How can I write to dmesg from command line?

Question

I'd like to write a statement to dmesg. How can I do this?

Two reasons I've wanted to do this before: (1) to see what "now is" in dmesg-timestamp format, and (2) to know when I last looked at dmesg. — notlesh, Apr 03 '12 at 20:39
I'm pretty sure `now` is seconds from boot, and I think it takes a kernel call to write to it so a userland program can't. — Kevin, Apr 03 '12 at 21:22
@Kevin, so you know without looking how many seconds have elapsed since you booted? — notlesh, Apr 04 '12 at 01:28
@Kevin it's the number of *microseconds* since boot, just formatted as seconds to make it easier to read for us humans. — Matthieu, Aug 02 '18 at 12:10

score 70 · Accepted Answer · edited Apr 13 '17 at 12:37

70

Write to /dev/kmsg (not /proc/kmsg as suggested by @Nils). See linux/kernel/printk/printk.c devkmsg_writev for the kernel-side implementation and systemd/src/journal/journald-kmsg.c server_forward_kmsg for an example of usage.

edited Apr 13 '17 at 12:37

Community

1

answered Apr 03 '12 at 21:42

ephemient

15,640
5
49
39

It's not allowed on some system for non-root users (e.g.: Android kernel) :-(. – pevik Jan 07 '16 at 11:20
25

E.g. `date | sudo tee /dev/kmsg` – sanmai Apr 11 '16 at 03:46
Worth noting, that while this will make the message appear in `dmesg`, however over a `netconsole` debugging utility *(where you'd expect to see the same content)* you will not see the message until you increase `dmesg` debugging level. For example `sudo dmesg -n 8` works. – Hi-Angel Mar 26 '23 at 21:37
As a n00b, I was pleasantly surprised that you have only to put a colon in your message if you want `dmesg` to highlight the prefix: `echo "BLAH: This is your message!" | sudo tee /dev/kmsg`. – AbuNassar Jun 29 '23 at 17:55

score 20 · Answer 2 · edited Jul 19 '20 at 17:20

20

For BSDs:

logger -p kern.notice MESSAGE

(courtesy Ian, freebsd-questions mailing list)

or other priorities.

For Linux:

su root -c 'echo MESSAGE > /dev/kmsg'

edited Jul 19 '20 at 17:20

Jeff Schaller

66,199
35
114
250

answered May 11 '16 at 06:39

xitij

381
3
6

score 1 · Answer 3 · answered Apr 03 '12 at 21:22

1

Assuming nobody else comes up with an official way to do this ...

You can write a kernel module that calls the printk function. There's an example here that might just do the job for you.

answered Apr 03 '12 at 21:22

ams

5,707
1
19
27

10

a. No need, others have written it already (e.g. [kecho](http://cgit.freedesktop.org/~halfline/kecho/)). b. **Really** no need for an extra module, see my answer. – ephemient Apr 03 '12 at 21:44

score 1 · Answer 4 · edited Mar 27 '20 at 00:08

--> You may write a C program as below:

test_mod.c

#include <linux/module.h>
#include <linux/kernel.h>

int init_module(void)
{
    printk("Hello All\n This is a test init\n");
    return 0;
}

void cleanup_module(void)
{
    printk("Good Bye");
}

--> make object file:

echo "obj-m := test_mod.o" > Makefile

--> compile by running :

make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules`

--> load your module as below:

insmod ./test_mod.ko

see the output:

dmesg | tail

--> unload module:

rmmod test_mod.ko

score 0 · Answer 5 · answered Jun 13 '22 at 11:20

0

echo "Add your message here" | sudo tee /dev/kmsg

Verify by running dmesg -T

answered Jun 13 '22 at 11:20

Sashank

121
4

3

This might have been better as an edit to [the accepted answer](https://unix.stackexchange.com/a/35647/116858), which already suggests writing to `/dev/kmsg`. – Kusalananda Jun 13 '22 at 11:27

score -1 · Answer 6 · edited Aug 02 '19 at 13:21

-1

If I understood man dmesg correctly, you should be able to write to /proc/kmsg.

edited Aug 02 '19 at 13:21

Rot-man

143
1
6

answered Apr 03 '12 at 20:47

Nils

18,202
11
46
82

4

# echo "test" >> /proc/kmsg [yields =>] -su: echo: write error: Input/output error – notlesh Apr 03 '12 at 20:50

How can I write to dmesg from command line?

6 Answers6