cmd module contains a base class for creating command interpreters.Module: cmd
Purpose: Create line-oriented command processors.
Python Version: 1.4 and later, with some additions in 2.3
Description:
The
cmd module contains one public class, Cmd, designed to be used as a base class for command processors such as interactive shells and other command interpreters. By default it uses readline for interactive prompt handling, command line editing, and command completion.Processing Commands:
The interpreter uses a loop to read all lines from its input, parse them, and then dispatch the command to an appropriate command handler. Input lines are parsed into two parts. The command, and any other text on the line. If the user enters a command
foo bar, and your class includes a method named do_foo(), it is called with “bar” as the only argument. The end-of-file marker is dispatched to
do_EOF(). If a command handler returns a true value, the program will exit cleanly. So to give a clean way to exit your interpreter, make sure to implement do_EOF() and have it return True.This simple example program supports the “greet” command:
import cmd
class HelloWorld(cmd.Cmd):
"""Simple command processor example."""
def do_greet(self, line):
print "hello"
def do_EOF(self, line):
return True
if __name__ == '__main__':
HelloWorld().cmdloop()
By running it interactively, we can demonstrate how commands are dispatched as well as show of some of the features included in
Cmd for free.$ python cmd_simple.py
(Cmd)
The first thing to notice is the command prompt, “(Cmd)“. The prompt can be configured through the attribute
prompt. If the prompt changes as the result of a command processor, the new value is used to query for the next command.(Cmd) help
Undocumented commands:
======================
EOF greet help
The help command is built into
Cmd. With no arguments, it shows the list of commands available. If you include a command you want help on, the output is more verbose and restricted to details of that command, when available.If we use the greet command,
do_greet() is invoked to handle it:(Cmd) greet
hello
If your class does not include a specific command processor for “foo”, the method
default() is called with the entire input line as an argument. The built-in implementation of default() reports an error.(Cmd) foo
*** Unknown syntax: foo
Since
do_EOF() returns True, hitting Ctrl-D will drop us out of the interpreter.(Cmd) ^D$
Notice that no newline is printed, so the results are a little messy.
Command Arguments:
This version of the example includes a few enhancements to eliminate some of the annoyances and add help for the greet command.
import cmd
class HelloWorld(cmd.Cmd):
"""Simple command processor example."""
def do_greet(self, person):
"""greet [person]
Greet the named person"""
if person:
print "hi,", person
else:
print 'hi'
def do_EOF(self, line):
return True
def postloop(self):
if __name__ == '__main__':
HelloWorld().cmdloop()
First, let’s look at the help. The docstring added to
do_greet() becomes the help text for the command:$ python cmd_arguments.py
(Cmd) help
Documented commands (type help):
========================================
greet
Undocumented commands:
======================
EOF help
(Cmd) help greet
greet [person]
Greet the named person
The optional argument to the greet command, “person”. There is a distinction between the argument to the command and the method. The method always takes the argument, but sometimes the value is an empty string. It is left up to the command processor to determine if an empty argument is valid, or do any further parsing and processing of the command. In this example, if a person’s name is provided then the greeting is personalized.
(Cmd) greet Alice
hi, Alice
(Cmd) greet
hi
Whether an argument is given by the user or not, the value passed to the command processor does not include the command itself. That simplifies parsing in the command processor, if multiple arguments are needed.
Live Help:
In the previous example, the formatting of the help text leaves something to be desired. Since it comes from the docstring, it retains the indentation from our source. Of course we could edit the source to remove the extra white-space, but that would leave our application looking poorly formatted. An alternative solution is to implement a help handler for the greet command, named
help_greet(). When present, it is called on to produce help text for the named command.import cmd
class HelloWorld(cmd.Cmd):
"""Simple command processor example."""
def do_greet(self, person):
if person:
print "hi,", person
else:
print 'hi'
def help_greet(self):
print '\n'.join([ 'greet [person]',
'Greet the named person',
])
def do_EOF(self, line):
return True
if __name__ == '__main__':
HelloWorld().cmdloop()
In this simple example, the text is static but formatted more nicely. It would also be possible to use previous command state to tailor the contents of the help text to the current context.
$ python cmd_do_help.py
(Cmd) help greet
greet [person]
Greet the named person
It is up to the help handler to actually output the help message, and not simply return the help text for handling elsewhere.
