kbd>-<kbd class="kbd docutils literal notranslate">P</kbd></kbd> scrolls back to the last command, <kbd class="kbd compound docutils literal notranslate"><kbd class="kbd docutils literal notranslate">Control</kbd>-<kbd class="kbd docutils literal notranslate">N</kbd></kbd> forward to the next one, <kbd class="kbd compound docutils literal notranslate"><kbd class="kbd docutils literal notranslate">Control</kbd>-<kbd class="kbd docutils literal notranslate">F</kbd></kbd> moves the cursor to the right non-destructively, <kbd class="kbd compound docutils literal notranslate"><kbd class="kbd docutils literal notranslate">Control</kbd>-<kbd class="kbd docutils literal notranslate">B</kbd></kbd> moves the cursor to the left non-destructively, etc.).</p> <p>An end-of-file on input is passed back as the string <code>'EOF'</code>.</p> <p id="index-0">An interpreter instance will recognize a command name <code>foo</code> if and only if it has a method <code>do_foo()</code>. As a special case, a line beginning with the character <code>'?'</code> is dispatched to the method <a class="reference internal" href="#cmd.Cmd.do_help" title="cmd.Cmd.do_help"><code>do_help()</code></a>. As another special case, a line beginning with the character <code>'!'</code> is dispatched to the method <code>do_shell()</code> (if such a method is defined).</p> <p>This method will return when the <a class="reference internal" href="#cmd.Cmd.postcmd" title="cmd.Cmd.postcmd"><code>postcmd()</code></a> method returns a true value. The <em>stop</em> argument to <a class="reference internal" href="#cmd.Cmd.postcmd" title="cmd.Cmd.postcmd"><code>postcmd()</code></a> is the return value from the command’s corresponding <code>do_*()</code> method.</p> <p>If completion is enabled, completing commands will be done automatically, and completing of commands args is done by calling <code>complete_foo()</code> with arguments <em>text</em>, <em>line</em>, <em>begidx</em>, and <em>endidx</em>. <em>text</em> is the string prefix we are attempting to match: all returned matches must begin with it. <em>line</em> is the current input line with leading whitespace removed, <em>begidx</em> and <em>endidx</em> 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.</p> </dd>
</dl> <dl class="py method"> <dt class="sig sig-object py" id="cmd.Cmd.do_help">
<code>Cmd.do_help(arg)</code> </dt> <dd>
<p>All subclasses of <a class="reference internal" href="#cmd.Cmd" title="cmd.Cmd"><code>Cmd</code></a> inherit a predefined <code>do_help()</code>. This method, called with an argument <code>'bar'</code>, invokes the corresponding method <code>help_bar()</code>, and if that is not present, prints the docstring of <code>do_bar()</code>, if available. With no argument, <code>do_help()</code> lists all available help topics (that is, all commands with corresponding <code>help_*()</code> methods or commands that have docstrings), and also lists any undocumented commands.</p> </dd>
</dl> <dl class="py method"> <dt class="sig sig-object py" id="cmd.Cmd.onecmd">
<code>Cmd.onecmd(str)</code> </dt> <dd>
<p>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 <a class="reference internal" href="#cmd.Cmd.precmd" title="cmd.Cmd.precmd"><code>precmd()</code></a> and <a class="reference internal" href="#cmd.Cmd.postcmd" title="cmd.Cmd.postcmd"><code>postcmd()</code></a> 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 <code>do_*()</code> method for the command <em>str</em>, the return value of that method is returned, otherwise the return value from the <a class="reference internal" href="#cmd.Cmd.default" title="cmd.Cmd.default"><code>default()</code></a> method is returned.</p> </dd>
</dl> <dl class="py method"> <dt class="sig sig-object py" id="cmd.Cmd.emptyline">
<code>Cmd.emptyline()</code> </dt> <dd>
<p>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.</p> </dd>
</dl> <dl class="py method"> <dt class="sig sig-object py" id="cmd.Cmd.default">
<code>Cmd.default(line)</code> </dt> <dd>
<p>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.</p> </dd>
