How can I write to dmesg from command line?

Question

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

Solution 1

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.

Solution 2

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'

Solution 3

--> 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

Solution 4

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.

View more solutions

Comments

[Admin]

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.

[Admin]

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.

[Admin]

@Kevin, so you know without looking how many seconds have elapsed since you booted?

[Admin]

@Kevin it's the number of microseconds since boot, just formatted as seconds to make it easier to read for us humans.

[notlesh]

# echo "test" >> /proc/kmsg [yields =>] -su: echo: write error: Input/output error

[ephemient]

a. No need, others have written it already (e.g. kecho). b. Really no need for an extra module, see my answer.

[pevik]

It's not allowed on some system for non-root users (e.g.: Android kernel) :-(.

[sanmai]

E.g. date | sudo tee /dev/kmsg

[Admin]

This might have been better as an edit to the accepted answer, which already suggests writing to /dev/kmsg.