Operating system shells have long been able to execute a sequence of commands saved in a text file. These script files simplify the repeated execution of long command sequences. cmd2 supports two similar mechanisms: command scripts and python scripts.

A command script contains a sequence of commands typed at the prompt of a cmd2 based application. Unlike operating system shell scripts, command scripts can't contain logic or loops.

Command scripts can be created in several ways:

• creating a text file using any method of your choice
• using the built-in edit command to create or edit an existing text file
• saving previously entered commands to a script file using history -s

If you create a text file from scratch, just include one command per line, exactly as you would type it inside a cmd2 application.

Command script files can be executed using the built-in run_script command or the @ shortcut (if your application is using the default shortcuts). Both ASCII and UTF-8 encoded Unicode text files are supported. The run_script command supports tab completion of file system paths. There is a variant _relative_run_script command or @@ shortcut (if using the default shortcuts) for use within a script which uses paths relative to the first script.

A command line is a comment if the first non-whitespace character is a #. This means any # character appearing later in the command will be treated as a literal. The same applies to a # in the middle of a multiline command, even if it is the first character on a line.

Comments are useful in scripts, but are generally not used within an interactive session.

(Cmd) # this is a comment
(Cmd) command # this is not a comment

If you require logic flow, loops, branching, or other advanced features, you can write a python script which executes in the context of your cmd2 app. This script is run using the run_pyscript command. Here's a simple example that uses the arg_printer.py pyscript:

(Cmd) run_pyscript examples/scripts/arg_printer.py foo bar 'baz 23'
Running Python script 'arg_printer.py' which was called with 3 arguments
arg 1: 'foo'
arg 2: 'bar'
arg 3: 'baz 23'

run_pyscript supports tab completion of file system paths, and as shown above it has the ability to pass command-line arguments to the scripts invoked.

If you as an app designer have not explicitly disabled the run_pyscript command, you should assume your application will be used for higher-level Python scripting. The following sections are meant as guidelines and highlight possible pitfalls with both production and consumption of API functionality. For clarity, a "scripter" writes pyscripts, and a "designer" is the cmd2 application author.

Without any work on the part of the designer, a scripter can take advantage of piecing together workflows using simple app calls. The result of a run_pyscript app call yields a [CommandResult][cmd2.CommandResult] object exposing four members: stdout, stderr, stop, and data.

stdout and stderr are fairly straightforward representations of normal data streams and accurately reflect what the user sees during normal cmd2 interaction. stop contains information about how the invoked command has ended its lifecycle. Lastly data contains any information the designer sets via self.last_result or self._cmd.last_result if called from inside a CommandSet.

Python scripts executed with run_pyscript can run cmd2 application commands by using the syntax:

app(‘command args’)

where:

• app is a configurable name which can be changed by setting the cmd2.Cmd.py_bridge_name attribute
• command and args are entered exactly like they would be entered by a user of your application.

Using f-strings tends to be the most straightforward and easily readable way to provide parameters.:

first = 'first'
second = 'second'

app(f'command {first} -t {second})

See python_scripting.py example and associated conditional.py script for more information.

If your cmd2 application follows the Unix design philosophy a scripter will have the most flexibility to create workflows using different commands. If the designer's application is more complete and less likely to be augmented in the future, a scripter can use simple serial scripts with little control flow. In either case, choices made by the designer will have effects on scripters.

The following diagram illustrates the different boundaries to keep in mind.

flowchart LR
    subgraph Py scripts
        direction TB
        subgraph cmd2 Application
            direction TB
            subgraph Class Library
                direction TB
                class1
                class2
                class3
                class4
            end
        end
    end

!!! note

As a designer, you should design from the inside out. Your code will be much easier to unit test than at the higher level. While there are regression testing extensions for cmd2, unit testing will always be faster for development.

!!! warning

It is bad design for a high-level pyscript to know about, let alone access, low-level class libraries of an application. Resist this urge as much as possible, unless it's necessary.

By default, cmd2 allows scripters to take advantage of all exposed do_* commands. As a scripter, you can easily interact with the application via stdout and stderr.

As a baseline, let's start with the following cmd2 application called FirstApp

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


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


    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))

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

Let's start with an example of what not to do:

app('speak'
print('Working')

SyntaxError: unexpected EOF while parsing
(Cmd) run_pyscript script.py
    File "<string>", line 2
    app('speak'
                ^
SyntaxError: unexpected EOF while parsing

cmd2 pyscripts require valid Python code as a first step.

!!! warning

It is a common misconception that all application exceptions will propagate up from below. This is not the case. `cmd2` catches all application exceptions and there are no means to handle them.

When executing the speak command without parameters you see the following error:

(Cmd) speak
Usage: speak [-h] [-p] [-s] [-r REPEAT] words [...]
Error: the following arguments are required: words

Even though this is a fully qualified cmd2 error, the pyscript must check for this error and perform error checking.:

app('speak')
print("Working")

(Cmd) run_pyscript script.py
Working
(Cmd)

You should notice that no error message is printed. Let's utilize the CommandResult object to inspect the actual returned data.:

result = app('speak')
print(result)

(Cmd) run_pyscript script.py
CommandResult(stdout='', stderr='Usage: speak [-h] [-p] [-s] [-r REPEAT] words [...]\nError: the following arguments are required: words\n\n', stop=False, data=None)

Now we can see that there has been an error. Let's rewrite the script to perform error checking.:

result = app('speak')

if not result:
    print(result.stderr)

(Cmd) run_pyscript script.py
Something went wrong

In Python development, it is good practice to fail fast after user input.:

import sys

result = app('speak TRUTH!!')

if not result:
    print("Something went wrong")
    sys.exit()

print("Continuing along..")

