EBC Exercise 21b systemd

Embedded Linux Class by Mark A. Yoder

Traditionally user space initialization has been done using init.d, however recently many distributions have been switching to systemd

systemd is a system and service manager for Linux, compatible with SysV and LSB init scripts. systemd provides aggressive parallelization capabilities, uses socket and D-Bus activation for starting services, offers on-demand starting of daemons, keeps track of processes using Linux control groups, supports snapshotting and restoring of the system state, maintains mount and automount points and implements an elaborate transactional dependency-based service control logic.

Here we'll see how to use systemd as an administration and how to create a simple service.

Administration

There are a few simple commands that show what's running under systemd and how to stop and start them. The examples here were inspired by the twenty part series on systemd administration and the three part intro by Carla Schroder

To see what's running, run

bone$ systemctl
 UNIT                                                                    >
 proc-sys-fs-binfmt_misc.automount                                       >
 sys-devices-platform-ocp-44c00000.interconnect-44c00000.interconnect:seg>
 sys-devices-platform-ocp-44c00000.interconnect-44c00000.interconnect:seg>
 sys-devices-platform-ocp-47400000.target\x2dmodule-47401400.usb-musb\x2d>
 sys-devices-platform-ocp-47400000.target\x2dmodule-47401400.usb-musb\x2d>
 sys-devices-platform-ocp-47400000.target\x2dmodule-47401c00.usb-musb\x2d>
 sys-devices-platform-ocp-48000000.interconnect-48000000.interconnect:seg>
 sys-devices-platform-ocp-48000000.interconnect-48000000.interconnect:seg>
 sys-devices-platform-ocp-48000000.interconnect-48000000.interconnect:seg>
 sys-devices-platform-ocp-48000000.interconnect-48000000.interconnect:seg>
 sys-devices-platform-ocp-48000000.interconnect-48000000.interconnect:seg>
 sys-devices-platform-ocp-48000000.interconnect-48000000.interconnect:seg>
 sys-devices-platform-ocp-48000000.interconnect-48000000.interconnect:seg>
 sys-devices-platform-ocp-48000000.interconnect-48000000.interconnect:seg>
 sys-devices-platform-ocp-48000000.interconnect-48000000.interconnect:seg>
 sys-devices-platform-ocp-48000000.interconnect-48000000.interconnect:seg>
 sys-devices-platform-ocp-48000000.interconnect-48000000.interconnect:seg>
 sys-devices-platform-ocp-48000000.interconnect-48000000.interconnect:seg>
 sys-devices-platform-ocp-48000000.interconnect-48000000.interconnect:seg>
 sys-devices-platform-ocp-48000000.interconnect-48000000.interconnect:seg>
 sys-devices-platform-ocp-48000000.interconnect-48000000.interconnect:seg>
 sys-devices-platform-ocp-48000000.interconnect-48000000.interconnect:seg>
 sys-devices-platform-ocp-48000000.interconnect-48000000.interconnect:seg>
 sys-devices-platform-ocp-48000000.interconnect-48000000.interconnect:seg>
 sys-devices-platform-ocp-4a000000.interconnect-4a000000.interconnect:seg>
 sys-devices-virtual-misc-rfkill.device                                  >
 sys-devices-virtual-net-docker0.device                                  >
 sys-devices-virtual-tty-ttyGS0.device                                   >
 sys-module-configfs.device                                              >
 sys-module-fuse.device                                                  >
 sys-subsystem-net-devices-can0.device                                   >
 sys-subsystem-net-devices-can1.device                                   >
 sys-subsystem-net-devices-docker0.device                                >
 sys-subsystem-net-devices-eth0.device                                   >
 sys-subsystem-net-devices-usb0.device                                   >
 sys-subsystem-net-devices-usb1.device                                   >
 sys-subsystem-net-devices-wlan0.device                                  >
 -.mount                                                                 >
 dev-mqueue.mount                                                        >
 run-docker-netns-default.mount                                          >
 run-user-1000.mount                                                     >
 sys-fs-fuse-connections.mount

and so on. Look through the list and see what you recognize. If you make your window bigger you will see more information. Here's how to find more details about a given process.

bone$ systemctl status nginx.service
● nginx.service - A high performance web server and a reverse proxy server
    Loaded: loaded (/lib/systemd/system/nginx.service; enabled; vendor preset: enab>
    Active: active (running) since Wed 2023-01-11 15:25:04 EST; 1 weeks 5 days ago
      Docs: man:nginx(8)
  Main PID: 750 (nginx)
     Tasks: 2 (limit: 1024)
    Memory: 3.7M
       CPU: 2.018s
    CGroup: /system.slice/nginx.service
            ├─750 nginx: master process /usr/sbin/nginx -g daemon on; master_proces>
            └─751 nginx: worker process

Jan 11 15:25:01 ece434 systemd[1]: Starting A high performance web server and a reve>
Jan 11 15:25:04 ece434 systemd[1]: Started A high performance web server and a rever>