</dl> <dl class="py method"> <dt class="sig sig-object py" id="cmd.Cmd.completedefault">
<code>Cmd.completedefault(text, line, begidx, endidx)</code> </dt> <dd>
<p>Method called to complete an input line when no command-specific <code>complete_*()</code> method is available. By default, it returns an empty list.</p> </dd>
</dl> <dl class="py method"> <dt class="sig sig-object py" id="cmd.Cmd.columnize">
<code>Cmd.columnize(list, displaywidth=80)</code> </dt> <dd>
<p>Method called to display a list of strings as a compact set of columns. Each column is only as wide as necessary. Columns are separated by two spaces for readability.</p> </dd>
</dl> <dl class="py method"> <dt class="sig sig-object py" id="cmd.Cmd.precmd">
<code>Cmd.precmd(line)</code> </dt> <dd>
<p>Hook method executed just before the command line <em>line</em> is interpreted, but after the input prompt is generated and issued. This method is a stub in <a class="reference internal" href="#cmd.Cmd" title="cmd.Cmd"><code>Cmd</code></a>; it exists to be overridden by subclasses. The return value is used as the command which will be executed by the <a class="reference internal" href="#cmd.Cmd.onecmd" title="cmd.Cmd.onecmd"><code>onecmd()</code></a> method; the <a class="reference internal" href="#cmd.Cmd.precmd" title="cmd.Cmd.precmd"><code>precmd()</code></a> implementation may re-write the command or simply return <em>line</em> unchanged.</p> </dd>
</dl> <dl class="py method"> <dt class="sig sig-object py" id="cmd.Cmd.postcmd">
<code>Cmd.postcmd(stop, line)</code> </dt> <dd>
<p>Hook method executed just after a command dispatch is finished. This method is a stub in <a class="reference internal" href="#cmd.Cmd" title="cmd.Cmd"><code>Cmd</code></a>; it exists to be overridden by subclasses. <em>line</em> is the command line which was executed, and <em>stop</em> is a flag which indicates whether execution will be terminated after the call to <a class="reference internal" href="#cmd.Cmd.postcmd" title="cmd.Cmd.postcmd"><code>postcmd()</code></a>; this will be the return value of the <a class="reference internal" href="#cmd.Cmd.onecmd" title="cmd.Cmd.onecmd"><code>onecmd()</code></a> method. The return value of this method will be used as the new value for the internal flag which corresponds to <em>stop</em>; returning false will cause interpretation to continue.</p> </dd>
</dl> <dl class="py method"> <dt class="sig sig-object py" id="cmd.Cmd.preloop">
<code>Cmd.preloop()</code> </dt> <dd>
<p>Hook method executed once when <a class="reference internal" href="#cmd.Cmd.cmdloop" title="cmd.Cmd.cmdloop"><code>cmdloop()</code></a> is called. This method is a stub in <a class="reference internal" href="#cmd.Cmd" title="cmd.Cmd"><code>Cmd</code></a>; it exists to be overridden by subclasses.</p> </dd>
</dl> <dl class="py method"> <dt class="sig sig-object py" id="cmd.Cmd.postloop">
<code>Cmd.postloop()</code> </dt> <dd>
<p>Hook method executed once when <a class="reference internal" href="#cmd.Cmd.cmdloop" title="cmd.Cmd.cmdloop"><code>cmdloop()</code></a> is about to return. This method is a stub in <a class="reference internal" href="#cmd.Cmd" title="cmd.Cmd"><code>Cmd</code></a>; it exists to be overridden by subclasses.</p> </dd>
</dl> <p>Instances of <a class="reference internal" href="#cmd.Cmd" title="cmd.Cmd"><code>Cmd</code></a> subclasses have some public instance variables:</p> <dl class="py attribute"> <dt class="sig sig-object py" id="cmd.Cmd.prompt">
<code>Cmd.prompt</code> </dt> <dd>
<p>The prompt issued to solicit input.</p> </dd>
</dl> <dl class="py attribute"> <dt class="sig sig-object py" id="cmd.Cmd.identchars">
<code>Cmd.identchars</code> </dt> <dd>
<p>The string of characters accepted for the command prefix.</p> </dd>
</dl> <dl class="py attribute"> <dt class="sig sig-object py" id="cmd.Cmd.lastcmd">
<code>Cmd.lastcmd</code> </dt> <dd>
<p>The last nonempty command prefix seen.</p> </dd>
</dl> <dl class="py attribute"> <dt class="sig sig-object py" id="cmd.Cmd.cmdqueue">
<code>Cmd.cmdqueue</code> </dt> <dd>