Auto-Completion:
Cmd includes support for command completion based on the names of the commands with processor methods. Completion is triggered by hitting the tab key on a line. When multiple completions are possible, pressing tab twice prints a list of the options.$ python cmd_do_help.py
(Cmd)
EOF greet help
(Cmd) h
(Cmd) help
Once the command is known, argument completion is handled by methods with the prefix complete_. This allows you to assemble a list of possible completions using your own criteria (query a database, look at at a file or directory on the filesystem, etc.). In this case, the program has a hard-coded set of “friends” who receive a less formal greeting than named or anonymous strangers. A real program would probably save the list somewhere, and either read it once and cache the contents to be scanned as needed.
import cmd
class HelloWorld(cmd.Cmd):
"""Simple command processor example."""
FRIENDS = [ 'Alice', 'Adam', 'Barbara', 'Bob' ]
def do_greet(self, person):
"Greet the person"
if person and person in self.FRIENDS:
greeting = 'hi, %s!' % person
elif person:
greeting = "hello, " + person
else:
greeting = 'hello'
print greeting
def complete_greet(self, text, line, begidx, endidx):
if not text:
completions = self.FRIENDS[:]
else:
completions = [ f
for f in self.FRIENDS
if f.startswith(text)
]
return completions
def do_EOF(self, line):
return True
if __name__ == '__main__':
HelloWorld().cmdloop()
When there is input text,
complete_greet() returns a list of friends that match. Otherwise, the full list of friends is returned.$ python cmd_arg_completion.py
(Cmd) greet
Adam Alice Barbara Bob
(Cmd) greet A
Adam Alice
(Cmd) greet Ad
(Cmd) greet Adam
hi, Adam!
If the name given is not in the list of friends, the formal greeting is given.
(Cmd) greet Joe
hello, Joe
Overriding Base Class Methods:
Cmd includes several methods that can be overridden as hooks for taking actions or altering the base class behavior. This example is not exhaustive, but contains many of the methods commonly useful.import cmd
class Illustrate(cmd.Cmd):
"Illustrate the base class method use."
def cmdloop(self, intro=None):
print 'cmdloop(%s)' % intro
return cmd.Cmd.cmdloop(self, intro)
def preloop(self):
print 'preloop()'
def postloop(self):
print 'postloop()'
def parseline(self, line):
print 'parseline(%s) =>' % line,
ret = cmd.Cmd.parseline(self, line)
print ret
return ret
def onecmd(self, s):
print 'onecmd(%s)' % s
return cmd.Cmd.onecmd(self, s)
def emptyline(self):
print 'emptyline()'
return cmd.Cmd.emptyline(self)
def default(self, line):
print 'default(%s)' % line
return cmd.Cmd.default(self, line)
def precmd(self, line):
print 'precmd(%s)' % line
return cmd.Cmd.precmd(self, line)
def postcmd(self, stop, line):
print 'postcmd(%s, %s)' % (stop, line)
return cmd.Cmd.postcmd(self, stop, line)
def do_greet(self, line):
print 'hello,', line
def do_EOF(self, line):
"Exit"
return True
if __name__ == '__main__':
Illustrate().cmdloop('Illustrating the methods of cmd.Cmd')
cmdloop() is the main processing loop of the interpreter. You can override it, but it is usually not necessary, since the preloop() and postloop() hooks are available.Each iteration through
cmdloop() calls onecmd() to dispatch the command to its processor. The actual input line is parsed with parseline() to create a tuple containing the command, and the remaining portion of the line.If the line is empty,
emptyline() is called. The default implementation runs the previous command again. If the line contains a command, first precmd() is called then the processor is looked up and invoked. If none is found, default() is called instead. Finally postcmd() is called.Here’s an example session with print statements added:
$ python cmd_illustrate_methods.py
cmdloop(Illustrating the methods of cmd.Cmd)
preloop()
Illustrating the methods of cmd.Cmd
(Cmd) greet Bob
precmd(greet Bob)
onecmd(greet Bob)
parseline(greet Bob) => ('greet', 'Bob', 'greet Bob')
hello, Bob
postcmd(None, greet Bob)
(Cmd) ^Dprecmd(EOF)
onecmd(EOF)
parseline(EOF) => ('EOF', '', 'EOF')
postcmd(True, EOF)
postloop()
Configuring Cmd Through Attributes:
In addition to the methods described above, there are several attributes for controlling command interpreters.
prompt can be set to a string to be printed each time the user is asked for a new command.
intro is the “welcome” message printed at the start of the program.
cmdloop() takes an argument for this value, or you can set it on the class directly.When printing help, the doc_header, misc_header, undoc_header, and ruler attributes are used to format the output.