(Cmd) run_pyscript script.py
Continuing along..

We changed the input to be a valid speak command, but there was no output. Again we must inspect the CommandResult:

import sys

#Syntax error
result = app('speak TRUTH!!!')
if not result:
    print("Something went wrong")
    sys.exit()

print(result.stdout)

(Cmd) run_pyscript script.py
TRUTH!!!

By just using stdout and stderr it is possible to chain commands with rudimentary control flow. In the next section we will show how to use cmd_result data.

So far, we haven't focused on the scripter's needs. Wouldn't it be nice if while creating pyscripts you did not have to parse data from stdout? We can accommodate the scripter by adding one small line at the end of our do_* commands.

self.last_result = <value>

Adding the above line enhances a cmd2 application and opens a new world of possibilities.

!!! tip

When setting results for a command function inside of a CommandSet use the private cmd instance:

```py
self._cmd.last_result = <value>
```

In the following command example we return a list containing directory elements.:

dir_parser = cmd2.Cmd2ArgumentParser()
dir_parser.add_argument('-l', '--long', action='store_true',
                        help="display in long format with one item per line")

@cmd2.with_argparser(dir_parser, with_unknown_args=True)
def do_dir(self, args, unknown):
    """List contents of current directory."""
    # No arguments for this command
    if unknown:
        self.perror("dir does not take any positional arguments:")
        self.do_help('dir')
        return

    # Get the contents as a list
    contents = os.listdir(self.cwd)

    for f in contents:
        self.poutput(f'{f}')
    self.poutput('')

    self.last_result = contents

The following script retrieves the array contents.:

result = app('dir')
print(result.data)

Results:

Cmd) run_pyscript script.py
['.venv', 'app.py', 'script.py']

As a rule of thumb, designers should return simple scalar types as command results instead of complex objects. If it is beneficial in providing class objects designers should choose immutable over mutable types and never provide direct access to class members as this could potentially lead to violation of the open-closed_principle.

When possible, a frozen dataclass is a lightweight solution ideal for data manipulation. Let's look at an example.

The following application has two commands: build and status. Let's assume that the build action happens somewhere else in the world at a REST API endpoint and has significant computational cost. The status command, will only show the current status of a build task. The application has provided everything that is needed for a user to start a build and then determine its status. However, the problem is that with a long running process the user may want to wait for it to finish. A designer may be tempted to create a command to start a build and then poll for status until finished, but this scenario is better solved as a script.

app.py:

#!/usr/bin/env python
"""A simple cmd2 application."""
import sys
from dataclasses import dataclass
from random import choice, randint
from typing import Optional

import cmd2
from cmd2.parsing import Statement


@dataclass(frozen=True)
class BuildStatus:
    id: int
    name: str
    status: str


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

    def __init__(self):
        self._status_cache = dict()

    def _start_build(self, name: str) -> BuildStatus:
        return BuildStatus(randint(10, 100), name, "Started")

    def _get_status(self, name: str) -> Optional[BuildStatus]:

        status = self._status_cache.get(name)

        status_types = ["canceled", "restarted", "error", "finished"]

        if status.status != "finished":
            status = BuildStatus(status.id, status.name, choice(status_types))
            self._status_cache[name] = status

        return status

    build_parser = cmd2.Cmd2ArgumentParser()
    build_parser.add_argument("name", help="Name of build to start")

    @cmd2.with_argparser(build_parser)
    def do_build(self, args: Statement):
        """Executes a long running process at an API endpoint"""
        status = self._start_build(args.name)
        self._status_cache[args.name] = status

        self.poutput(
            f"Build {args.name.upper()} successfully started with id : {status.id}"
        )
        self.last_result = status

    status_parser = cmd2.Cmd2ArgumentParser()
    status_parser.add_argument("name", help="Name of build determine status of")

    @cmd2.with_argparser(status_parser)
    def do_status(self, args: Statement):
        """Shows the current status of a build"""

        status = self._get_status(args.name)

        self.poutput(f"Status for Build: {args.name} \n {status.status}")
        self.last_result = status


if __name__ == "__main__":
    import sys

    c = FirstApp()
    sys.exit(c.cmdloop())

Below is a possible solution via pyscript:

import sys
import time

# start build
result = app('build tower')

# If there was an error then exit
if not result:
    print('Build failed')
    sys.exit()

# This is a BuildStatus dataclass object
build = result.data

print(f"Build {build.name} : {build.status}")

# Poll status
while True:

    # Perform status check
    result = app('status tower')

    #error checking
    if not result:
        print("Unable to determine status")
        break

    build_status = result.data

    # If the status shows complete then the script is done
    if build_status.status in ['finished', 'canceled']:
        print(f"Build {build.name} has completed")
        break

    print(f"Current Status: {build_status.status}")
    time.sleep(1)