Desktop entry to stop a service, launch an application and restart the service when the application is closed

Question

I have an application (OS is Ubuntu 16.04) that basically open the camera do some stuff with the video stream. It's managed by a systemd service:

[Unit]
Description=My Application
After=network.target
StartLimitInterval=0

[Service]
Type=simple
Restart=always
RestartSec=5
User=myuser
WorkingDirectory=/opt/foo/
ExecStart=/opt/foo/myapplication

[Install]
WantedBy=multi-user.target

Then I have a desktop entry that stop the service and re-run my application with an argument:

[Desktop Entry]
Name=Myapp
Type=Application
Exec=/opt/foo/launcher.sh
Terminal=true
Path=/opt/foo/
Categories=None;

This is my launcher:

#!/bin/bash
service myservice stop
/opt/foo/myapplication --debug

This is working, anyway I canno find a way to restart the service when I close the application launched by the desktop entry. I tried to add a service restart at the end of the launcher but it doesn't work, because when I close the terminal in which the application run, everything I presume is killed. How can I do it?

Answer 1

You can write a unit file in, say, /etc/systemd/system/myprocess.service:

[Unit]
Description=My Process

[Service]
ExecStart=/bin/myprocess
Restart=always

[Install]
WantedBy=multi-user.target

And then in the desktop entry create a launcher to do

systemctl start myprocess.service

But remember not to enable that service in startup so can you start it manually from you executable entry.
EDIT:
If you also need to run in debug mode than create another service unit with the --debug parameter. You can create another desktop entry to point a script that look like(just representative idea)

systemctl stop myprocess.service // to stop previous service
systemctl start processwithdebug.servce // to debug

Answer 2

You can have systemd do some of the work for you with a second unit B by saying that it Conflicts with the first unit A; then when you start B, it will stop A. Sadly, when B stops, A is not restarted by default, but you can add a OnFailure line; if B exits with a failure, A will be started. To ensure failure in case of normal exit of B you can use an ExecStop that sets the return code to non-zero.

A.service:

[Service]
Type=simple
ExecStart=/opt/foo/myapplication

B.service:

[Unit]
Conflicts=A.service
OnFailure=A.service
[Service]
Type=simple
ExecStart=/opt/foo/myapplication --debug
ExecStop=/usr/bin/false

Answer 3

You could use a lock file to ensure that only one instance of your application runs. Instead of systemctl stop myservice in your launcher, run systemctl restart myservice, and in the 5 seconds it needs to restart the application, start your debug version after acquiriing a lock file:

systemctl restart myservice
flock /opt/foo/mylock /opt/foo/myapplication --debug

and change the unit file to use the same lock:

ExecStart=/usr/bin/flock /opt/foo/mylock /opt/foo/myapplication

It will restart after 5 seconds then wait to acquire the lock, and hold it whilst running the application. The lock file can be anywhere. You can actually use /opt/foo/myapplication as the lock file, but if you are recompiling it or something you will lose the lock when it changes.