Android - How do I get my device detected by ADB on Linux?

Enable USB debugging on the device

This is done in Settings › Development. If you don't have that entry in your settings menu, go to Settings › About, scroll to the "Build number", and hammer it like a monkey until your device congratulates you having become a developer. Go back to the main page of the Settings menu, and close to the bottom you should see the "Development" (or "Developers") settings now. Enter it, and enable USB Debugging here.

Identify the device

First we need to know how the device identifies on the USB bus. For that, with the Android device NOT connected, grab a shell and run the command lsusb. Then connect the device and run the command again. Spot the new line. For the Wileyfox Swift this is a "nameless device":

Bus 004 Device 003: ID 2970:2282

Setting up the rules for ADB

We now need the numbers at the end of the above line: 2970:2282. These specify the vendor (2970) and the device itself (2282). Having those details, we need a root shell on our Linux machine to edit (or create, if it doesn't yet exist) the /etc/udev/rules.d/51-android.rules file. In there, add a line for your device. Following example line shows how it looks for the Wileyfox Swift:¹

SUBSYSTEMS=="usb", ATTRS{idVendor}=="2970", ATTRS{idProduct}=="2282", MODE="0666" GROUP="androiddev", SYMLINK+="android%n"

If you have a different device, replace the vendor and product IDs with what you've found above when running lsusb. A short explanation of the line:

• SUBSYSTEMS=="usb": obviously this rule is for USB only ;)
• ATTRS{idVendor}=="2970": the vendor ID of the device this rule is for
• ATTRS{idProduct}=="2282": the device ID
• MODE="0666": permissions the device node shall get. 0666 is quite lax, giving every user on your system read and write permission – so if you're concerned, you might try replacing that with a 0660 (giving only owner and group read-write, and deny everything to others).
• GROUP="androiddev": which group the device node should belong to. This should be a group the users intended to work with the device belong to.
• SYMLINK+="android%n": just to give the node a nice name, so you can find it easier in /dev (in my case, it later showed up there as /dev/android5)

That rule entered in /etc/udev/rules.d/51-android.rules, we must tell udev to make use of it. Safest way (next to a reboot ;) is restarting the udev service. Depending on your Linux distro, this can be done either via service udev restart or /etc/init.d/udev restart.

Done that, leave the root shell. Disconnect and reconnect your Android device, try adb devices again. Most devices showed up now, but not the Wileyfox Swift – which obviously wants some extra cuddles. If you're in that situation, open (or create if it doesn't exist) the file ~/.android/adb_usb.ini and add a single line to it, naming the vendor you've found out with lsusb above; for the Swift that would be 0x2970 (yupp, here you need to prefix it by 0x to point out it's a hexadecimal number). Then restart the ADB server: adb kill-server && adb start-server. Disconnect and reconnect the device again. Now adb devices should see it.

Connecting the device

You might have noticed adb devices told you something like 0123456789ABCDEF unauthorized. That's OK and for your (devices) safety: your computer must be authorized first to be able to access the device. So simply issue adb shell now – which will be quit with an error: device unauthorized. Please check the confirmation dialog on your device. Follow that advice (optionally mark the check-box to permanently authorize your computer), and you're done: Now you can use adb to access your device.

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Updates:

¹ Note that in later Linux versions, syntax for the UDEV rules has slightly changed, as e.g. jcomeau_ictx pointed out in his comment. For the values we found above that would be:

SUBSYSTEM=="usb", ATTR{idVendor}=="2970", ATTR{idProduct}=="2282", MODE="0666", GROUP="plugdev", SYMLINK+="android%n"

Two differences: it's now SUBSYSTEM (no plural), and the group has changed from androiddev to plugdev (the former does not exist on recent systems, the latter does and usually is assigned at least to the first user).

Additionally, you might need to add the vendorID to your ~/.android/adb_usb.ini (one ID per line, in hex notation):

# ANDROID 3RD PARTY USB VENDOR ID LIST
# 1 USB VENDOR ID PER LINE.
0x2970

A few comments from a newer Linux distribution. Fedora 29 with a Nexus 5X or the Nokia 7.1 (Android One) phone.

First disconnect the phone, if already connected.

1. Install android-tools which will supply ADB (sudo dnf install android-tools)
2. Copy udev rules (sudo cp /usr/share/doc/android-tools/51-android.rules /etc/udev/rules.d)
3. Reload udev rules (sudo udevadm control --reload-rules)
4. Restart ADB to be sure (sudo systemctl restart adb)

Now connect the phone and run adb devices from the command line. You will probably see a device listed with "no permissions". This is fine.
IF it is not listed, you will have to add your device to the udev rules file, but for me, the tested devices just worked with the pre-defined rules.

Run adb shell and hopefully you will get a security notification on the phone asking if you want to trust the computer, select yes.
IF instead your computer says "error:insufficient permissions for device", you need to make sure that on the phone you have set your USB port to "Transfer files" mode, and not "Charge this device". On Android 8.1 this is located in settings under "Connected devices" > "USB".

I have noticed that even if you have everything working today, that tomorrow it may suddenly break for no apparent reason. If that happens, first check the USB port setting on the device, which may have changed back to charging mode, and if that fails, revoke USB debugging authorisations on the device (in settings under Developer options), and you should hopefully get the pop-up again when you run adb shell.

With this, I am able to run Android Studio and run on the connected device.