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First Application

Here's a quick walkthrough of a simple application which demonstrates 8 features of cmd2:

If you don't want to type as we go, here is the complete source (you can click to expand and then click the Copy button in the top-right):

??? example

```py
{%
    include "../../examples/first_app.py"
%}
```

Basic Application

First we need to create a new cmd2 application. Create a new file first_app.py with the following contents:

#!/usr/bin/env python
"""A simple cmd2 application."""
import cmd2


class FirstApp(cmd2.Cmd):
    """A simple cmd2 application."""


if __name__ == '__main__':
    import sys
    c = FirstApp()
    sys.exit(c.cmdloop())

We have a new class FirstApp which is a subclass of [cmd2.Cmd][]. When we tell python to run our file like this:

$ python first_app.py

it creates an instance of our class, and calls the cmd2.Cmd.cmdloop method. This method accepts user input and runs commands based on that input. Because we subclassed cmd2.Cmd, our new app already has a bunch of features built in.

Congratulations, you have a working cmd2 app. You can run it, and then type quit to exit.

Create a New Setting

Before we create our first command, we are going to add a setting to this app. cmd2 includes robust support for Settings. You configure settings during object initialization, so we need to add an initializer to our class:

def __init__(self):
    super().__init__()

    # Make maxrepeats settable at runtime
    self.maxrepeats = 3
    self.add_settable(cmd2.Settable('maxrepeats', int, 'max repetitions for speak command', self))

In that initializer, the first thing to do is to make sure we initialize cmd2. That's what the super().__init__() line does. Next create an attribute to hold the setting. Finally, call the [cmd2.Cmd.add_settable][] method with a new instance of a [cmd2.utils.Settable][] class. Now if you run the script, and enter the set command to see the settings, like this:

$ python first_app.py
(Cmd) set

you will see our maxrepeats setting show up with it's default value of 3.

Create A Command

Now we will create our first command, called speak which will echo back whatever we tell it to say. We are going to use an argument processor so the speak command can shout and talk piglatin. We will also use some built in methods for generating output. Add this code to first_app.py, so that the speak_parser attribute and the do_speak() method are part of the CmdLineApp() class:

speak_parser = cmd2.Cmd2ArgumentParser()
speak_parser.add_argument('-p', '--piglatin', action='store_true', help='atinLay')
speak_parser.add_argument('-s', '--shout', action='store_true', help='N00B EMULATION MODE')
speak_parser.add_argument('-r', '--repeat', type=int, help='output [n] times')
speak_parser.add_argument('words', nargs='+', help='words to say')

@cmd2.with_argparser(speak_parser)
def do_speak(self, args):
    """Repeats what you tell me to."""
    words = []
    for word in args.words:
        if args.piglatin:
            word = '%s%say' % (word[1:], word[0])
        if args.shout:
            word = word.upper()
        words.append(word)
    repetitions = args.repeat or 1
    for _ in range(min(repetitions, self.maxrepeats)):
        # .poutput handles newlines, and accommodates output redirection too
        self.poutput(' '.join(words))

Up at the top of the script, you'll also need to add:

import argparse

There's a bit to unpack here, so let's walk through it. We created speak_parser, which uses the argparse module from the Python standard library to parse command line input from a user. There is nothing thus far that is specific to cmd2.

There is also a new method called do_speak(). In both cmd and cmd2, methods that start with do_ become new commands, so by defining this method we have created a command called speak.

Note the cmd2.decorators.with_argparser decorator on the do_speak() method. This decorator does 3 useful things for us:

  1. It tells cmd2 to process all input for the speak command using the argparser we defined. If the user input doesn't meet the requirements defined by the argparser, then an error will be displayed for the user.
  2. It alters our do_speak method so that instead of receiving the raw user input as a parameter, we receive the namespace from the argparser.
  3. It creates a help message for us based on the argparser.

You can see in the body of the method how we use the namespace from the argparser (passed in as the variable args). We build an array of words which we will output, honoring both the --piglatin and --shout options.

At the end of the method, we use our maxrepeats setting as an upper limit to the number of times we will print the output.