This example class shows a command processor to let the user control the prompt for the interactive session.
import cmd
class HelloWorld(cmd.Cmd):
"""Simple command processor example."""
prompt = 'prompt: '
intro = "Simple command processor example."
doc_header = 'doc_header'
misc_header = 'misc_header'
undoc_header = 'undoc_header'
ruler = '-'
def do_prompt(self, line):
self.prompt = line + ': '
def do_EOF(self, line):
return True
if __name__ == '__main__':
HelloWorld().cmdloop()
$ python cmd_attributes.py
Simple command processor example.
prompt: prompt hello
hello: help
doc_header
----------
prompt
undoc_header
------------
EOF help
hello:
Shelling Out:
To supplement the standard command processing,
Cmd includes 2 special command prefixes. A question mark (?) is equivalent to the built-in help command, and can be used in the same way. An exclamation point (!) maps to do_shell(), and is intended for shelling out to run other commands, as in this example.import cmd
import os
class ShellEnabled(cmd.Cmd):
last_output = ''
def do_shell(self, line):
"Run a shell command"
print "running shell command:", line
output = os.popen(line).read()
print output
self.last_output = output
def do_echo(self, line):
"Print the input, replacing '$out' with the output of the last shell command"
# Obviously not robust
print line.replace('$out', self.last_output)
def do_EOF(self, line):
return True
if __name__ == '__main__':
ShellEnabled().cmdloop()
$ python cmd_do_shell.py
(Cmd) ?
Documented commands (type help):
========================================
echo shell
Undocumented commands:
======================
EOF help
(Cmd) ? shell
Run a shell command
(Cmd) ? echo
Print the input, replacing '$out' with the output of the last shell command
(Cmd) shell pwd
running shell command: pwd
/Users/dhellmann/Documents/PyMOTW/in_progress/cmd
(Cmd) ! pwd
running shell command: pwd
/Users/dhellmann/Documents/PyMOTW/in_progress/cmd
(Cmd) echo $out
/Users/dhellmann/Documents/PyMOTW/in_progress/cmd
(Cmd)
Alternative Inputs:
While the default mode for
Cmd is to interact with the user through the readline library, it is also possible to pass a series of commands in to standard input using standard Unix shell redirection.$ echo help | python cmd_do_help.py
(Cmd)
Documented commands (type help):
========================================
greet
Undocumented commands:
======================
EOF help
(Cmd)
If you would rather have your program read the script file directly, a few other changes may be needed. Since
readline interacts with the terminal/tty device, rather than the standard input stream, you should disable it if you know your script is going to be reading from a file. Also, to avoid printing superfluous prompts, you can set the prompt to an empty string. This example shows how to open a file and pass it as input to a modified version of the HelloWorld example.import cmd
class HelloWorld(cmd.Cmd):
"""Simple command processor example."""
# Disable rawinput module use
use_rawinput = False
# Do not show a prompt after each command read
prompt = ''
def do_greet(self, line):
print "hello,", line
def do_EOF(self, line):
return True
if __name__ == '__main__':
import sys
input = open(sys.argv[1], 'rt')
try:
HelloWorld(stdin=input).cmdloop()
finally:
input.close()
With use_rawinput set to
False and prompt set to an empty string, we can all the script on this input file:greet
greet Alice and Bob
to produce output like:
$ python cmd_file.py cmd_file.txt
hello,
hello, Alice and Bob
Commands from sys.argv:
If, instead of reading commands from stdin or a file, you want to process command line arguments to the program as a command for your interpreter class, that is also possible. In that case, you can call
onecmd() directly, as in this example.import cmd
class InteractiveOrCommandLine(cmd.Cmd):
"""Accepts commands via the normal interactive prompt or on the command line."""
def do_greet(self, line):
print 'hello,', line
def do_EOF(self, line):
return True
if __name__ == '__main__':
import sys
if len(sys.argv) > 1:
InteractiveOrCommandLine().onecmd(' '.join(sys.argv[1:]))
else:
InteractiveOrCommandLine().cmdloop()
Since
onecmd() takes a single string as input, the arguments to the program need to be joined together before being passed in.$ python cmd_argv.py greet Command Line User
hello, Command Line User
$ python cmd_argv.py
(Cmd) greet Interactive User
hello, Interactive User
(Cmd)
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