# Scripting

Operating system shells have long had the ability to execute a sequence of commands saved in a text file. These script files make long sequences of commands easier to repeatedly execute. `cmd2` supports two similar mechanisms: command scripts and python scripts.

## Command Scripts

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

### Creating Command Scripts

Command scripts can be created in several ways:

- creating a text file using any method of your choice
- using the built-in [edit](./builtin_commands.md#edit) command to create or edit an existing text file
- saving previously entered commands to a script file using [history -s](./history.md#for-users)

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

### Running Command Scripts

Command script files can be executed using the built-in [run_script](./builtin_commands.md#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](./builtin_commands.md#run_script) command supports tab completion of file system paths. There is a variant [\_relative_run_script](./builtin_commands.md#_relative_run_script) command or `@@` shortcut (if using the default shortcuts) for use within a script which uses paths relative to the first script.

### Comments

Any command line input where the first non-whitespace character is a `\#` will be treated as a comment. 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 would be pointless within an interactive session.

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

## Python Scripts

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](./builtin_commands.md#run_pyscript) command. Here's a simple example that uses the [arg_printer](https://github.com/python-cmd2/cmd2/blob/master/examples/scripts/arg_printer.py) script:

    (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](./builtin_commands.md#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.

## Developing a cmd2 API

If you as an app designer have not explicitly disabled the `run_pyscript` command it must be assumed that your application is structured for use in 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 when speaking of "scripter" we are referring to those writing scripts to be run by pyscript and "designer" as the `cmd2` application author.

### Basics

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` object exposing four members: `Stdout`, `Stderr`, `Stop`, and `Data`.

`Stdout` and `Stderr` are fairly straightforward representations of normal data streams and accurately reflect what is seen by the user 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](./builtin_commands.md#run_pyscript) can run `cmd2` application commands by using the syntax:

```py
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 fstrings tends to be the most straight forward and easily readable way to provide parameters.:

```py
first = 'first'
second = 'second'

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

See [python_scripting](https://github.com/python-cmd2/cmd2/blob/master/examples/python_scripting.py) example and associated [conditional](https://github.com/python-cmd2/cmd2/blob/master/examples/scripts/conditional.py) script for more information.

### Design principles

If the cmd2 application follows the [unix_design_philosophy](https://en.wikipedia.org/wiki/Unix_philosophy) a scriptor will have the most flexibility to piece together workflows using different commands. If the designers' application is more complete and less likely to be augmented in the future a scripter may opt for 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.

```mermaid
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 it is preferable to design from the inside to out. Your code will be infinitely far easier to unit test than at the higher level. While there are regression testing extensions for cmd2 UnitTesting will always be faster for development.

!!! warning

    It is bad design or a high level py_script to know about let alone access low level class libraries of an application. Resist this urge at all costs, unless it's necessary.

### Developing a Basic API

CMD2 out of the box allows scripters to take advantage of all exposed `do_*` commands. As a scripter one can easily interact with the application via `stdout` and `stderr`.

As a baseline lets start off with the familiar FirstApp

```py
#!/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())
```

Lets start off on the wrong foot:

```py
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 "bubble" up from below. Unfortunately or fortunately this is not the case. `cmd2` sinkholes 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 py[script]{#script} must look for this error and perform error checking.:

```py
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.:

```py
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 re write the script to perform error checking.:

```py
result = app('speak')

if not result:
    print(result.stderr)
```

    (Cmd) run_pyscript script.py
    Something went wrong

In python development is good practice to fail and exit quickly after user input.:

```py
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 no output. Again we must inspect the `CommandResult`:

```py
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 string together commands with rudimentary control flow. In the next section we will show how to take advantage of `cmd_result` data.

### Developing an Advanced API

Until now the application designer has paid little attention to scripters and their needs. Wouldn't it be nice if while creating py*scripts one did not have to parse data from `stdout`? We can accommodate the weary scripter by adding one small line at the end of our `do*\*` commands.

`self.last_result = <value>`

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

!!! note

    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 an array containing directory elements.:

```py
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.:

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

Results:

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

As a rule of thumb it is safer for the designer to return simple scalar types as command results instead of complex objects. If there is benefit 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](https://en.wikipedia.org/wiki/Open%E2%80%93closed_principle).

When possible, a dataclass is a lightweight solution perfectly suited for data manipulation. Lets dive into an example.

The following fictitional application has two commands: `build` and `status`. We can pretend that the build action happens somewhere else in the world at an REST API endpoint and has significant computational cost. The status command for all intents and purposes will only show the current status of a build task. The application has provided all that is needed for a user to start a build and then determine it's status. The problem however 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 an extensible script.

app.py:

```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 stared 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())
```

The below is a possible solution via pyscript:

```py
import sys
import time

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

# If there was an error then quit now
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 (it would be wise to NOT hang here)
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 we are 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)
```