The last thing you'll notice is that we used the self.poutput() method to display our output. poutput() is a method provided by cmd2, which I strongly recommend you use anytime you want to generate output. It provides the following benefits:

  1. Allows the user to redirect output to a text file or pipe it to a shell process
  2. Gracefully handles BrokenPipeWarning exceptions for redirected output
  3. Makes the output show up in a transcript
  4. Honors the setting to strip embedded ansi sequences (typically used for background and foreground colors)

Go run the script again, and try out the speak command. Try typing help speak, and you will see a lovely usage message describing the various options for the command.

With those few lines of code, we created a command, used an Argument Processor, added a nice help message for our users, and generated some output.

Shortcuts

cmd2 has several capabilities to simplify repetitive user input: Shortcuts and Aliases. Let's add a shortcut to our application. Shortcuts are character strings that can be used instead of a command name. For example, cmd2 has support for a shortcut ! which runs the shell command. So instead of typing this:

(Cmd) shell ls -al

you can type this:

(Cmd) !ls -al

Let's add a shortcut for our speak command. Change the __init__() method so it looks like this:

def __init__(self):
    shortcuts = cmd2.DEFAULT_SHORTCUTS
    shortcuts.update({'&': 'speak'})
    super().__init__(shortcuts=shortcuts)

    # Make maxrepeats settable at runtime
    self.maxrepeats = 3
    self.add_settable(cmd2.Settable('maxrepeats', int, 'max repetitions for speak command', self))

Shortcuts are passed to the cmd2 initializer, and if you want the built-in shortcuts of cmd2 you have to pass them. These shortcuts are defined as a dictionary, with the key being the shortcut, and the value containing the command. When using the default shortcuts and also adding your own, it's a good idea to use the .update() method to modify the dictionary. This way if you add a shortcut that happens to already be in the default set, yours will override, and you won't get any errors at runtime.

Run your app again, and type:

(Cmd) shortcuts

to see the list of all of the shortcuts, including the one for speak that we just created.

Multiline Commands

Some use cases benefit from the ability to have commands that span more than one line. For example, you might want the ability for your user to type in a SQL command, which can often span lines and which are terminated with a semicolon. Let's add a multiline command to our application. First we'll create a new command called orate. This code shows both the definition of our speakcommand, and theorate command:

@cmd2.with_argparser(speak_parser)
def do_speak(self, args):
    """Repeats what you tell me to."""
    words = []
    for word in args.words:
        if args.piglatin:
            word = '%s%say' % (word[1:], word[0])
        if args.shout:
            word = word.upper()
        words.append(word)
    repetitions = args.repeat or 1
    for _ in range(min(repetitions, self.maxrepeats)):
        # .poutput handles newlines, and accommodates output redirection too
        self.poutput(' '.join(words))

# orate is a synonym for speak which takes multiline input
do_orate = do_speak

With the new command created, we need to tell cmd2 to treat that command as a multi-line command. Modify the super initialization line to look like this:

super().__init__(multiline_commands=['orate'], shortcuts=shortcuts)

Now when you run the example, you can type something like this:

(Cmd) orate O for a Muse of fire, that would ascend
> The brightest heaven of invention,
> A kingdom for a stage, princes to act
> And monarchs to behold the swelling scene! ;

Notice the prompt changes to indicate that input is still ongoing. cmd2 will continue prompting for input until it sees an unquoted semicolon (the default multi-line command termination character).

History

cmd2 tracks the history of the commands that users enter. As a developer, you don't need to do anything to enable this functionality, you get it for free. If you want the history of commands to persist between invocations of your application, you'll need to do a little work. The History page has all the details.

Users can access command history using two methods:

From the prompt in a cmd2-based application, you can press Control-p to move to the previously entered command, and Control-n to move to the next command. You can also search through the command history using Control-r. The GNU Readline User Manual has all the details, including all the available commands, and instructions for customizing the key bindings.

The history command allows a user to view the command history, and select commands from history by number, range, string search, or regular expression. With the selected commands, users can:

  • re-run the commands
  • edit the selected commands in a text editor, and run them after the text editor exits
  • save the commands to a file
  • run the commands, saving both the commands and their output to a file

Learn more about the history command by typing history -h at any cmd2 input prompt, or by exploring Command History For Users.

Conclusion

You've just created a simple, but functional command line application. With minimal work on your part, the application leverages many robust features of cmd2. To learn more you can: