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executor 9.9

Programmer friendly subprocess wrapper

The executor package is a simple wrapper for Python’s subprocess module that makes it very easy to handle subprocesses on UNIX systems with proper escaping of arguments and error checking:

  • An object oriented interface is used to execute commands using sane but customizable (and well documented) defaults.
  • Remote commands (executed over SSH) are supported using the same object oriented interface.
  • There’s also support for executing a group of commands concurrently in what’s called a “command pool”. The concurrency level can be customized and of course both local and remote commands are supported.

The package is currently tested on Python 2.6, 2.7, 3.4, 3.5 and PyPy. For usage instructions please refer to following sections and the documentation.

Installation

The executor package is available on PyPI which means installation should be as simple as:

$ pip install executor

There’s actually a multitude of ways to install Python packages (e.g. the per user site-packages directory, virtual environments or just installing system wide) and I have no intention of getting into that discussion here, so if this intimidates you then read up on your options before returning to these instructions ;-).

Usage

There are two ways to use the executor package: As the command line program executor and as a Python API. The command line interface is described below and there are also some examples of simple use cases of the Python API.

Command line

Usage: executor [OPTIONS] COMMAND …

Easy subprocess management on the command line based on the Python package with the same name. The “executor” program runs external commands with support for timeouts, dynamic startup delay (fudge factor) and exclusive locking.

You can think of “executor” as a combination of the “flock” and “timelimit” programs with some additional niceties (namely the dynamic startup delay and integrated system logging on UNIX platforms).

Supported options:

Option Description
-t, --timeout=LIMIT Set the time after which the given command will be aborted. By default LIMIT is counted in seconds. You can also use one of the suffixes “s” (seconds), “m” (minutes), “h” (hours) or “d” (days).
-f, --fudge-factor=LIMIT This option controls the dynamic startup delay (fudge factor) which is useful when you want a periodic task to run once per given interval but the exact time is not important. Refer to the --timeout option for acceptable values of LIMIT, this number specifies the maximum amount of time to sleep before running the command (the minimum is zero, otherwise you could just include the command “sleep N && …” in your command line :-).
-e, --exclusive Use an interprocess lock file to guarantee that executor will never run the external command concurrently. Refer to the --lock-timeout option to customize blocking / non-blocking behavior. To customize the name of the lock file you can use the --lock-file option.
-T, --lock-timeout=LIMIT By default executor tries to claim the lock and if it fails it will exit with a nonzero exit code. This option can be used to enable blocking behavior. Refer to the --timeout option for acceptable values of LIMIT.
-l, --lock-file=NAME Customize the name of the lock file. By default this is the base name of the external command, so if you’re running something generic like “bash” or “python” you might want to change this :-).
-v, --verbose Make more noise than usual (increase logging verbosity).
-q, --quiet Make less noise than usual (decrease logging verbosity).
-h, --help Show this message and exit.

Python API

Below are some examples of how versatile the execute() function is. Refer to the API documentation on Read the Docs for (a lot of) other use cases.

Checking status codes

By default the status code of the external command is returned as a boolean:

>>> from executor import execute
>>> execute('true')
True

If an external command exits with a nonzero status code an exception is raised, this makes it easy to do the right thing (never forget to check the status code of an external command without having to write a lot of repetitive code):

>>> execute('false')
Traceback (most recent call last):
  File "executor/__init__.py", line 124, in execute
    cmd.start()
  File "executor/__init__.py", line 516, in start
    self.wait()
  File "executor/__init__.py", line 541, in wait
    self.check_errors()
  File "executor/__init__.py", line 568, in check_errors
    raise ExternalCommandFailed(self)
executor.ExternalCommandFailed: External command failed with exit code 1! (command: bash -c false)

The ExternalCommandFailed exception exposes command and returncode attributes. If you know a command is likely to exit with a nonzero status code and you want execute() to simply return a boolean you can do this instead:

>>> execute('false', check=False)
False

Providing input

Here’s how you can provide input to an external command:

>>> execute('tr a-z A-Z', input='Hello world from Python!\n')
HELLO WORLD FROM PYTHON!
True

Getting output

Getting the output of external commands is really easy as well:

>>> execute('hostname', capture=True)
'peter-macbook'

Running commands as root

It’s also very easy to execute commands with super user privileges:

>>> execute('echo test > /etc/hostname', sudo=True)
[sudo] password for peter: **********
True
>>> execute('hostname', capture=True)
'test'

Enabling logging

If you’re wondering how prefixing the above command with sudo would end up being helpful, here’s how it works:

>>> import logging
>>> logging.basicConfig()
>>> logging.getLogger().setLevel(logging.DEBUG)
>>> execute('echo peter-macbook > /etc/hostname', sudo=True)
DEBUG:executor:Executing external command: sudo bash -c 'echo peter-macbook > /etc/hostname'

Running remote commands

To run a command on a remote system using SSH you can use the RemoteCommand class, it works as follows:

>>> from executor.ssh.client import RemoteCommand
>>> cmd = RemoteCommand('localhost', 'echo $SSH_CONNECTION', capture=True)
>>> cmd.start()
>>> cmd.output
'127.0.0.1 57255 127.0.0.1 22'

Running remote commands concurrently

The foreach() function wraps the RemoteCommand and CommandPool classes to make it very easy to run a remote command concurrently on a group of hosts:

>>> from executor.ssh.client import foreach
>>> from pprint import pprint
>>> hosts = ['127.0.0.1', '127.0.0.2', '127.0.0.3', '127.0.0.4']
>>> commands = foreach(hosts, 'echo $SSH_CONNECTION')
>>> pprint([cmd.output for cmd in commands])
['127.0.0.1 57278 127.0.0.1 22',
 '127.0.0.1 52385 127.0.0.2 22',
 '127.0.0.1 49228 127.0.0.3 22',
 '127.0.0.1 40628 127.0.0.4 22']

Contact

The latest version of executor is available on PyPI and GitHub. The documentation is hosted on Read the Docs. For bug reports please create an issue on GitHub. If you have questions, suggestions, etc. feel free to send me an e-mail at peter@peterodding.com.

License

This software is licensed under the MIT license.

© 2016 Peter Odding.

 
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