user70982: I've got Debian 10 as OS and for a Project, I need my arduino to always be connected as /dev/ttyUSB0, not any other ttyUSB-number, and I need it to be connected with the 0777 rights for a script to work.
I've read some things about udev, but nothing I tried yet worked.
I tried creating
/etc/udev/rules.d/51-my_usb_device
with the content
SUBSYSTEM=="tty", MODE="0777", KERNELS=="ttyUSB0"
but it does not work, even after reboot (like nothing changed). I must admit though that I do not really understand udev.
How to do this?

user48740: Have you considered not trying to force ttyUSB0 but instead creating a symlink to whatever tty is assigned? Your software then just uses the symlink.
A problem with forcing a particular tty is what if you boot without the Arduino but another device is attached? You then need to convince the other device to skip ttyUSB0.
If you would consider a symlink then something like:
ACTION=="add", KERNEL=="ttyUSB*", ATTRS{idVendor}=="1a86", ATTRS{idProduct}=="7523", SYMLINK+="arduino"

might get you started. Check the VID and PID using lsusb or similar. To set permissions as well you can use e.g. MODE="0777" as you already have.
There is an example at https://wiki.debian.org/udev that might give more background.

70982: <p>I've got Debian 10 as OS and for a Project, I need my arduino to always be connected as /dev/ttyUSB0, not any other ttyUSB-number, and I need it to be connected with the 0777 rights for a script to work.</p>
<p>I've read some things about udev, but nothing I tried yet worked.</p>
<p>I tried creating</p>
<p>/etc/udev/rules.d/51-my_usb_device</p>
<p>with the content</p>
<p>SUBSYSTEM==&quot;tty&quot;, MODE=&quot;0777&quot;, KERNELS==&quot;ttyUSB0&quot;</p>
<p>but it does not work, even after reboot (like nothing changed). I must admit though that I do not really understand udev.</p>
<p>How to do this?</p>
 48740: <p>Have you considered not trying to force ttyUSB0 but instead creating a symlink to whatever tty is assigned? Your software then just uses the symlink.</p>
<p>A problem with forcing a particular tty is what if you boot without the Arduino but another device is attached? You then need to convince the other device to skip ttyUSB0.</p>
<p>If you would consider a symlink then something like:</p>
<pre><code>ACTION==&quot;add&quot;, KERNEL==&quot;ttyUSB*&quot;, ATTRS{idVendor}==&quot;1a86&quot;, ATTRS{idProduct}==&quot;7523&quot;, SYMLINK+=&quot;arduino&quot;
</code></pre>
<p>might get you started. Check the VID and PID using <code>lsusb</code> or similar. To set permissions as well you can use e.g. <code>MODE=&quot;0777&quot;</code> as you already have.</p>
<p>There is an example at <a href="https://wiki.debian.org/udev" rel="nofollow noreferrer">https://wiki.debian.org/udev</a> that might give more background.</p>
