pystemd

pystemd

This library allows you to talk to systemd over D-Bus from Python, without actually thinking that you are talking to systemd over D-Bus. This allows you to programmatically start/stop/restart/kill and verify service status from systemd's point of view, avoiding executing subprocess.Popen(['systemctl', ... and then parsing the output to know the result.

Show don't tell

In software as in screenwriting, it's better to show how things work instead of tell. So this is how you would use the library from an interactive shell.

In [1]: from pystemd.systemd1 import Unit
In [2]: unit = Unit(b'postfix.service')
In [3]: unit.load()

Note: you need to call unit.load() because by default Unit will not load the unit information as that would require doing some I/O (and we don't like doing I/O in a class constructor). You can autoload the unit by Unit(b'postfix.service', _autoload=True) or using the unit as a context manager like with Unit(b'postfix.service'): ...

Once the unit is loaded, you can interact with it by accessing its systemd interfaces:

In [4]: unit.Unit.ActiveState
Out[4]: b'active'

In [5]: unit.Unit.StopWhenUnneeded
Out[5]: False

In [6]: unit.Unit.Stop(b'replace') # requires a privileged account
Out[6]: b'/org/freedesktop/systemd1/job/6601531'

In [7]: unit.Unit.ActiveState
Out[7]: b'inactive'

In [8]: unit.Unit.SubState
Out[8]: b'running'

In [9]: unit.Unit.Start(b'replace') # requires a privileged account
Out[9]: b'/org/freedesktop/systemd1/job/6601532'

In [10]: unit.Unit.ActiveState
Out[10]: b'active'

In [11]: unit.Service.GetProcesses() # requires systemd v238 and above
Out[11]:
[(b'/system.slice/postfix.service',
    1754222,
    b'/usr/libexec/postfix/master -w'),
 (b'/system.slice/postfix.service', 1754224, b'pickup -l -t fifo -u'),
 (b'/system.slice/postfix.service', 1754225, b'qmgr -l -t fifo -u')]

In [12]: unit.Service.MainPID
Out[12]: 1754222

The systemd1.Unit class provides shortcuts for the interfaces in the systemd namespace. As shown above, we have Service (org.freedesktop.systemd1.Service) and Unit (org.freedesktop.systemd1.Unit). Others can be found in unit._interfaces:

In [12]: unit._interfaces
Out[12]:
{'org.freedesktop.DBus.Introspectable': <org.freedesktop.DBus.Introspectable of /org/freedesktop/systemd1/unit/postfix_2eservice>,
 'org.freedesktop.DBus.Peer': <org.freedesktop.DBus.Peer of /org/freedesktop/systemd1/unit/postfix_2eservice>,
 'org.freedesktop.DBus.Properties': <org.freedesktop.DBus.Properties of /org/freedesktop/systemd1/unit/postfix_2eservice>,
 'org.freedesktop.systemd1.Service': <org.freedesktop.systemd1.Service of /org/freedesktop/systemd1/unit/postfix_2eservice>,
 'org.freedesktop.systemd1.Unit': <org.freedesktop.systemd1.Unit of /org/freedesktop/systemd1/unit/postfix_2eservice>}

 In [13]: unit.Service
 Out[13]: <org.freedesktop.systemd1.Service of /org/freedesktop/systemd1/unit/postfix_2eservice>

Each interface has methods and properties that you can access directly as unit.Service.MainPID. The list of properties and methods is in .properties and .methods of each interface.

The above code operates on system (a.k.a root) units by default. To operate on user units, explicitly pass in a user mode D-Bus instance:

from pystemd.dbuslib import DBus
with DBus(user_mode=True) as bus:
    unit = Unit(b"postfix.service", bus=bus)
    unit.load()

Alongside systemd1.Unit, we also have systemd1.Manager, which allows you to interact with the systemd manager.

In [14]: from pystemd.systemd1 import Manager

In [15]: manager = Manager()

In [16]: manager.load()

In [17]: manager.Manager.ListUnitFiles()
Out[17]:
...
(b'/usr/lib/systemd/system/rhel-domainname.service', b'disabled'),
 (b'/usr/lib/systemd/system/fstrim.timer', b'disabled'),
 (b'/usr/lib/systemd/system/getty.target', b'static'),
 (b'/usr/lib/systemd/system/systemd-user-sessions.service', b'static'),
...

In [18]: manager.Manager.Architecture
Out[18]: b'x86-64'

In [19]: manager.Manager.Virtualization
Out[19]: b'kvm'

Extras

We also include pystemd.run, the spiritual port of systemd-run to Python. Example of usage:

# run this as root
>>> import pystemd.run, sys
>>> pystemd.run(
    [b'/usr/bin/psql', b'postgres'],
    machine=b'db1',
    user=b'postgres',
    wait=True,
    pty=True,
    stdin=sys.stdin, stdout=sys.stdout,
    env={b'PGTZ': b'UTC'}
)

will open a PostgreSQL interactive prompt in a local nspawn machine.

You also get an interface to sd_notify in the form of pystemd.daemon.notify docs.

# run this as root
>>> import pystemd.daemon
>>> pystemd.daemon.notify(False, ready=1, status='Gimme! Gimme! Gimme!')

And access to listen file descriptors for socket-activated scripts.

# run this as root
>>> import pystemd.daemon
>>> pystemd.daemon.LISTEN_FDS_START
3
>>> pystemd.daemon.listen_fds()
1 # you normally only open 1 socket

And access to check if watchdog is enabled and ping it.

import time
import pystemd.daemon

watchdog_usec = pystemd.daemon.watchdog_enabled()
watchdog_sec = watchdog_usec/10**6

if not watchdog_usec:
  print('watchdog was not enabled!')

for i in range(20):
    pystemd.daemon.notify(False, watchdog=1, status=f'count {i+1}')
    time.sleep(watchdog_sec*0.5)

print('sleeping for 30 seconds')
time.sleep(watchdog_sec*2)
print('you will never reach me in a watchdog env')

We also provide basic journal interaction with pystemd.journal docs

import logging
import pystemd.journal

pystemd.journal.sendv(
  f"PRIORITY={logging.INFO}",
  MESSAGE="everything is awesome",
  SYSLOG_IDENTIFIER="tegan"
)

will result in the following message (shortened for the sake of the example).

{

  "SYSLOG_IDENTIFIER" : "tegan",
  "PRIORITY" : "20",
  "MESSAGE" : "everything is awesome",
  ...
}

Install

If you like what you see, the simplest way to install pystemd is:

$ pip install pystemd

pystemd is packaged in a few distros like Fedora and Debian. It is available in Fedora 32+ and in EPEL 8+.

It can be installed with:

$ sudo dnf install python3-pystemd # fedora
$ sudo apt install python3-pystemd # debian

which will also take care of installing any required dependencies. Keep in mind that most distros manage their own repos and versions, so you may be getting older versions.

Build from source

You'll need to have:

• Python headers: Just use your distro's package (e.g. python-dev).
• systemd headers: Chances are you already have this. Normally, it is called libsystemd-dev or systemd-devel. You need to have at least v237. Please note that CentOS 7 ships with version 219. To work around this, please read this.
• systemd library: check if pkg-config --cflags --libs libsystemd returns -lsystemd if not you can install systemd-libs or libsystemd depending on your distribution, version needs to be at least v237.
• gcc: or any compiler that setup.py will accept.
• pkg-config command. Depending on your distro, the package is called "pkg-config", "pkgconfig" or a compatible substitute like "pkgconf"

If you want to install from source, after cloning this repo all you need to do is pip install .

In addition to the previous requirements, you'll need:

• setuptools: Just use your distro's package (e.g. python-setuptools).
• Cython: at least version 0.21a1. Just pip install it or use the official installation guide from the Cython homepage to get the latest version: http://cython.readthedocs.io/en/latest/src/quickstart/install.html.

Learning more

This project has been covered in a number of conference talks:

• Using systemd in high level languages at All Systems Go! 2018
• systemd: why you should care as a Python developer at PyCon 2018
• Better security for Python with systemd at Pyninsula #10

A Vagrant-based demo was also developed for PyCon 2018.

License

pystemd is licensed under LGPL 2.1 or later.