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.
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.
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 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.
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
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 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 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 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.
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 can run cmd2
application commands by using the syntax:
app(‘command args’)where:
appis a configurable name which can be changed by setting thecmd2.Cmd.py_bridge_nameattributecommandandargsare 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.:
first = 'first'
second = 'second'
app(f'command {first} -t {second})See python_scripting example and associated conditional script for more information.
If the cmd2 application follows the unix_design_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.
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.
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
#!/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:
app('speak'
print('Working')
SyntaxError: unexpected EOF while parsing
(Cmd) run_pyscript script.py
File "<string>", line 2
app('speak'
^
SyntaxError: unexpected EOF while parsingcmd2 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.:
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 re write 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 is good practice to fail and exit quickly 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 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 string together commands with rudimentary
control flow. In the next section we will show how to take advantage of cmd_result data.
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.:
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 = contentsThe 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 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.
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:
#!/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:
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)