Here's a quick walkthrough of a simple application which demonstrates 8
features of cmd2
:
- :ref:`features/settings:Settings`
- :ref:`features/commands:Commands`
- :ref:`features/argument_processing:Argument Processing`
- :ref:`features/generating_output:Generating Output`
- :ref:`features/help:Help`
- :ref:`features/shortcuts_aliases_macros:Shortcuts`
- :ref:`features/multiline_commands:Multiline Commands`
- :ref:`features/history:History`
If you don't want to type as we go, you can download the complete source for this example.
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
:class:`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 :meth:`~cmd2.Cmd.cmdloop` method. This method accepts user input and runs commands based on that input. Because we subclassed :class:`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.
Before we create our first command, we are going to add a setting to this app.
cmd2
includes robust support for :ref:`features/settings: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'))
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 :meth:`~cmd2.Cmd.add_settable`
method with a new instance of a :meth:`~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
.
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 :ref:`argument processor
<features/argument_processing:Argument Processing>` so the speak
command
can shout and talk piglatin. We will also use some built in methods for
:ref:`generating output <features/generating_output: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 = argparse.ArgumentParser() 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 :func:`~cmd2.decorators.with_argparser` decorator on the
do_speak()
method. This decorator does 3 useful things for us:
- It tells
cmd2
to process all input for thespeak
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. - 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. - 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 :ref:`generate output
<features/generating_output:Generating Output>`. It provides the following
benefits:
- Allows the user to redirect output to a text file or pipe it to a shell process
- Gracefully handles
BrokenPipeWarning
exceptions for redirected output - Makes the output show up in a :ref:`transcript <features/transcripts:Transcripts>`
- Honors the setting to :ref:`strip embedded ansi sequences <features/settings:allow_style>` (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 :ref:`command <features/commands:Commands>`, used an :ref:`Argument Processor <features/argument_processing:Argument Processing>`, added a nice :ref:`help message <features/help:Help>` for our users, and :ref:`generated some output <features/generating_output:Generating Output>`.
cmd2
has several capabilities to simplify repetitive user input:
:ref:`Shortcuts, Aliases, and Macros
<features/shortcuts_aliases_macros:Shortcuts, Aliases, and Macros>`. 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'))
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.
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 :ref:`multiline command
<features/multiline_commands:Multiline Commands>` to our application. First
we'll create a new command called orate
. This code shows both the
definition of our speak
command, and the orate
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).
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
:ref:`features/history:History` page has all the details.
Users can access command history using two methods:
- the readline library which provides a python interface to the GNU readline library
- the
history
command which is built-in tocmd2
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 :ref:`Command History For Users
<features/history:For Users>`.
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:
- Dive into all of the :doc:`../features/index` that
cmd2
provides - Look at more :doc:`../examples/index`
- Browse the :doc:`../api/index`