cmd2 is designed to make it easy for you to create new commands. These
commmands form the backbone of your application. If you started writing your
application using cmd, all the commands you have built will work when you move
cmd2. However, there are many more capabilities available in
which you can take advantage of to add more robust features to your commands,
and which makes your commands easier to write. Before we get to all the good
stuff, let’s briefly discuss how to create a new command in your application.
cmd2 application looks like this:
#!/usr/bin/env python """A simple cmd2 application.""" import cmd2 class App(cmd2.Cmd): """A simple cmd2 application.""" if __name__ == '__main__': import sys c = App() sys.exit(c.cmdloop())
This application subclasses
cmd2.Cmd but has no code of it’s own, so all
functionality (and there’s quite a bit) is inherited. Lets create a simple
command in this application called
echo which outputs any arguments given
to it. Add this method to the class:
def do_echo(self, line): self.poutput(line)
When you type input into the
cmd2 prompt, the first space delimited word is
treated as the command name.
cmd2 looks for a method called
do_commandname. If it exists, it calls the method, passing the rest of the
user input as the first argument. If it doesn’t exist
cmd2 prints an error
message. As a result of this behavior, the only thing you have to do to create
a new command is to define a new method in the class with the appropriate name.
This is exactly how you would create a command using the cmd module which is
part of the python standard library.
See Generating Output if you are
unfamiliar with the
A command is passed one argument: a string which contains all the rest of the
user input. However, in
cmd2 this string is actually a
object, which is a subclass of
str to retain backwards compatibility.
cmd2 has a much more sophsticated parsing engine than what’s included in
the cmd module. This parsing handles:
- quoted arguments
- output redirection and piping
- multi-line commands
- shortcut, macro, and alias expansion
In addition to parsing all of these elements from the user input,
has code to make all of these items work; it’s almost transparent to you and to
the commands you write in your own application. However, by passing your
Statement object instead of just a plain string, you can get
visibility into what
cmd2 has done with the user input before your command
got it. You can also avoid writing a bunch of parsing code, because
gives you access to what it has already parsed.
Statement object is a subclass of
str that contains the following
- Name of the command called. You already know this because of the method
cmd2called, but it can sometimes be nice to have it in a string, i.e. if you want your error messages to contain the command name.
- A string containing the arguments to the command with output redirection or
piping to shell commands removed. It turns out that the “string” value of
Statementobject has all the output redirection and piping clauses removed as well. Quotes remain in the string.
- A string of just the command and the arguments, with output redirection or piping to shell commands removed.
- A list of arguments a-la
sys.argv, including the command as
argvand the subsequent arguments as additional items in the list. Quotes around arguments will be stripped as will any output redirection or piping portions of the command.
- Full input exactly as typed by the user.
- Character used to end a multiline command. You can configure multiple termination characters, and this attribute will tell you which one the user typed.
For many simple commands, like the
echo command above, you can ignore the
Statement object and all of it’s attributes and just use the passed value
as a string. You might choose to use the
argv attribute to do more
sophisticated argument processing. Before you go too far down that path, you
should check out the Argument Processing
functionality included with
Most commands should return nothing (either by omitting a
return None. This indicates that your command is finished (with or
without errors), and that
cmd2 should prompt the user for more input.
If you return
True from a command method, that indicates to
it should stop prompting for user input and cleanly exit.
quit command, but if you wanted to make another one called
finis you could:
def do_finis(self, line): """Exit the application""" return True
cmd2 has basic infrastructure to support sh/ksh/csh/bash type exit codes.
cmd2.Cmd object sets an
exit_code attribute to zero when it is
instantiated. The value of this attribute is returned from the
call. Therefore, if you don’t do anything with this attribute in your code,
cmdloop() will (almost) always return zero. There are a few built-in
cmd2 commands which set
-1 if an error occurs.
You can use this capability to easily return your own values to the operating system shell:
#!/usr/bin/env python """A simple cmd2 application.""" import cmd2 class App(cmd2.Cmd): """A simple cmd2 application.""" def do_bail(self, line): """Exit the application"" self.perror("fatal error, exiting") self.exit_code = 2 return true if __name__ == '__main__': import sys c = App() sys.exit(c.cmdloop())
If the app was run from the bash operating system shell, then you would see the following interaction:
(Cmd) bail fatal error, exiting $ echo $? 2
You may choose to catch and handle any exceptions which occur in
a command method. If the command method raises an exception,
catch it and display it for you. The debug setting controls how the exception is displayed. If
debug is false, which is the default,
cmd2 will display the exception
name and message. If debug is true,
cmd2 will display a traceback, and
then display the exception name and message.