How to debug an udev rule (in /etc/udev/rules.d/...)

Question

I'm creating a new basic rule

/etc/udev/rules.d/10-myrule.rules

containing:

KERNEL!="sdb*", GOTO="auto_mount_end"
ACTION=="add", RUN+="/usr/bin/mount /dev/sdb1 /media"
LABEL="auto_mount_end"

I saved, rebooted, and inserted a SD card (recognized by /dev/sdb1, I see it with dmesg), but nothing happens. When I do manually mount /dev/sdb1 /media, it works.

How can I troubleshoot / debug such an udev rule?

Note: I'm using ArchLinux, but it should be the same on any distro?

Answer 1

Update

• Reference: udev_237 - man udev (Ubuntu_18.04)

  RUN{type}

   ︙

  Note that running programs that access the network or mount/unmount filesystems is not allowed inside of udev rules, due to the default sandbox that is enforced on systemd-udevd.service.

---

Original Answer Debugging hint are valid for other udev rule applications.

• 10- as mentioned by jasonwryan, use high numbering (90's good). So your rule is not going to be overridden by another one.

• Use the minimum keys just as you really need. Example, != & GOTO/LABEL, instead use directly ==

    ACTION=="add", KERNEL=="sdb*", RUN+="/usr/bin/mount /dev/sdb1 /media"
  

• Your target was sdb1 with fixed command, minimize the blind match using KERNEL=="sdb1"

• I find it useful to create a shadow debugging rule, I called shadow because I always leave it there in same file, so I use it when I need it.

    ACTION=="add", KERNEL=="sdb*", RUN+="/bin/sh -c 'echo == >> /home/user/Desktop/udev-env.txt; env >> /home/user/Desktop/udev-env.txt'"
    #ACTION=="add", KERNEL=="sdb*", RUN+="/usr/bin/mount /dev/sdb1 /media"
  

  Notes:

  • udev-env.txt is created then the rule is triggered anyway. Line == corresponding to one matching node. The ENV recorded in that file could be mixture between 2 node or more, created almost in same time, it's a stdout buffering problem.
  • Some environment variables that showed up in this debug, may not be used for condition because at by that time udev processing matches they are not yet populated (from previous rules). See https://www.suse.com/support/kb/doc/?id=000016106 (Mentioned by @clonejo in comments)

• Use udevadm monitor -u, udevadm test ... and udevadm trigger ... to verify which rules processed the events.

• Inside the scripts is up to you to make debug log and catch failed commands, by saving their return value also stdout & stderr messages.

Answer 2

This command will allow one to observe the execution of rules:

udevadm control --log-priority=debug
journalctl -f

(and udevadm control --log-priority=info to set it back to sane levels)

Answer 3

I think the command you're looking for here is udevadm. You'll use the trigger and test parameters to trigger a rescan of the udev events, and to test a specific event, respectively.

I learned this the hard way when putzing around with the new network device naming in EL 7. Good luck!

Answer 4

1. Create a udev rules file

   sudo nano /etc/udev/rules.d/99-removable-sd.rules
   

2. Add rule that tells udisks to automount it

   SUBSYSTEM=="block", SUBSYSTEMS=="mmc", DRIVERS=="mmcblk", ATTRS{type}=="SD", ENV{UDISKS_AUTO}="1", ENV{UDISKS_SYSTEM}="0"
   

   ATTRS{type}=="SD" may be not required if you are using different types.

3. Reload rules

   sudo udevadm control -R
   

4. Eject it then put back.

Reference: Archlinux Wiki: Some devices, that should be treated as removable, are not

Answer 5

I was having the same problem with RASPBERRY PI 3 B+, it could be possible that above commands may help you. But it DID NOT help me. I was trying to invoke a script on inserting a USB storage device. The rules do not get logged in syslog, so it becomes very difficult to understand which rule worked or which rule failed.

So I did the following:

(1) I made my rule file in /etc/udev/rules.d/100-myrule.rules

(2) then I ran the command sudo /etc/init.d/udev restart

then I checked it worked. A piece of information, may be useful to you or may not be, but the filesystems are readonly for udev until the command at (2) is executed.