Stopping and Starting

You can stop a process with

bone$ sudo systemctl stop nginx.service

Now try accessing the web server (192.168.7.2). It isn't there. You can start it with

bone$ systemctl start nginx.service
bone$ systemctl status nginx.service
● nginx.service - A high performance web server and a reverse proxy server
    Loaded: loaded (/lib/systemd/system/nginx.service; enabled; vendor preset: enabled)
    Active: active (running) since Tue 2023-01-24 10:07:23 EST; 38s ago
      Docs: man:nginx(8)
   Process: 16409 ExecStartPre=/usr/sbin/nginx -t -q -g daemon on; master_process on; (code=exited, status=0/SUCCESS)
   Process: 16410 ExecStart=/usr/sbin/nginx -g daemon on; master_process on; (code=exited, status=0/SUCCESS)
  Main PID: 16411 (nginx)
     Tasks: 2 (limit: 1024)
    Memory: 5.7M
       CPU: 392ms
    CGroup: /system.slice/nginx.service
            ├─16411 nginx: master process /usr/sbin/nginx -g daemon on; master_process on;
            └─16412 nginx: worker process

Jan 24 10:07:12 ece434 systemd[1]: Starting A high performance web server and a reverse proxy server...
Jan 24 10:07:23 ece434 systemd[1]: Started A high performance web server and a reverse proxy server.

Notice the log messages have changed.

Now, when you browse to 192.168.7.2 you will get an answer.

Autostart at boot time

You can use enable and disable to make a service start (or not start) at boot time.

bone$ sudo systemctl disable nginx.service
Synchronizing state of nginx.service with SysV service script with /lib/systemd/system-sysv-install.
Executing: /lib/system/systemd-sysv-install disable nginx
Removed /etc/system/system/multi-user.target.wants/nginx.service.

bone$ sudo systemctl enable nginx.service
Synchronizing state of nginx.service with SysV service script with /lib/systemd/system-sysv-install.
Executing: /lib/system/systemd-sysv-install enable nginx
Created symlink /etc/systemd/system/multi-user.target.wants/nginx.service → /lib/systemd/system/nginx.service.

Watch out though, if some other service needs the service you disabled, it will start anyway.

Stopping no matter what

If you want to stop a service NO MATTER WHAT.

beagle$ ln -s /dev/null /etc/systemd/system/servicename.service
beagle$ systemctl daemon-reload

systemd first looks in /etc/systemd/system and then looks in /lib/systemd/system. The command above places an empty file in /etc/systemd/system, so the real file in /lib/systemd/system is never seen.

Running your own service

If you check in exercises/flask/server you find app4.py a server for some flask demos. You run it with:

beagle$ cd exercises/flask/server
beagle$ ./app4.py
 * Serving Flask app "app4" (lazy loading)
 * Environment: production
   WARNING: This is a development server. Do not use it in a production deployment.
   Use a production WSGI server instead.
 * Debug mode: on
 * Running on http://0.0.0.0:8081/ (Press CTRL+C to quit)
 * Restarting with stat
 * Debugger is active!
 * Debugger PIN: 103-331-254

Now point a browser to 192.168.7.2:8081. You'll see the demo served up by flask. Suppose you want the app4.py to automatically start every time the Beagle boots. Here is how to do it.

We need to create a service file and the quickest way is to find one that does similar things.

bone$ systemctl | grep nginx
 nginx.service  loaded active running   A high performance web server and a reverse proxy server

Check its status.

bone$ systemctl status nginx.service
● nginx.service - A high performance web server and a reverse proxy server
    Loaded: loaded (/lib/systemd/system/nginx.service; enabled; vendor preset: enabled)
    Active: active (running) since Tue 2023-01-24 10:07:23 EST; 3h 13min ago
...

Looks like the file is in /lib/systemd/system/nginx.service copy it to a handy place and take a look.

beagle$ cp /lib/systemd/system/nginx.service flask.service
beagle$ cat flask.service
[Unit]
Description=A high performance web server and a reverse proxy server
Documentation=man:nginx(8)
After=network. Target nss-lookup.target

[Service]
Type=forking
PIDFile=/run/nginx.pid
ExecStartPre=/usr/sbin/nginx -t -q -g 'daemon on; master_process on;'
ExecStart=/usr/sbin/nginx -g 'daemon on; master_process on;'
ExecReload=/usr/sbin/nginx -g 'daemon on; master_process on;' -s reload
ExecStop=-/sbin/start-stop-daemon --quiet --stop --retry QUIT/5 --pidfile /run/nginx.pid
TimeoutStopSec=5
KillMode=mixed

[Install]
WantedBy=multi-user.target

Modify the file so it will work for your server. Make sure to modify 'Description' as well or your service will confuse itself with the original nginx.service. Here's what mine looks like:

[Unit]
Description=flask server 

[Service]
WorkingDirectory=/home/debian/exercises/flask/server
ExecStart=/home/debian/exercises/flask/server/app4.py
SyslogIdentifier=flask

[Install]
WantedBy=multi-user.target

Once your flask. Service file is ready, copy it to the right place

bone$ sudo cp flask.service /lib/systemd/system

and start the server

beagle$ sudo systemctl start flask

Point your browser to 192.168.7.2:8081 and see if it works.

To make it work after rebooting

bone$ sudo systemctl enable flask
ln -s '/lib/systemd/system/flask.service' '/etc/systemd/system/multi-user.target.wants/boneServer.service'

Notice it copies your file to another place. Try rebooting and see if it works.

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Embedded Linux Class by Mark A. Yoder