<p>A list of queued input lines. The cmdqueue list is checked in <a class="reference internal" href="#cmd.Cmd.cmdloop" title="cmd.Cmd.cmdloop"><code>cmdloop()</code></a> when new input is needed; if it is nonempty, its elements will be processed in order, as if entered at the prompt.</p> </dd>
</dl> <dl class="py attribute"> <dt class="sig sig-object py" id="cmd.Cmd.intro">
<code>Cmd.intro</code> </dt> <dd>
<p>A string to issue as an intro or banner. May be overridden by giving the <a class="reference internal" href="#cmd.Cmd.cmdloop" title="cmd.Cmd.cmdloop"><code>cmdloop()</code></a> method an argument.</p> </dd>
</dl> <dl class="py attribute"> <dt class="sig sig-object py" id="cmd.Cmd.doc_header">
<code>Cmd.doc_header</code> </dt> <dd>
<p>The header to issue if the help output has a section for documented commands.</p> </dd>
</dl> <dl class="py attribute"> <dt class="sig sig-object py" id="cmd.Cmd.misc_header">
<code>Cmd.misc_header</code> </dt> <dd>
<p>The header to issue if the help output has a section for miscellaneous help topics (that is, there are <code>help_*()</code> methods without corresponding <code>do_*()</code> methods).</p> </dd>
</dl> <dl class="py attribute"> <dt class="sig sig-object py" id="cmd.Cmd.undoc_header">
<code>Cmd.undoc_header</code> </dt> <dd>
<p>The header to issue if the help output has a section for undocumented commands (that is, there are <code>do_*()</code> methods without corresponding <code>help_*()</code> methods).</p> </dd>
</dl> <dl class="py attribute"> <dt class="sig sig-object py" id="cmd.Cmd.ruler">
<code>Cmd.ruler</code> </dt> <dd>
<p>The character used to draw separator lines under the help-message headers. If empty, no ruler line is drawn. It defaults to <code>'='</code>.</p> </dd>
</dl> <dl class="py attribute"> <dt class="sig sig-object py" id="cmd.Cmd.use_rawinput">
<code>Cmd.use_rawinput</code> </dt> <dd>
<p>A flag, defaulting to true. If true, <a class="reference internal" href="#cmd.Cmd.cmdloop" title="cmd.Cmd.cmdloop"><code>cmdloop()</code></a> uses <a class="reference internal" href="functions#input" title="input"><code>input()</code></a> to display a prompt and read the next command; if false, <a class="reference internal" href="sys#sys.stdout" title="sys.stdout"><code>sys.stdout.write()</code></a> and <a class="reference internal" href="sys#sys.stdin" title="sys.stdin"><code>sys.stdin.readline()</code></a> are used. (This means that by importing <a class="reference internal" href="readline#module-readline" title="readline: GNU readline support for Python. (Unix)"><code>readline</code></a>, on systems that support it, the interpreter will automatically support <strong class="program">Emacs</strong>-like line editing and command-history keystrokes.)</p> </dd>
</dl> </section> <section id="cmd-example"> <span id="id2"></span><h2>Cmd Example</h2> <p>The <a class="reference internal" href="#module-cmd" title="cmd: Build line-oriented command interpreters."><code>cmd</code></a> module is mainly useful for building custom shells that let a user work with a program interactively.</p> <p>This section presents a simple example of how to build a shell around a few of the commands in the <a class="reference internal" href="turtle#module-turtle" title="turtle: An educational framework for simple graphics applications"><code>turtle</code></a> module.</p> <p>Basic turtle commands such as <a class="reference internal" href="turtle#turtle.forward" title="turtle.forward"><code>forward()</code></a> are added to a <a class="reference internal" href="#cmd.Cmd" title="cmd.Cmd"><code>Cmd</code></a> subclass with method named <code>do_forward()</code>. The argument is converted to a number and dispatched to the turtle module. The docstring is used in the help utility provided by the shell.</p> <p>The example also includes a basic record and playback facility implemented with the <a class="reference internal" href="#cmd.Cmd.precmd" title="cmd.Cmd.precmd"><code>precmd()</code></a> method which is responsible for converting the input to lowercase and writing the commands to a file. The <code>do_playback()</code> method reads the file and adds the recorded commands to the <a class="reference internal" href="#cmd.Cmd.cmdqueue" title="cmd.Cmd.cmdqueue"><code>cmdqueue</code></a> for immediate playback:</p> <pre data-language="python">import cmd, sys
