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I'm familiar with creating ordinary systemd services. However, when learning how to configure wireguard, I came across the '@' symbol in service names and was wondering exactly how it works.

For example, in wireguard, you can configure a connection in /etc/wireguard/wg0.conf and then control that connection as if it had it's own service file by running sudo systemctl [enable|disable|start|stop|whatever] [email protected]. If you create a second config file called wg1.conf, you can control it via the [email protected].

this is all very handy, but how does it work? what is it called?

user3243135
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  • Relating: https://unix.stackexchange.com/q/588629/315749 - about systemd instance units in general, does not address the specific WireGuar case. – fra-san Nov 27 '22 at 14:01
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    `man systemd.unit`, look for "template file" – Panki Nov 27 '22 at 14:02

1 Answers1

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@ symbols in SystemD services are used to create services that take a variable so as you can have multiple of the same service with different configs/targets.

So in this case it allows you to distinguish between configurations, but it's used by many other things such as ceph to allow you to run multiple of the same service but serving up different resources.

Basically when you create your systemd service file you can use %i and it will substitute in whatever comes after the @ symbol.

So you could create a oneshot service that just echos whatever is after the @ like this

[Unit]
Description=Echo '%I'

[Service]
Type=oneshot
ExecStart=/bin/echo %i
StandardOutput=syslog

And anything you put after the @ would be put in syslog.

Hope that helps!

Greenonline
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Insanemal
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    You explained `%i`, but in the code you used `%i` and `%I`. Are they equivalent? – Kamil Maciorowski Nov 27 '22 at 23:35
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    @KamilMaciorowski - Isn't it just a description? Note that the `Echo` is also capitalised, with a `E` rather than a `e`. Maybe the capitalisation is used to differentiate the description from the actual command. Even so, it is a bit misleading, I must admit. – Greenonline Nov 27 '22 at 23:46
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    @Greenonline It is a description and I want to know why it's not `Echo '%i'`. I know a description can be almost anything, but is there a technical reason to use `%I` or not to use `%i`? – Kamil Maciorowski Nov 27 '22 at 23:51
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    @KamilMaciorowski Just read the manual `systemd.unit(5)`. The lower case `%i` has its special characters escaped. – yt7b97q- Nov 28 '22 at 03:58