from turtle import *
class TurtleShell(cmd.Cmd):
intro = 'Welcome to the turtle shell. Type help or ? to list commands.\n'
prompt = '(turtle) '
file = None
# ----- basic turtle commands -----
def do_forward(self, arg):
'Move the turtle forward by the specified distance: FORWARD 10'
forward(*parse(arg))
def do_right(self, arg):
'Turn turtle right by given number of degrees: RIGHT 20'
right(*parse(arg))
def do_left(self, arg):
'Turn turtle left by given number of degrees: LEFT 90'
left(*parse(arg))
def do_goto(self, arg):
'Move turtle to an absolute position with changing orientation. GOTO 100 200'
goto(*parse(arg))
def do_home(self, arg):
'Return turtle to the home position: HOME'
home()
def do_circle(self, arg):
'Draw circle with given radius an options extent and steps: CIRCLE 50'
circle(*parse(arg))
def do_position(self, arg):
'Print the current turtle position: POSITION'
print('Current position is %d %d\n' % position())
def do_heading(self, arg):
'Print the current turtle heading in degrees: HEADING'
print('Current heading is %d\n' % (heading(),))
def do_color(self, arg):
'Set the color: COLOR BLUE'
color(arg.lower())
def do_undo(self, arg):
'Undo (repeatedly) the last turtle action(s): UNDO'
def do_reset(self, arg):
'Clear the screen and return turtle to center: RESET'
reset()
def do_bye(self, arg):
'Stop recording, close the turtle window, and exit: BYE'
print('Thank you for using Turtle')
self.close()
bye()
return True
# ----- record and playback -----
def do_record(self, arg):
'Save future commands to filename: RECORD rose.cmd'
self.file = open(arg, 'w')
def do_playback(self, arg):
'Playback commands from a file: PLAYBACK rose.cmd'
self.close()
with open(arg) as f:
self.cmdqueue.extend(f.read().splitlines())
def precmd(self, line):
line = line.lower()
if self.file and 'playback' not in line:
print(line, file=self.file)
return line
def close(self):
if self.file:
self.file.close()
self.file = None
def parse(arg):
'Convert a series of zero or more numbers to an argument tuple'
return tuple(map(int, arg.split()))
if __name__ == '__main__':
TurtleShell().cmdloop()
</pre> <p>Here is a sample session with the turtle shell showing the help functions, using blank lines to repeat commands, and the simple record and playback facility:</p> <pre data-language="none">Welcome to the turtle shell. Type help or ? to list commands.
(turtle) ?
Documented commands (type help <topic>):
========================================
bye color goto home playback record right
circle forward heading left position reset undo
(turtle) help forward
Move the turtle forward by the specified distance: FORWARD 10
(turtle) record spiral.cmd
(turtle) position
Current position is 0 0
(turtle) heading
Current heading is 0
(turtle) reset
(turtle) circle 20
(turtle) right 30
(turtle) circle 40
(turtle) right 30
(turtle) circle 60
(turtle) right 30
(turtle) circle 80
(turtle) right 30
(turtle) circle 100
(turtle) right 30
(turtle) circle 120
(turtle) right 30
(turtle) circle 120
(turtle) heading
Current heading is 180
(turtle) forward 100
(turtle)
(turtle) right 90
(turtle) forward 100
(turtle)
(turtle) right 90
(turtle) forward 400
(turtle) right 90
(turtle) forward 500
(turtle) right 90
(turtle) forward 400
(turtle) right 90
(turtle) forward 300
(turtle) playback spiral.cmd
Current position is 0 0
Current heading is 0
Current heading is 180
(turtle) bye
Thank you for using Turtle
</pre> </section> <div class="_attribution">
<p class="_attribution-p">
© 2001–2023 Python Software Foundation<br>Licensed under the PSF License.<br>
<a href="https://docs.python.org/3.12/library/cmd.html" class="_attribution-link">https://docs.python.org/3.12/library/cmd.html</a>
</p>
</div>
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